Bridge Design Manual-Texas Department of Transportation

Bridge Design ManualDecember 2001 © by Texas Department of Transportation (512) 416-2055 all rights reserved Manual Notice 2001-1 To: From: Manual: Districts, Divisions and Offices Steven E. Simmons, P.E. Deputy Executive Director Bridge Design Manual Effective Date: December 1, 2001 Purpose This manual provides policies and guidelines set forth by TxDOT regarding the design of bridges. It has been developed to help bridge designers working on TxDOT projects. Instructions This is a new manual containing new and significantly reorganized material. It supersedes the 1990 Bridge Design Guide and the 1990 Bridge Design Examples, both first editions. Contents The manual contains ten chapters – Organizational Overview, TxDOT and Bridge Design, Design Specifications, Geometric Restraints, Preliminary Considerations, General Design Controls, Superstructure Design, Substructure Design, Special Designs, and Foundation Design. The manual also has four appendices. Contact For more information regarding any chapter or section in this manual, please contact the Design Section of the Bridge Division. Chapter 1 Organizational Overview Contents: Section 1 — This Manual .....................................................................................................1-2 Section 2 — Evolution of the TxDOT Bridge Division .......................................................1-4 Bridge Design Manual 1-1 TxDOT 12/2001 Chapter 1 — Organizational Overview Section 1 — This Manual Section 1 This Manual Overview This manual was developed to provide bridge designers working on Texas Department of Transportation (TxDOT) projects with the policies and guidelines set forth by TxDOT regarding the design of bridges. Its purpose is to improve the bridge design and detailing process by promoting uniformity among bridge designers working on TxDOT projects. This manual is subject to revision as conditions, experience, or research data warrant. Changes will be issued by Manual Notice. Changes may be in the form of new sheets to be added, revised sheets to replace superceded ones, or sheets to be deleted. The manual is not intended to be a complete substitute for engineering experience, knowledge, or judgment. Special situations may arise that appear to call for variation from the policy requirements herein. Such variation will be subject to approval of the administration of the TxDOT Bridge Division. Direct any questions or comments on the content of the manual to the Director of the Design Section of the Bridge Division, Texas Department of Transportation. Bridge Design Manual Format The manual begins with this overview of the manual and a description of the evolution of the TxDOT Bridge Division. The chapters that follow include information on TxDOT Divisions/Sections, design specifications, geometric restraints, preliminary considerations, general design controls, superstructure design, substructure design, special designs, and foundation design. The following paragraphs briefly discuss these chapters: Chapter 2 presents a description of the TxDOT Divisions/Sections primarily involved in bridge design, planning, construction, and maintenance and provides descriptions of the responsibilities of the TxDOT Bridge Division’s Bridge Design Section. Chapter 3 lists and briefly describes the governing design specifications, or “Rule Books,” involved in bridge design. The chapter includes information on mandatory specifications, guide specifications, and industry recommendations. Chapter 4 discusses the common roadway geometric restraints inherent in bridge design. Bridge widths, span lengths, clearances, and alignment are discussed. A section on the constraints involved during stage construction is also included. Chapter 5 presents some common aspects a designer/planner must consider during the preliminary planning and design process. These aspects include materials, structure type, Bridge Design Manual 1-2 TxDOT 12/2001 Chapter 1 — Organizational Overview Section 1 — This Manual economics, and aesthetics. A discussion on bridge railing and use of corrosion protection is also included. Chapter 6 discusses in greater detail some of the more common design specifications involved during the design of a bridge, giving the designer additional information on the application and usage of common design specifications and criteria. Chapter 7 presents design criteria and design guidance for the most commonly used superstructure types, including cast-in-place, precast, and steel superstructures. Background information on the development of each superstructure type is also included. Chapter 8 presents design criteria and design guidance for the most commonly used substructure items, including caps, columns, and foundations. Background information on the development of some of these items is also included. Chapter 9 presents design criteria and design guidance for designs that inherently involve unique aspects, culverts and drainage, bridge appurtenances, sign bridges, and some common bridge items. Background information on the development of some of these designs is also included. Chapter 10 discusses in greater detail the relationship between structural design and geotechnical design. Some guidance on bridge foundation designs and retaining wall designs is included, as well as background information on the development of some of these items. Bridge Design Manual 1-3 TxDOT 12/2001 Chapter 1 — Organizational Overview Section 2 — Evolution of the TxDOT Bridge Division Section 2 Evolution of the TxDOT Bridge Division Origin of the TxDOT Bridge Division The Texas Highway Department was established in 1917 and is responsible to the Governor of Texas to design, construct, and maintain an adequate system of highways in the state. In 1918, a Bridge Office was created with the primary responsibility of preparing standard designs and drawings in an attempt to bring some uniformity to the bridges being constructed by the counties. The Bridge Division appeared in 1928, retaining bridge design as a big part of its mission. The Bridge Division continued to maintain standards and design non-standard bridges. In time, advance planning, railroad negotiations, and plan review capabilities were developed. Construction management was provided for some of the more complicated structures. 1940s and 1950s Activities were curtailed during the war years, but in the late 1940s and 1950s increased demand for improved infrastructure produced a large volume of expressways, for which special design offices were established in the affected cities. Some of the groups adopted their own design and detailing standards. When welding began to replace rivets for field splices in steel beams and girders in the early 1950s, the Bridge Division sent qualified welders to the larger projects to help with quality assurance and quality control. In the middle 1950s the Bridge Division, with the cooperation of precast manufacturers, developed a group of standard pretensioned concrete beams, which quickly proved to be the most economical way to construct medium-span length bridges. When the Interstate Highway System was inaugurated in the middle 1950s, the design workload increased dramatically and has remained generally good to date. Between the expressway offices, district design groups, and the Bridge Division, plan preparation was handled for several years with a minimum of help from consulting engineers. Recent Years In the early 1980s, consulting engineers began to do a significant portion of the highway plans and a somewhat smaller portion of the bridge plans. After a period of reduced activity, consulting engineers are now preparing a significant portion of highway and bridge plans. Meanwhile, the use of TxDOT “bridge standards” has become more uniform, as many district design groups have abandoned their own plan preparation activities. Currently, the Bridge Design Manual 1-4 TxDOT 12/2001 Chapter 1 — Organizational Overview Section 2 — Evolution of the TxDOT Bridge Division Bridge Division continues to prepare its share of structure plans while attending to a growing number of non-engineering responsibilities. Bridge Design Manual 1-5 TxDOT 12/2001 Chapter 2 TxDOT and Bridge Design Contents: Section 1 — Coordinating with TxDOT Divisions and Sections .........................................2-2 Section 2 — Primary Responsibilities of the Bridge Design Section...................................2-3 Section 3 — Coordination Responsibilities of the Bridge Design Section...........................2-7 Section 4 — Contractive Responsibilities of the Bridge Design Section ...........................2-10 Bridge Design Manual 2-1 TxDOT 12/2001 Chapter 2 — TxDOT and Bridge Design Section 1 — Coordinating with TxDOT Divisions and Sections Section 1 Coordinating with TxDOT Divisions and Sections Overview Before a bridge is designed. Bridge Design Manual 2-2 TxDOT 12/2001 . Maintenance Operations Section Within the Bridge Division. The planning and design of a bridge project involves several divisions and sections within the Texas Department of Transportation (TxDOT). Some of the contributing entities are: ♦ ♦ ♦ ♦ ♦ ♦ ♦ Bridge Division Design Division. the Bridge Design Section is responsible for functions that include engineering and non-engineering aspects of bridge design. These responsibilities are discussed in this chapter. Field Coordination Section Transportation Planning and Programming Division Traffic Operations Division. Railroad Section Environmental Affairs Division Construction Division. critical preliminary functions described in the Bridge Project Development Manual must be completed. Materials Section Maintenance Division. the procedure for preparation of plans by the Bridge Design Section is as follows. These layouts are usually complete with geometric controls. but the routine for consultants will be similar. Consultation between the Bridge Design Section. At this time. specifications. timing for the plan work is re-negotiated with the district and the job of bridge plan preparation is given to the bridge design engineer or to the consultant. soil test boring data. bridge project development manager. Bridge Plan Preparation. type of foundation should be proposed and conveyance of water through stream crossings and scour analysis should be addressed and coordinated with the Hydraulics Section. These primary responsibilities involve a procedure that begins with a concept to construct a highway facility and concludes with the submission of finalized plans. Generally. required clearances. The procedure includes many steps and it is important to know these steps to fully grasp the responsibilities of the Bridge Design Section. and primarily on its ability to complete the plans in the required length of time. The area engineer or the project’s designated consulting engineer prepare preliminary bridge layouts. and/or area engineer should precede determination of structure type and scheduling of the letting. The following steps apply to the Bridge Design Section. When approval has been secured from all the appropriate agencies. The layouts are sent to the bridge project development manager who will forward them to the Design Division.Chapter 2 — TxDOT and Bridge Design Section 2 — Primary Responsibilities of the Bridge Design Section Section 2 Primary Responsibilities of the Bridge Design Section Overview The primary responsibilities of the Bridge Design Section are structural design and the preparation of working drawings or plans. they are encouraged to contact the director of the Bridge Design Section. Consultation. Preliminary Bridge Layouts. length of spans. Note: If consultants are unsure about the current design or detailing standards for an item. Note: Area engineers and consulting engineers are encouraged to contact the Geotechnical Branch for advice if there is any question regarding the proper foundation. or other agencies that may exercise review authority. the Federal Highway Administration (FHWA) on federal oversight projects. type. ♦ The director of the Bridge Design Section assigns the work to a Design Group according to its particular expertise in that type of design. and estimates (PS&E). hydraulic data. district bridge engineer. 2-3 TxDOT 12/2001 Bridge Design Manual . size. and projected traffic. classification of highway. and the district design engineer. Note: This is a good time to make a preliminary decision about who will prepare the bridge plans. Reviewed prints are returned to the bridge project manager and the Design Group makes any revisions required by the district review. optional designs for prestressed concrete beams are submitted by the fabricator and checked early. traffic signal supports. Any revision required by the pre-letting review are made by the Design Group. construction problems arise that require review for structural adequacy. otherwise they are called underpasses. Stream Crossings. movable bridges. Highway over highway separations are called overpasses if the project highway passes over. systems. Design is under strict control by the railroad companies. Grade Separation Structures.Chapter 2 — TxDOT and Bridge Design Section 2 — Primary Responsibilities of the Bridge Design Section ♦ ♦ The Design Group leader schedules engineering and detailing work for the job according to the target completion date and the Design Group’s other commitments. channels. the bridge project manager sends prints to the district. Also. Project plans. pedestrian overpasses. They should budget their labor accordingly. Clearance for the overpassed traffic is critical. Other shop drawings follow. Clearances for marine traffic may be required. occasionally. Note: Consulting engineers should be aware that shop drawings require a significant amount of checking time. and bays. Occasionally. Miscellaneous Structures. utility bridges. Hydraulic considerations are usually involved. The following items give examples of potential areas of consideration. bayous. When the project is finished. ♦ ♦ ♦ ♦ ♦ ♦ ♦ Structural Design Structural design involves selection of appropriate materials. When foundation loads have been determined. Railroad underpasses are where a highway passes under an intersecting railroad. fender systems. Railroad Underpasses. rivers. Grade separation structures occur where one roadway must cross over another. Bridge Design Manual 2-4 TxDOT 12/2001 . Originals. Stream crossings carry highway traffic over creeks. including all reproducible standard drawings are sent to the district. ferry boat landings. specifications. the Geotechnical Branch will be asked to establish founding elevations. bridge members that were fabricated beyond specification tolerances are reviewed by the Bridge Design Section for structural adequacy when properly repaired. Miscellaneous structures include sign bridges. illumination poles. and radio towers. The Geotechnical Branch is contacted early if there is any doubt about the foundation type. When the plans are complete. After letting. geometric calculations are discarded and design notes are assembled and filed. and details for the structure and performing calculations of stress and strain in each component caused by the prescribed loading. and estimates are sent to the Design Division. ) drawings as completely and accurately as necessary to allow the structure to be built according to the design. Most culverts are constructed from standard drawings. Shop Drawings. Preliminary bridge layouts are reviewed and approved by the Bridge Design Section. Half-size sheets are preferred by the department. Geotechnical Design. The Bridge Design Section assists the Field Operations Section when necessary and will check shop drawings for structures that they have designed. Culverts carry storm water under highways. Bridge standards are maintained by the Bridge Standards Branch of the Technical Services Section. (See http://www. Bridge Plans. Plans. Layouts are initiated by the district. A special group within the Bridge Technical Services Section performs geotechnical design. Preparation and Approval of Working Drawings Preparation and approval of working drawings involve assuring that the various requirements of the design are shown on plan-size (22 x 34 in. Consultants must check their own shop plans.Chapter 2 — TxDOT and Bridge Design Section 2 — Primary Responsibilities of the Bridge Design Section Culverts and End Treatment. sometimes with assistance by a consulting engineer. Drawings for construction projects must contain accurate quantities of the various items of work so that the contractor can be adequately reimbursed according to the unit bid prices. consulting engineers. In any case a review is necessary to determine that the structure is safe and reasonably economical. Mechanically stabilized earth (MSE) wall designs require mostly geotechnical considerations. Preliminary Bridge Layouts. The larger of these culverts are reinforced concrete boxes. Bridge Design Manual 2-5 TxDOT 12/2001 . PS&E.state.htm. coordinate with the Bridge Design Section through the project development manager as early as possible. or the Bridge Design Section. The use of fullsize sheets is being phased out. These conditions are outlined in the TxDOT Roadway Design Manual.tx. Detailed plans for MSE walls are prepared by the successful wall supplier. Some types of retaining walls require structural design.us/insdtdot/orgchart/cmd/cserve/standard/disclaim. See the Bridge Project Development Manual for the submittal process. Standard drawings contain often-used systems and details that can be used in bridge plans without modification. Bridge Standards. Note: For major structures.) The website contains instructions about the use of the graphics files. The Field Operations Section of the Bridge Division coordinates and checks shop drawings.) or half-size (11 x 17 in. and estimates contain structural details prepared by the district. Under some conditions Safety-End Treatment is required to protect errant vehicles. Standards are indexed on the main TxDOT website under Business/TxDOT CAD Standard Plan Files. specifications. Retaining Walls.dot. sealed expansion joints.Chapter 2 — TxDOT and Bridge Design Section 2 — Primary Responsibilities of the Bridge Design Section The specifications and special provisions dictate shop drawing submittal requirements. The following is a partial list of items that require shop drawings: ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ Various prestressed concrete beams Deck panels Preformed metal deck forms Structural steel Segmental prestressed concrete Retaining wall systems requiring shop drawings by specification Sound barrier walls Bearing pads and other structural bearings Various bridge joints (armor joints. etc.) Bridge protective assemblies Overhead sign bridges Concrete piling Prefabricated pedestrian bridges Bridge Design Manual 2-6 TxDOT 12/2001 . finger joints. especially when bridge plans are to be prepared by the districts or the consulting engineers. When requested by the permitting agency. In the area of Structures and Hydraulics. this program has generated a significant library of reports on various aspects of structural and hydraulic design. Once the PS&E are submitted it is usually too late to change systems or major details. Note: For major bridges. defined coordinative responsibilities are those functions that are not considered a part of the fundamental duties of the Bridge Design Section but that are important and are carried on daily. Bridge Design Manual 2-7 TxDOT 12/2001 . makes an assessment of the effects of these vehicles on the bridges along the proposed route. Overweight/Oversize Permits There are increasing requests for permits to move overweight/oversize loads over state highways. the Bridge Construction Section.Chapter 2 — TxDOT and Bridge Design Section 3 — Coordination Responsibilities of the Bridge Design Section Section 3 Coordination Responsibilities of the Bridge Design Section Overview All responsibilities of the Bridge Design Section are actually coordinative because another entity is always involved. Structures Research Management Committee (RMC 5) TxDOT carries on a very extensive research program. especially. The Bridge Construction Section will approve or disapprove accordingly. interaction with the Bridge Design Section is desirable during the preparation of preliminary layouts. However. can alleviate misunderstandings and avert delays. with occasional assistance from the Bridge Design Section or a consultant. Consultation During Plan Preparation by Others Consultation during plan preparation by others. especially a consulting engineer. Preliminary Consultation Regarding Structure Type Preliminary consultation regarding structure type is a very important duty. the Bridge Design Section will try to make someone available for consultation in a timely manner. Note: No matter how busy the workload. primarily through the Highway Planning and Research Program. Over many years. Many of the findings have been incorporated in the design specifications or procedures. the Bridge Design Section and some districts provide technical support for the various projects. Oversight The determination of federal or state oversight for highway projects is discussed in Chapter 3 of the TxDOT Bridge Project Development Manual. Additionally. ♦ The American Association of State Highway and Transportation Officials (AASHTO) The director of the Bridge Division represents Texas on the prestigious American Association of State Highway and Transportation Officials Highway Subcommittee on Bridges and Structures. each with a specific structural system to monitor for possible improvements to the specification. They have maintained strong bridge sections in Washington (Headquarters). National Committees. Interaction with Outside Agencies Outside agencies are often involved in various aspects of bridge planning and design. Chapter 3. The group is divided into several subcommittees. One of the most important interactions since the advent of the Interstate Highway System has been with the Federal Highway Administration. They publish construction specifications also. Section 3 of the TxDOT Bridge Project Development Manual lists and describes many of these agencies. Streets. but these are modified heavily by our own TxDOT Standard Specifications for Construction of Highways. 2-8 TxDOT 12/2001 ♦ Bridge Design Manual . This organization is responsible for writing and revising the structural design specifications to be followed by all 50 states. The Bridge Design Section will consider workload and available personnel. the Bridge Design Section has close working relationships with the following agencies: Federal Highway Administration. Texas has been able to influence the specifications by the results of local research. and Austin (Texas Division). the Bridge Design Section will exercise oversight of all aspects of bridge structural design. The Bridge Design Section represents TxDOT on several national committees and organizations that furnish information and develop procedures for structural design. Chapter 5 of the TxDOT Bridge Project Development Manual contains suggested lead times for submitting bridge layouts.Chapter 2 — TxDOT and Bridge Design Section 3 — Coordination Responsibilities of the Bridge Design Section Scheduling Plan Work Scheduling plan work is usually a negotiation process with the district. When the state has oversight responsibilities. Specification revisions usually originate in these subcommittees. and Bridges. Atlanta (Southern Resource Center). The American Railway Engineering and Maintenance-of-Way Association (AREMA) AREMA is the organization that controls everything associated with railway engineering and maintenance and publishes a specification that must be followed when designing structures on or over a railroad. allowing the details to be completed to meet a realistic target date. The director of the Bridge Division and/or their representatives meet annually with the full bridge committee. and suppliers can meet to further refine current design. This is a very important institute. AASHTO sometimes draws on the experience of ACI in revising its specification. AISC is active in trying to keep AASHTO current in steel design and publishes a manual that contains much useful information regarding availability and capability of steel components. The American Segmental Bridge Institute (ASBI) The American Segmental Bridge Institute is a nonprofit organization that provides a forum where owners. with contributors from many universities nationwide. and construction management procedures. construction. The American Society of Testing and Materials (ASTM) The American Society of Testing and Materials develops and publishes specifications for all types of materials used in highway construction. designers. Post-Tensioning Institute (PTI) The Post-Tensioning Institute publishes a manual and keeps up-to-date on developments in post-tensioned concrete. constructors. Prestressed Concrete Institute (PCI) The Prestressed Concrete Institute publishes a manual and keeps up-to-date on developments in prestressed concrete.S. and Canada. The American Iron and Steel Institute (AISI) The American Iron and Steel Institute is a nonprofit service organization for the fabricated steel industry in the United States and is dedicated to presenting the most advanced information available to the technical professions.Chapter 2 — TxDOT and Bridge Design Section 3 — Coordination Responsibilities of the Bridge Design Section ♦ Transportation Research Board (TRB) The Transportation Research Board is a federal agency that manages transportation research projects contracted by universities and other research organizations in the U. ♦ ♦ ♦ ♦ ♦ ♦ ♦ Bridge Design Manual 2-9 TxDOT 12/2001 . and evolve new techniques that will advance the quality and use of segmental concrete bridges. The American Institute of Steel Construction (AISC) The American Institute of Steel Construction publishes a specification for structural steel design that is widely used in building construction. The American Concrete Institute (ACI) The American Concrete Institute publishes a specification for reinforced concrete that is widely used for building construction. The contracting process is subject to rules governing all TxDOT engineering contracts. Bridge Design Manual 2-10 TxDOT 12/2001 .Chapter 2 — TxDOT and Bridge Design Section 4 — Contractive Responsibilities of the Bridge Design Section Section 4 Contractive Responsibilities of the Bridge Design Section Overview The Bridge Design Section manages a pool of consultants that expands its capabilities to meet the demands of large letting volumes. ..................................................................................................................................................................................................3-2 Section 2 — Guide Specifications ....................3-6 Bridge Design Manual 3-1 TxDOT 12/2001 .........Chapter 3 Design Specifications Contents: Section 1 — Mandatory Specifications ....3-4 Section 3 — Industry Recommendations.................... The Federal Highway Administration (FHWA) may. AASHTO Standard Specifications for Highway Bridges The Standard Specifications for Highway Bridges adopted by the American Association of State Highway and Transportation Officials (AASHTO) is the most important control over bridge design.org/. review designs and details for compliance with these specifications for projects using federal money. Bridge design is no better than the details that are generated. as well as other AASHTO publications. This specification contains a more comprehensive treatment of wind effects on structures. may be purchased from AASHTO by calling 1-800-231-3475. and structural steel is particularly detail oriented because of fatigue considerations. In the intervening years. Note: Although these specifications are considered mandatory. Bridge Design Manual 3-2 TxDOT 12/2001 .Chapter 3 — Design Specifications Section 1 — Mandatory Specifications Section 1 Mandatory Specifications Overview There are many specifications available that have a bearing on the design of bridges and other highway structures.5) The American National Standards Institute (ANSI)/AASHTO/American Welding Society (AWS) Bridge Welding Code combines the recommendations of the three agencies with regard to welding details.transportation. which contain the revisions approved on a ballot following the last meeting of the AASHTO Highway Subcommittee on Bridges and Structures.1) Standard Specifications for Structural Supports for Highway Signs. methods. and quality tests. Luminaires and Traffic Signals (D1. AASHTO Standard Specifications for Structural Supports for Highway Signs. This section identifies those specifications that the Texas Department of Transportation (TxDOT) considers mandatory for use. ANSI/AASHTO/AWS Bridge Welding Code (D1. a few deviations are made based on long-time local practice or research. Copies of these specifications. Luminaires and Traffic Signals is also published by AASHTO. It is usually published in full every four years. or at their website at http://www. Interim Specifications are distributed. at any time. which should be investigated for each project. including the design of bridges and culverts that carry railway traffic. Individual Railroad Company Requirements Individual railway companies may have their own supplemental requirements. Bridge Design Manual 3-3 TxDOT 12/2001 .Chapter 3 — Design Specifications Section 1 — Mandatory Specifications AREMA Specifications The American Railway Engineering and Maintenance-of-Way Association (AREMA) Specifications cover many aspects of railway engineering. This document is a complete rewrite of the bridge specification. researchers. Guide Specifications for Design and Construction of Segmental Concrete Bridges. This places it in the guidance category. and manuals published by AASHTO. Switzerland. composed of consulting engineers. There are several other guide specifications. “Design and Construction Specifications for Segmental Concrete Bridges. They are formulated and based on both observed performance of bridges of this type and on recent research conducted in the United States and abroad. bridges in Texas will be designed using this specification. but may be useful or even vital to the design process are referred to as guide specifications. In coming years. with load and resistance factors based on probability analyses. and federal agency representatives from throughout the United States as well as representatives from Canada.Chapter 3 — Design Specifications Section 2 — Guide Specifications Section 2 Guide Specifications Overview Specifications that are not binding to bridge design. This document contains guidelines for the design and construction of segmental concrete bridges. It was subsequently studied and approved as a guide specification by the AASHTO Highway Subcommittee on Bridges and Structures in 1989. contractors. state highway agencies. This document was originally prepared by the Post-Tensioning Institute under National Cooperative Highway Research Program (NCHRP) Project 20-7/32 with the title. other than those discussed in Section 1. and Germany. Mandatory Specifications. academicians. Some of those include: ♦ ♦ ♦ ♦ Guide Specifications for Alternate Load Factor Design Procedures for Steel Beam Bridges Using Braced Compact Sections Guide Specifications for Fracture Critical Non-Redundant Steel Bridge Members Guide Specifications for Horizontally Curved Highway Bridges Guide Specifications for Bridge Railings 3-4 TxDOT 12/2001 Bridge Design Manual . standard specifications. This section identifies and describes guide specifications recommended by TxDOT. The guidelines are the recommendations of a team of nationally recognized experts. France. Additional AASHTO Publications. relating to bridge design that may be useful to the designer.” in February 1988. Standard Specifications for Highway Bridges — Load and Resistance Factor Design. AASHTO Guide Specifications AASHTO publishes a number of specifications and manuals. The guidelines are comprehensive in nature and embody several new concepts that are significant departures from previous design and construction provisions. Chapter 3 — Design Specifications Section 2 — Guide Specifications ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ Guide Specifications for Fatigue Design of Steel Bridges Guide Specifications for Fatigue Evaluation of Existing Steel Bridges Guide Specifications for Strength Evaluation of Existing Steel and Concrete Bridges Guide Specification and Commentary for Vessel Collision Design of Highway Bridges Guide Specifications for Strength Design of Truss Bridges (Load Factor Design) Guide Specifications for Distribution of Loads for Highway Bridges Guide Specifications for Structural Design of Sound Barriers Guide Specifications for Aluminum Highway Bridges Guide Design Specifications for Bridge Temporary Works Guide Specifications for Design of Pedestrian Bridges Standard Specifications for Movable Highway Bridges Manual for Maintenance Inspection of Bridges Manual for Condition Evaluation of Bridges Bridge Design Manual 3-5 TxDOT 12/2001 . The specifications produced usually have the backing of leaders in the industry and also experts from universities and other research agencies nationwide. Concrete Concrete design is the subject of these publications: ♦ ♦ ♦ Steel Structural steel design is the subject of these publications: ♦ ♦ American Institute of Steel Construction Specification and Manual (AISC) American Iron and Steel Institute Manual (AISI) American Concrete Institute Specifications(ACI) Prestressed Concrete Institute Manual (PCI) Post-Tensioning Institute Manual (PTI) Materials Specifications for a wide range of materials are contained in several volumes of American Society of Testing and Materials publications.Chapter 3 — Design Specifications Section 3 — Industry Recommendations Section 3 Industry Recommendations Overview Industry recommendations are valuable to the structural designer. Bridge Design Manual 3-6 TxDOT 12/2001 . ...4-2 Section 2 — Bridge Span Length ............................................................................................4-8 Section 5 — Stage Construction .............4-4 Section 3 — Horizontal and Vertical Clearances ................................................................................................Chapter 4 Geometric Restraints Contents: Section 1 — Bridge Width..............................................................................................................4-13 Bridge Design Manual 4-1 TxDOT 12/2001 ..........................................4-5 Section 4 — Alignment............................................................................................. alignment.2 m to 13.0 m) wide between inside lane edges. including shoulders. This need makes bridge widths. bridges are just a small part of the highway. it became evident that many vehicular accidents happened at the beginning of bridges where the horizontal clearance became restricted. For several years. unless the median is less than 30 ft. A concrete traffic barrier is usually constructed at the center of the median. (3. With continuing increases of highway speed. to 48 ft. but the recommendation did not appear in the American Association of State Highway and Transportation Officials (AASHTO) Specifications until 1969 because of the considerable cost of the additional bridge width. Background Roadway widths covered by Bridge Design Standards have ranged from 16 ft. Current Status Today.Chapter 4 — Geometric Restraints Section 1 — Bridge Width Section 1 Bridge Width Overview Bridge width depends solely on the width of the highway except in unusual cases. Design Practice On divided highways. (0. Texas began a campaign for shoulder width bridges in the 1950s. From the driver’s standpoint.2 m during the past 80 years. bridge surfaces are usually flush with the pavement across the median. a longitudinal open joint is recommended when the bridge width Bridge Design Manual 4-2 TxDOT 12/2001 . Geometrically. bridges need to blend inconspicuously into the perception of the road.0 m) depending on traffic volume and structure function. The Bridge Division provides standard bridge details for a few of the most repetitive widths that are less likely to have complicated geometry. Curbs are not used except to protect a pedestrian walkway on a low-speed highway.6 m) to 10 ft. The resulting bridge can be quite wide when there are multiple lanes in each direction. (9. separate bridges are used for each direction of traffic. and from 7. Refer to the Texas Department of Transportation Department (TxDOT) Roadway Design Manual for guidance on highway design. Traffic lanes are usually 12 ft. This results in a large number of bridge widths. In this case. Shoulders vary from 2 ft. and clearances subject to the requirements of the highway engineer.6 m) wide but may be reduced for low-volume or extremely crowded conditions and may be increased for sight distance around a horizontal curve. outside of the traffic lane. To ensure against transverse expansion and contraction problems. virtually all bridges in Texas are as wide as the approach roadway. The nominal face of bridge railing is located at the outside edge of the shoulder. (3. Texas began providing graveled shoulders on major highways earlier than most states. bridges had curbs located 2 ft. (0.3 m) from the deck edge to allow the use of different railing with the same standard details. Figure 4-1. (36. (0. For simplicity of design.) Bridge Design Manual 4-3 TxDOT 12/2001 . but this is not considered sufficient encroachment to affect operation of the shoulder. (0. Bridge widths for the current Bridge Design Standards are shown in Figure 4. Except for unusual situations. which might justify moving the joint to one side of the barrier. differential deflection can cause cracking in the concrete traffic barrier. This dimension is also recommended for all non-standard bridges. the joint should be at the median centerline. the overall width of bridge decks is 2 ft.1.3 m) of deck width. Standard Bridge Widths (Online users can click here to view this illustration in PDF.0 m).Chapter 4 — Geometric Restraints Section 1 — Bridge Width exceeds 120 ft.6 m) more than the distance between the nominal faces of outside railing. The nominal face of railing is set at 1 ft. On long span structures. Most standard bridge railings occupy less than 1 ft. The safety-shape or straight-sided traffic railings are slightly wider in their lower part. Marine Bridges Intracoastal canal and international shipping lane main spans are subject to U.Chapter 4 — Geometric Restraints Section 2 — Bridge Span Length Section 2 Bridge Span Length Overview Bridge length depends on terrain. hydraulics. prescribed clearances. Grade Separations Prescribed clearances and header slopes govern highway grade separation span lengths. no bents or piers should be placed in erodable streambeds. and is controlled by economics and the capability of available structural systems. Coast Guard regulations. Bridge Design Manual 4-4 TxDOT 12/2001 . There are no warrants for determining the length of this span. For small streams. The larger prestressed concrete beams have become so economical that they are used where shorter spans would suffice. River crossings almost always have a main span across the center of flood flow. Navigable rivers as determined by the Federal Highway Administration (FHWA) are also subject to U. the bridge is often divided into equal spans for ease of construction. Sometimes aesthetic considerations may dictate longer-than-necessary spans to give the separation a more open look underneath.S. However. Approach spans are determined by economics and/or aesthetics. it is a matter of engineering judgment. Stream Crossings Stream crossing span lengths usually have a main span that straddles the stream. the purpose being to discourage an accumulation of drift on the piers.S. where hydraulic considerations are minimal. or aesthetics. Coast Guard Regulations. Highway Grade Separations Clearance measurements for highway grade separations are depicted in Figure 4-2. Clearances to railroads are specified by AREMA. Highway Grade Separation Clearances 14'-6" (4. lowvolume roadways 16'-0" (4. Vertical Stream Crossings Vertical Horizontal Stream Crossing Clearances 2'-0" (0.61 m) Desired. Coast Guard and. U. Minimum horizontal and vertical clearances for highway bridges are established in the TxDOT Roadway Design Manual.Chapter 4 — Geometric Restraints Section 3 — Horizontal and Vertical Clearances Section 3 Horizontal and Vertical Clearances Overview Clearances are established by AASHTO.30 m) Absolute minimum. above design high water 1'-0" (0. Coast Guard. and tabulated here for ready reference. Intracoastal canal clearances are determined by the U.42 m) Absolute minimum 16'-6" (5. For a complete listing of horizontal and vertical clearances for specific highway functional classifications.88 m) From edge of travel lane on medium-volume roadways and freeway ramps 30'-0" (9. FHWA. International shipping lane clearances must be negotiated with the U.S. Army Corps of Engineers. local authorities.S. above design high water As determined by topography and hydraulics Bridge Design Manual 4-5 TxDOT 12/2001 . U.S.S.46 m) Absolute minimum from face of curb or Horizontal barrier 10'-0" (3. Army Corps of Engineers.05 m) From edge of travel lane on low-speed. see the Roadway Design Manual. to some extent.03 m) To be provided over all roadways if possible and mandatory for new construction over interstate highways 1'-6" (0. repeated in the TxDOT Bridge Project Development Manual.14 m) From edge of travel lane on high-volume roadways and all freeway main lanes Note: Special conditions that would severely increase structure cost may justify negotiation of these clearances with the TxDOT Bridge Division. American Railway Engineering and Maintenanceof-Way Association (AREMA). this practice will greatly reduce bridge cost by reducing the required span length.33 m) 2'-0" (0.92 m) 9'-0" (2.39 m) May be required for electric powered trains to 26'-0" (7. Vertical Horizontal Railroad Overpass Clearances 23'-0" (7. Pedestrian Bridges and Non-redundant Bridge Supports Pedestrian Bridge and Non-redundant Bridge Support Clearances 17'-6" (5. Given the high cost of this structure type. consideration should be given to using appropriate barrier railing through the railroad underpass.Chapter 4 — Geometric Restraints Section 3 — Horizontal and Vertical Clearances Railroad Overpasses See Figure 4-2. allowing for a reduced horizontal clearance.59 m) Absolute minimum (12' crash walls required) 12'-0" (3.33 m) Minimum Vertical Desirably greater than highway grade separation structures Horizontal Sign Bridges Minimum Absolute minimum from face of barrier railing Note:: Additional vertical clearance may be required by some area offices based on experience.62 m) Minimum to eliminate crash walls Railroad Underpasses Railroad Underpass Clearances Same as for Highway Grade Separation Structures Vertical Same as for Highway Grade Separation Structures Horizontal Note: Although the horizontal clearance criteria for a railroad underpass are the same as for a highway grade separation structure.66 m) Desirable minimum (6' crash walls required) 25'-0" (7.61 m) Intracoastal Canal Bridges Bridge Design Manual 4-6 TxDOT 12/2001 .01 m) Absolute minimum 24'-3" (7. Vertical Horizontal Sign Bridge Clearances 17'-6" (5. Figure 4-2. 3. Coast Guard. Horizontal clearance from outside edge of exterior lane (uncurbed) to obstruction.25 m) Above mean high water 125'-0" (38.) Explanatory Notes for Figure 4-2 1.S. Refer to the TxDOT Roadway Design Manual for specific criteria of roadway functional classification. Minimum horizontal clearance of 12'-0" with crash walls that are 6'-0" above track elevation. Horizontal clearance from the centerline of tracks to obstruction.Chapter 4 — Geometric Restraints Section 3 — Horizontal and Vertical Clearances Intracoastal Canal Bridge Clearances 73'-0" (22. Clearance Measurements (see following explanatory notes) (Online users can click here to view this illustration in PDF. Minimum horizontal clearance of 9'-0" with crash walls that are 12'-0" above track elevation. Horizontal clearance from edge of curbed roadway to obstruction. 2.10 m) From center of channel Vertical Horizontal Bridges over International Shipping Lanes Clearances are subject to negotiation with the U. Refer to the TxDOT Roadway Design Manual for specific criteria of roadway functional classification. Use 25'-0" or greater to avoid the need for crash walls. Bridge Design Manual 4-7 TxDOT 12/2001 . Since the beams must be straight. Thicker slab spans are more sensitive than deck slabs. Curves up to 20 degrees have occasionally been used on “button hook” ramps and turnarounds. can be handled gracefully in cast-in-place structures. The concrete tends to flow downhill during finishing operations. but these have not been economical in Texas for many years. and crown rollouts are normal. although with considerably more complexity in the details. Vertical Curvature. such as the use of curved steel girders. highway engineers were satisfied with bridges that were straight. Extreme vertical curvature can seriously affect forming methods for deck slabs on prestressed beams. overhang width to the curved deck edge may limit the span length. Current Practice Highway alignment follows the guidelines given in the TxDOT Roadway Design Manual. If the deck slab is cast on removable forms. If the curvature/span length combination exceeds the capability of the deck slab. It gradually became evident that bridges could handle other types of alignment. this extra depth can be accommodated in the haunch depth over each Bridge Design Manual 4-8 TxDOT 12/2001 . Presently. Background In the early days.Chapter 4 — Geometric Restraints Section 4 — Alignment Section 4 Alignment Overview The subject of alignment covers horizontal and vertical curvature of the profile and/or station line and the cross-slope of the deck surface. especially if precast concrete deck panels are used. Extreme vertical grade can cause construction problems but seldom influences structure type. Crest curves cause extra deck depth in the middle of the span. Grades over 5 percent call for extra care during concrete placement. curves. Horizontal curvature up to 5 degrees on wide bridges and 10 degrees on narrow connection structures can be expected. variable widths. but they are usually approximated by three centered circles for use in bridge framing. About the only alignment that is not compatible with bridges is the spiral curve. with the exception of pan form girders. Design Recommendations Horizontal Curvature. Bridge alignment conforms to these alignments and is usually a “given” on the preliminary layouts. Figure 4-3 shows this relationship. Elastometric bearing details for prestressed concrete beams require special consideration for grades over 5 percent. Sag curves cause extra depth at the ends. skews. Horizontal curvature of beams. square. the span must be decreased or other measures must be considered. Spirals are still used occasionally for highway alignment. The preferred support system is precast prestressed beams. and relatively flat. 5 percent to facilitate drainage. Based on AASHTO Slab Grade 60 Reinforcing *Adjust to 1/4 pt. Superelevation above 5 percent can cause problems with concrete placement the same as steep grades. It may not be possible to cover all these variations in the design stage. Current practice is to set haunch depths that will keep the top of beam at or below the bottom of the slab.) Gradeline. Guidelines for Horizontal Curvature Using Prestressed Beams (Online users can click here to view this illustration in PDF. If such deck slopes cannot be avoided. Variable camber in prestressed beams aggravates the problem. (75 mm). the problem is more critical. Extra reinforcing is required if the haunch depth exceeds 3. Special grading details may be required to accommodate tall haunches. Even so. it is necessary to set the haunch depth carefully to avoid construction problems. the outer lanes are usually sloped 2. the construction engineers should be alerted to the possible need for special concrete placement requirements. If the structure is more than two lanes wide. Cross-slope or crown for bridges is 1 percent minimum. 2 percent desirable. Contractors have become accustomed to adjusting the gradeline after taking elevations on the tops of the erected panels. of Flanges for Steel Beams Figure 4-3. Bearing seat elevations may require lowering for sag curves. If precast concrete deck panels are used. Cross-slopes can transition into superelevations as much as 8 percent on curved structures. Bridge Design Manual 4-9 TxDOT 12/2001 .0 in.Chapter 4 — Geometric Restraints Section 4 — Alignment beam. The program has also been made compatible with metric dimensions. The Roadway Design System (RDS) is a geometric computer program. Other Software. especially with sag curves and panel deck construction. is being used in most highway applications. When vertical curvature and superelevation exist. beams. the alignment file must be duplicated in the RDS format for use in bridge framing. ♦ ♦ ♦ Contour plotting is also available in the program. If there are significant bridges in a project. However. There is a company that maintains the RDS program for national use. All of this can be accommodated. and girders and is used exclusively by the Bridge Design Section for prestressed concrete beam spans on curves. Deck dimensions. It is very useful in performing the usual highway plan functions. but extreme caution should be exercised and detailed geometric computations made. Computes and tabulates edge dimensions and areas for deck slabs of all configurations. It is used to calculate vertical clearances and to check beam haunch within a span. VCLR. Superelevation creates an apparent sag vertical curve along the prestressed beam. A new geometry program. Computes vertical distance from a roadway surface to chorded beam lines. This calculation is the responsibility of the bridge designers. originally developed in Texas and formerly used nationwide. Consultants should be careful to use the most recent version of the program. surface elevations.Chapter 4 — Geometric Restraints Section 4 — Alignment Superelevation also affects clearances between deck slab and beam. The more important bridge routines in RDS are the following: ♦ ♦ SLAB. drastic measures may be required. and bottom of slab plus dead load deflection along the boundaries of the slab. Computes and tabulates framing dimensions for beam spans or continuous girders according to one of several programmed options. IGRDS. Roadway Design System. and the effect of beam camber is added. beam framing. Slab edges can be plotted. is an AASHTOWARE product available from AASHTO. and bent locations must be accurately calculated to fit the prescribed alignment. the Information Systems Division of TxDOT has performed most of the maintenance of the program’s bridge commands in the past few years. especially when precast concrete panels are used. Will produce a tabulation of surface elevations. which is a chord to the curvature. though the company has not yet been able to provide good bridge routines. SLEL. called GEOPAK. bearing seat elevations. bottom of slab elevations. a computer software roadway design system. FOPT. BMGD. The program has several bridge oriented capabilities for slabs. web cutting. and bottom of slab elevations plus dead load deflection along the centerline of each beam. Bridge Design Manual 4-10 TxDOT 12/2001 . Refer to the Roadway Design System Manual for further details. Will produce a tabulation of distances. Framing diagrams can be plotted. bottom of slab elevations. This problem is usually corrected by highway engineers. Superelevation Transition. Superelevation transitions can have an adverse effect on beam haunch. where ramps are converging or diverging. but it appears often to be overlooked or loosely handled. Under certain conditions. This effect can be minimized by starting and ending a transition at a bent. Contours can be used to advantage in this situation. This usually occurs where ramps enter or leave the main structure. Figure 4-4 shows the built-in and recommended optional methods for handling superelevation transition in RDS. it may be necessary to adjust cross-slopes at close intervals along the main profile grade line to provide a smooth transition to the ramp grade and avoid edge profile problems. Bridge Design Manual 4-11 TxDOT 12/2001 . Highway engineers are better able to work out this problem. Superelevation transition across a varying width roadway can cause unsightly lines on the outside railing.Chapter 4 — Geometric Restraints Section 4 — Alignment One problem with RDS is that the roadway surface must be defined by radial cross-slopes from only one profile grade line. Both are unsightly. a combination of superelevation transition and vertical curvature with a constant roadway width can cause sags or humps on the outside of the bridge. In varying roadway widths. but it would be advisable for the bridge designers to verify the outside lines by contours or pavement edge profile plots. Relative grades between the two also have an influence. and sags can pond water on the roadway surface. Contour plots and a plot option of SLEL can be useful in these considerations. It is recommended that bridge engineers consider this situation carefully before setting cross-slopes for framing computations. Chapter 4 — Geometric Restraints Section 4 — Alignment Figure 4-4.) Bridge Design Manual 4-12 TxDOT 12/2001 . Superelevation Transition According to Roadway Design System (Online users can click here to view this illustration in PDF. Bridge Design Manual 4-13 TxDOT 12/2001 . This equipment is quite expensive. Item 496 “Removing Old Structures. New Substructure The following are guidelines for the design of the new substructure. below the proposed ground.Chapter 4 — Geometric Restraints Section 5 — Stage Construction Section 5 Stage Construction Overview Stage construction is required when traffic must be diverted onto a portion of an existing bridge while part of the new structure is built. If necessary. The partial removal of the existing structure begins with the cutting and removal of the slab. This section is provided to give the bridge planner/designer some guidelines that generally apply for all staged construction. The breakback is generally located over a beam and must be supported by a stable substructure. For widenings. avoid the location of the existing foundations that remain. and should be avoided. the beams are removed and the substructure. minimum horizontal clearance from edge of foundation to the obstruction. This process is seldom practical or cost effective. and temporary railing.. Topics include existing structure removal. of headroom. The location of the cut is called the breakback. Ideally. and placement of reinforcing steel and concrete is very difficult. Both drilled shafts and piling require a 1 ft. After the slab is cut and removed. then moved over for reconstruction of the first part. The exact breakback point should be determined by the bridge designer and is based on the structural capacity of the existing structure.” outlines requirements for the removal of existing structure. footings are removed and drilled shafts and piles are cut and removed to a distance a minimum of 2 ft. Existing Structure Removal Texas Standard Specifications.Geotechnical Branch for information on the practicality and cost of these types of shafts. there should be no vertical obstruction above either type of foundation. is demolished. new substructure. The approximate location of the breakback is determined through coordination with the traffic and highway engineer and is based on lane width requirements of both the new structure and the partial structure to remain in place. foundations should be of similar type as those remaining in use. If possible. or as specified in the plans. Consideration must be given to the room required for drilled shaft and piling installation. The only way to install piling in limited headroom is to drive and splice short sections of steel piling. Contact the TxDOT Bridge Division . Foundations. or a portion thereof. Special drilled shaft rigs are now available that can work with as little as 6 ft. new superstructure. However. Instead. or provide a sealed expansion joint. locate the joint at the quarter point of the beam spacing. Shorter laps might be justified based on the AASHTO provision (As required /As provided) in areas where the slab has excess capacity. the reinforcing steel can be spliced together using a lap or mechanically coupled together. Consideration of raising the grade a few inches to allow the top mat to be lapped should be given. Joints should be located so that space for minimum reinforcing steel laps and 1 in. of cover beyond the ends of the bars is provided.Chapter 4 — Geometric Restraints Section 5 — Stage Construction Abutments. If splicing is used. When placing the joint over a supporting prestressed beam. use mechanical couplers or butt welds. beyond the centerline of the beam to grab the R-bars with the first pour. The exposed reinforcement must be protected. use independent bents. placing the stage construction joint over a supporting beam is the preferred method. Bridge Design Manual 4-14 TxDOT 12/2001 . However. locate foundations (drilled shafts or piling) close to the stage construction joint and dowel the two sides of the cap together. mechanical couplers can be utilized. If possible. New Superstructure The following are guidelines for the design of the new superstructure. If the clear distance is inadequate. it is difficult to leave reinforcing steel projecting from the abutments for splicing because of the conflicts with the temporary shoring that must retain the fill. When placing the joint between beams. adequate horizontal and vertical clearances must be provided to account for the projecting reinforcement. be sure the appropriate specifications are supplied. Prestressed concrete panels are typically not allowed in the second placement in the bay adjacent to the construction joint. Due to the complexity of couplers and welds. accurate details and proper structural detail notes are essential. Interior Bents. there are concerns about the performance of a construction joint using couplers in both mats. If available clearances are limited. If couplers are used. the joint must be located 2 in. If a single structure is required. The location of the stage construction joint in the slab and the available clear distance for splicing the mat reinforcing are critical factors in the slab design. At the stage construction joint. particularly in salt areas. The stage construction joint can be placed over a supporting beam or in a bay between beams. The available construction clear distance may limit the available length required for an adequate lap length. ................Chapter 5 Preliminary Considerations Contents: Section 1 — Materials....................................................................................................................................................5-26 Section 4 — Aesthetics ........5-28 Section 6 — Railing..................................................................................................................................................................5-9 Section 3 — Economics.............................................................................................5-2 Section 2 — Structure Type...................................5-29 Bridge Design Manual 5-1 TxDOT 12/2001 .......................................................................................................................................5-27 Section 5 — Corrosion Problems.......... plastics are used for small diameter pipe. around 13. Silica fume has also been used. It has been demonstrated that high strength concrete. Greater lengths are possible with the use of high performance concrete.” When the steel is stretched before the concrete is placed. Except for reinforcing steel.Chapter 5 — Preliminary Considerations Section 1 — Materials Section 1 Materials Overview Availability of materials is generally not a factor in determining the most suitable type of structure for a given location. but pretensioned precast concrete beams are the mainstay of Texas bridge construction. in special circumstances. These ingredients are also being used to develop a “high performance concrete” (HPC) with emphasis on density to provide better resistance to chloride attack. Streets. which identifies its strength. The use of fly ash to augment or replace some of the cement is gaining acceptance. Use of long beams. and coarse aggregate type according to the item “Portland Cement Concrete” of the Texas Department of Transportation (TxDOT) Standard Specifications for Construction of Highways. The concrete is usually mixed nearby and trucked to the job site. asphalt is used for overlays. long are possible to manufacture and transport. Concrete may be made from many different sources of cement. depends on the accessibility of the bridge site by transporting trucks. the member is said to be “post-tensioned. water/cement ratio. Bridge Design Manual 5-2 TxDOT 12/2001 . Type IV beams up to 135 ft. When the concrete is placed before the steel is stretched. fine and coarse aggregate. cement content. only brief descriptions are given here. Reinforced concrete is the term applied to concrete containing reinforcing bars designed to resist any tension that may occur in the member. and butyl rubber is used for railroad underpass waterproofing.000 psi compressive strength. neoprene is used for bearings. While bridges are primarily concrete and steel. Concrete and steel are the basic ingredients of most structures. can be produced from Texas aggregates and successfully placed in the forms for certain bridge members. the member is said to be “pretensioned. Various additives are allowed or required for certain conditions of use. however. Type VI (MOD) beams can be used for spans up to 175 ft. but all materials must meet the requirements of the specification. Prestressed concrete is the term applied to high-strength concrete containing very highstrength steel that has been stretched and anchored to the concrete with sufficient force to significantly reduce tension from occurring in the member.” Posttensioned structures are used sparingly. aluminum is used very sparingly in railing and pipe. Virtually all bridges contain some reinforced concrete. and water. Concrete Concrete is described by class. and they are available to every county in the state. and Bridges. The 1973 American Association of State Highway and Transportation Officials (AASHTO) specification ushered in highstrength reinforcing steel and put a limit on stress range to avoid fatigue problems. Deformations were a big concern of the 1940s but the questions were put to rest by ASTM 305-47T. Grade 75 bars were considered for concrete but abandoned because of the absence of a yield plateau in the stress/strain diagram. but soon outlawed by the specifications. There have been many changes in the strength and configuration of reinforcing bars in the history of TxDOT. Load factor design. Ductile structural grade steel was used until the early 1950s. columns. #9. which has become the standard method. For nonspecification work it was possible to find anything from barbed wire to old car parts reinforcing the concrete. Oil well sucker rods were used occasionally during World War II because of a scarcity of regular reinforcing bars. For service load design. Further discussion of structural steel can be found in Chapter 7 of this manual. beams. and #11 bars were square. even into the late 1940s. Prestressing Steel Prestressed steel is a very high-strength material.Chapter 5 — Preliminary Considerations Section 1 — Materials Structural Steel Structural steel is available in many shapes and sizes. allows the reinforcing steel to reach yield under the action of loads factored up to provide safety. Bridge Design Manual 5-3 TxDOT 12/2001 . only to be removed in the late 1970s and added back in the 1980s. Grade 75. The early 1960s saw the availability of #14 and #18 bars established. but fabrication is usually performed in or near the state. Weldable reinforcing steel was covered by ASTM A706. #10. and footings. Design of reinforcing steel requires analysis of the complex interaction with concrete slabs. Epoxy coating of reinforcing bars was introduced in the late 1970s. Rail steel was added. Much of the structural steel is manufactured elsewhere. Smooth bars cold twisted to improve bond were used early. service load stresses must usually be calculated to insure that crack width and fatigue stress limits are not exceeded. which is discussed further in of this manual. With load factor design. All bars were square for awhile and. Reinforcing Steel Background. Variations through the years in the specification requirements for reinforcing bars are shown in tables for years 1918-1953 and 1953-1988. size #18S bars were used for anchor bolts by one light pole manufacturer. an allowable stress is specified that accounts for a reasonable factor of safety. Chapter 5 — Preliminary Considerations Section 1 — Materials AASHTO Specification 1918 (T. or Int.. = ASTM Spec.H. Deform. = Open hearth Twist = Cold twisted plain bars Deform = Deformations to improve bond 1953 Bridge Design Manual 5-4 TxDOT 12/2001 . Str. or Medium Plain. or Int.. Deform approved by engineer No Twist O. etc. Only No Twist O. 18 Int..H. Str. & Hard 20 1949 M31-38 (Deform) M31-52 (Billet) M42-48 (Rail) A305-50T (Deform) T.) 1926 (T.H.. 18 Int..H. Mild. All Deform No Twist Str.H. = Intermediate grade Hard = Hard grade Mild = Structural grade Med. M31.H. 20 Str.) A15 O. Str.) 1931 1935 1941 Chronology of Reinforcing Steel Specifications (1918-1953) Material Specification Special Requirements Yield ≥ 33 ksi Bend 180° over one diameter pin A15-14 A15-30 A15-33 (Mod..H.D.H. O. Deform approved by engineer No Twist O. All Deform No Twist O. Only No Twist O. = Texas Highway Department A15. etc.H.. or Int. Str. and Deform O. = Structural grade Int. Str. or Plain Twist with engineer’s approval O. = AASHTO Spec..H.. 20 Str.D. = Intermediate grade fs allowable (ksi) 16 16 LL 16 DL 24 16 18 1944 M31-42 Str..H. 18 Int.H.D. Twist. = Intermediate grade Hard = Hard grade Gr. Bridge Design Manual 5-5 TxDOT 12/2001 .. 18 Int.. bars were lapped enough to develop the allowable working stress in bond.. The studies continued until the early 1980s. 40/20 Gr.1 The design specification in 1977 may have over reacted to the first research and splice lengths became long and variable.H.O.O. 50 Special Bend Requirements for #14 & #18 Gr. M31. but splice lengths still came out long and variable.O.F.. & Hard 20 1961 1965 Str. 40/20 Gr.H. 60/24 1977 1983 A615 (Billet) A616 (Rail) A706 (Weldable) A15. 40) A615 (Gr. 60) A42 (Rail Gr. etc. Refer to the TxDOT Bridge Detailing Manual for tables of development lengths and lap splices. 40/20 Gr.H. The later research reexamined previous findings and declared the problem less severe. Current AASHTO Specifications are similar to the ACI Code. 50) M31 (Billet Only) A706 (Weldable) A615 (Billet Only) A706 (Weldable) O. & Hard 20 Str. No Twist fs allowable (ksi) Str. 60 = 60 ksi yield stress 20 Gr. = Open hearth E. 40/20 Gr. 50 Supplementary Requirements S1 for Bending A616 Str. E. 50) A615 (Billet Gr. & Hard 20 1969 1973 O. Other factors began to be studied in the late 1960s. B.F. 18 Int. = Structural grade Int. = AASHTO Spec. Allowable bond stresses changed often and were finally eliminated from the specification. 60/24 Gr.H.O. B. E.. This research was taken into account for the 1989 American Concrete Institute (ACI) Code. No Twist O. 40 = 40 ksi yield stress Gr. = ASTM Spec. E. No Twist Special Bend Requirements For #14 & #18 Gr.F.H.. 18 Int. = Electric furnace B.H. All Deform No Twist O. 60/24 Gr. For years.F. = Basic oxygen Twist = Cold twisted plain bars Deform = Deformations to improve bond 1988 Interim Probably the most volatile part of the design specification is tensile lap splice lengths.Chapter 5 — Preliminary Considerations Section 1 — Materials AASHTO Specification 1957 Chronology of Reinforcing Steel Specifications (1957-1988 Interim) Material Specification Special Requirements A15-54T (Billet) A16-54T (Rail) A305-53T (Deform) A15 (Billet) A16 (Rail) A408 (#14 & #18) A305/408 (Deform) A15 (Billet) A16 (Rail) A408 (#14 & #18) A305/408 (Deform) A615 (Billet Gr. 60/24 Gr. All Deform No Twist O. O. B. etc. 40) A42 (Rail Gr. so that there are now several acceptable alternatives for tension splices. and Bridges. In direct load and flexural calculations. which refers to all appropriate American Society of Testing and Materials (ASTM) specifications. linear variation of strain is assumed. and welded both sides. so truss bars are no longer recommended for any type of deck. mechanical splices were usually confined to bars in compression. as well as spirals for columns and drilled shafts. Metal filled sleeve splices became acceptable for tension splices. This problem was supposedly corrected by Supplementary Requirement S1 to ASTM A616. all reinforcing steel should be Grade 60 (420 Mpa) except that longitudinal bars in drilled shafts may be designed as Grade 40 (300 Mpa) to reduce required lap and development lengths. but it is sometimes difficult to get properly coated bars from the factory into the bridge. and weldable reinforcing bars are all subject to the requirements of Item 440 of the TxDOT Specification for Construction of Highways.000 ksi. especially rail steel. ♦ ♦ The modulus of elasticity of all reinforcing steel should be taken as 29. There is currently no acceptable concrete reinforcing material that will not rust in the presence of moisture and air. Bars have been known to break when thrown off the truck. Controlled chemistry is usually required for welds. Current Status. Maintenance problems have been caused by design errors.Chapter 5 — Preliminary Considerations Section 1 — Materials Welded splices have been used under some conditions. Streets. Design Recommendations. Adjacent bridge decks on a large stage construction project used dowels in tapered thread couplers. For bridge members. Lap splice lengths must be increased as much as 50 percent for epoxy coated bars. Epoxy coating has been developed to slow down the corrosion process. prestressed concrete beam dowels. This steel. and rust. 5-6 TxDOT 12/2001 Bridge Design Manual . floating cages. The truss bar problem was usually not discovered until construction was in progress. The design guidelines for reinforcing steel are covered here because it is common to most structural components constructed on Texas highways. Truss bars on curved decks with chorded beams invariably gave trouble. Requirements for mechanical couplers are covered by special provision to Texas Standard Specification. especially with truss bars formerly used in deck slabs. More manufacturers developed mechanical splice systems. Mechanical splices are allowed or required by plan note. Reinforcing steel design is well covered in the AASHTO Specifications and the sections that follow. caps. Originally. after an extensive test program to verify their fatigue performance. Bars #7 and smaller are lapped 4 in. Congested reinforcement has been a frequent complaint in columns. mislocation of splices. Bending tolerance has sometimes been a problem. Failure to adequately tie a mat or cage sometimes resulted in movement of bars due to foot traffic or concrete placement. Fabrication problems have occurred with bending of the harder steels. lack of cover. Weldable ASTM A706 reinforcing bars are now readily available. Larger bars are butt welded. Item 440 “Reinforcing Steel” and by Departmental Specification DMS-4510. A few observations are made here for emphasis. and prestressed concrete members. Spiral reinforcing steel used for ties in columns should be #3 at 6 in.5 in. Precast Members — 1. #18 Bars — 3. Substructure — 3. as an alternate to lap splices.5 in. Bridge Design Manual 5-7 TxDOT 12/2001 . Splicing in regions of maximum stress is not recommended. Recently timber was used in the rehabilitation of the approach spans to an off-system suspension span. are no longer recommended. Designed compression splices in flexural members are not recommended.5 in. Longitudinal reinforcing steel in new columns should have an area of at least 1 percent of the gross concrete area. welded splices should be allowed as an alternate to mechanical splices. pitch for columns 30 in. Tension lap splice lengths shall conform to the requirements of the current AASHTO specifications for reinforcing steel. Bending is discouraged and lap splicing is prohibited. stress in the steel is the modulus of elasticity times the calculated strain except that. Hooks and bends must always conform to the requirements of the AASHTO Specification to prevent excessive breakage of Grade 60 (420) bars. Requirements for weldable steel and mechanical couplers are covered by Texas Standard Specifications Item 440 “Reinforcing Steel” and the special provisions thereto. pitch for all others. If possible. In consideration of possible coarse aggregate size and to avoid congestion. the following minimum clear spacings should be observed: • • • • Superstructure — 2. regardless of the relationship between actual and required capacity. the stress is at yield. 60 ksi. stress in the steel is the modulus of elasticity times the calculated strain. in large epoxy coated bars and should be required in all bars where clearance is doubtful. #18 bars should only be used when the project will require more than 40. but is permissible. ♦ ♦ ♦ ♦ Timber Fender systems and ferry landings use timber because of its resilience. In load factor design. ♦ ♦ ♦ ♦ ♦ Avoid splicing in an area where reinforcing steel from an intersecting member exists. Splicing cap bars over a column should be avoided.0 in. Mechanical splices should be allowed.00207.000 pounds of these bars. where strain exceeds 0. Timber pilings.Chapter 5 — Preliminary Considerations Section 1 — Materials ♦ ♦ ♦ ♦ In service load design. Congestion is bound to occur because of spacing tolerances. used for many years in Texas bridges. round or less and #4 at 9 in. has been used for bridge bearings since the advent of prestressed concrete beams in the middle of the 1950s. silane. Some experimental projects using both structural shapes and concrete reinforcing bars have been developed. linseed oil. Neoprene Neoprene. Bridge Design Manual 5-8 TxDOT 12/2001 . siloxane. Teflon. the aluminum industry developed an alloy that had sufficient toughness to be cast into rail posts. Early aluminum bridge railing was mostly ornamental because it had very little impact strength. asphalt. polyester. With the advent of the present railing design specification.Chapter 5 — Preliminary Considerations Section 1 — Materials Aluminum Aluminum has yet to become useful for structural members in bridges. This type of railing was used extensively for several years and is still available as a standard. which is a polychloroprene polymer originally patented by DuPont. Extruded semi-elliptical rail members are fastened to the cast posts. Fiber Reinforced Polymer (FRP) Composites Glass and carbon fiber reinforced composites are being studied for use in highway structures. coal tar. and concrete additives such as fly ash and silica flume. rubber fabric. Miscellaneous Materials Many other materials have been used in conjunction with primary construction materials. Some of these are epoxy. Slab Beam Spans Figure 5-8: Prestressed . Simple Slab Span Figure 5-2: Cast-In-Place. Continuous Slab Unit Figure 5-3: Cast-In-Place. Currently.Precast. or their consultants usually make the choice of superstructure type. as they prepare the preliminary bridge layouts. area engineers. I-Beam Units Figure 5-13: Steel. Double Tee Beam Spans Figure 5-6: Prestressed . Simple Span Box Girder Units Figure 5-12: Steel. District design engineers.Precast. If there is any doubt as to the proper design for the situation.Precast. The most common types of superstructure in use.Precast. In some cases the district may have a preference for certain structure types. Bridge Design Manual 5-9 TxDOT 12/2001 .Precast. TxDOT Box Beam Spans Figure 5-7: Prestressed . Pan Form Spans Figure 5-4: Prestressed .Continuous Slab Units Figure 5-9: Prestressed . over 30. U-Beam Spans Figure 5-10: Segmental.Chapter 5 — Preliminary Considerations Section 2 — Structure Type Section 2 Structure Type Superstructure Selecting an appropriate superstructure type is a critical factor in the planning and design process. and a quantity breakdown. district personnel should contact the bridge project development manager or the director of Bridge Design for assistance in determining structure type and span lengths. The following figures illustrate the most common superstructure types currently used by TxDOT: ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ Figure 5-1: Cast-In-Place. Continuous Plate Girder Units Figure 5-14: Steel. are shown in "Types of Superstructure and Quantity Breakdown of On-System Bridges in Texas (FY 2000)" table to further aid in selecting a superstructure type. Continuous Trapezoidal Girder Units The figures show the economical and practical span limits. I-Beam Slab Span Figure 5-5: Prestressed . Continuous Box Girder Units Figure 5-11: Segmental. and some advantages and disadvantages of each superstructure type.000 bridges and bridge class culverts exist on the state highway system. 458 1.250 1.Chapter 5 — Preliminary Considerations Section 2 — Structure Type Figure 5-1.333 1.460 Span (ft) 20 25 to 26.) Explanatory notes for Figure 5-1 Approximate Superstructure Depth (ft) Skew 0° 15° 30° ° ° ° 1. Not the most economical solution 4. Requires formwork support 5. Simple Slab Span (see following explanatory notes) (Online users can click here to view this illustration in PDF.083 1. Ease of design and detail 3.080 Advantages 1.960 1. Complicated for skews over 15 degrees 3.040 1. Deck joints at each bent 2. Limited span length Bridge Design Manual 5-10 TxDOT 12/2001 .040 1.083 1. Minimum depth for short spans 2.120 1. Aesthetic for small stream crossings Disadvantages 1.250 30 35 40 45° ° 1.333 1.458 1.250 1.120 1. Cast-in-Place.460 60° ° 0. 12 30 1. Cast-in-Place.33 1. Aesthetic for small stream crossings Disadvantages 1. No deck joints 3.) Explanatory Notes for Figure 5-2 Approximate Superstructure Depth (Ft) Skew Span (ft) 0° 30° ° ° 20 1.12 1. Not the most economical solution 3.50 1. Continuous Slab Unit (see following explanatory notes) (Online users can click here to view this illustration in PDF.33 35 1.50 Advantages 1. Limited to 30 degree skew 2.Chapter 5 — Preliminary Considerations Section 2 — Structure Type Figure 5-2.00 25 1. Limited span length 4.00 1. Requires formwork support Bridge Design Manual 5-11 TxDOT 12/2001 . Absolute minimum depth 2. Pan Form Spans (see following explanatory notes) (Online users can click here to view this illustration in PDF. Limited span capabilities 2. No shoring required Disadvantages 1.75 2.75 Advantages 1.75 45° ° 2. Tendency to maintenance problems 3.00 2.167 40 to 40. Absolute minimum cost for short spans 2.833 0° ° 2. Not aesthetically pleasing Bridge Design Manual 5-12 TxDOT 12/2001 .00 2.00 2.) Explanatory Notes for Figure 5-3 Approximate Superstructure Depth (ft) Skew 14°02'00" 26°34'00" 36°52'00" ° ° ° 2.00 2.Chapter 5 — Preliminary Considerations Section 2 — Structure Type Figure 5-3.75 Span (ft) 30 to 34.00 2.75 2. Standard details available 3. Cast-in-Place. I-Beam Slab Span (see following explanatory notes) (Online users can click here to view this illustration in PDF.333 5.333 6.667 4.833 6. Long beams sensitive to handling stresses Bridge Design Manual 5-13 TxDOT 12/2001 . and 145 ft.Chapter 5 — Preliminary Considerations Section 2 — Structure Type Figure 5-4.833 Advantages 1. Cannot be curved or cut to fit extreme geometry 3.167 4.) Explanatory Notes for Figure 5-4 Economical and Practical Span Lengths (ft) Beam Type A B C 45 55 75 60 80 90 Economical Limit Practical limit IV 115 135 VI (MOD) 150 175 Span (ft) A 45 3.333 5. Structure depths include 2" estimated beam haunch Approximate Superstructure Depth (ft) * Beam Type B C 3.667 3. Not a minimum depth structure 2.Invariably the most economical for spans between 45 ft.167 IV VI (MOD) 5.167 55 75 115 125 135 150 * For skews up to 65°.333 5.833 6.Design computerized and beam details standardized 3.Adaptable to most geometric conditions Disadvantages 1. Prestressed – Precast. 2. Prestressed – Precast.75 3.42 Advantages 1.Minimal diaphragm formwork to remove underneath Disadvantages 1.) Explanatory Notes for Figure 5-5 Span (ft) 30-35 30-50 30-60 * For skews up to 30° Approximate Superstructure Depth * Beam Type T21 or 22 T27 or 28 T35 or 36 Depth (ft) 2. span range 2.Not appropriate for flared or curved structures or skews Bridge Design Manual 5-14 TxDOT 12/2001 .Chapter 5 — Preliminary Considerations Section 2 — Structure Type Figure 5-5. Double Tee Beam Spans (see following explanatory notes) (Online users can click here to view this illustration in PDF.17 2.Probably the most economical precast bridge in the 30 ft. to 40 ft.Overlay thickness varies with camber 2. decreases the economical and practical span lengths.750 2. Economical and Practical Span Lengths (ft) Beam Type 20" 28" 34" Economical Limit Practical Limit 2 2 40" 115 65 80 100 For spans utilizing 5" concrete slab and shear keys.) Explanatory Notes for Figure 5-6 Approximate Superstructure Depth (ft) Beam Type 20" 28" 34" 2.083 2. Advantages 1. Not appropriate for curved or flared structures 6. Subject to longitudinal and transverse cracking 4. Not economical 3. Complicated for skews Bridge Design Manual 5-15 TxDOT 12/2001 . instead of the 5" reinforced concrete slab. Depths include 5" reinforced concrete slab. Use of 2" asphalt overlay. instead of the 5" reinforced concrete slab. Not aesthetic 5.Chapter 5 — Preliminary Considerations Section 2 — Structure Type Figure 5-6.Absolute minimum depth of precast bridge for short and intermediate spans 2.250 1 Span (ft) 35 65 80 100 115 1 40" 3.750 3. Prestressed – Precast.750 3. decreases the structure depths.250 3. Use of 2" asphalt overlay. TxDOT Box Beam Spans (see following explanatory notes) (Online users can click here to view this illustration in PDF. all skews are discouraged. Difficult to manufacture 2. however.083 2.750 For skews up to 45°.Expedites stage construction Disadvantages 1. Absolute minimum depth of precast bridge for short spans 2.) Explanatory Notes for Figure 5-7 Approximate Superstructure Depth (ft) * Span (ft) 30 1. Slab Beam Spans (see following explanatory notes) (Online users can click here to view this illustration in PDF.Chapter 5 — Preliminary Considerations Section 2 — Structure Type Figure 5-7.Speed of construction Disadvantages 1.417 55 1.Low-cost alternative for off-system short span bridges 3. Complicated for skews Bridge Design Manual 5-16 TxDOT 12/2001 .667 * For skews up to 30° Advantages 1. Not appropriate for curved or flared structures 2. Prestressed – Precast. 000 Advantages 1.000 80 2.500 3.000 100 2. Design and detailing more complicated 2. Prestressed.Deflections controlled by prestressing Disadvantages 1. Requires falsework for form support Bridge Design Manual 5-17 TxDOT 12/2001 . Slightly less economical than constant depth 3.500 5.Can be aesthetically pleasing 3. Continuous Slab Units (see following explanatory notes) (Online users can click here to view this illustration in PDF.000 4.) Explanatory Notes for Figure 5-8 Approximate Superstructure Depth (ft) Interior Span (ft) ¼ Point Bent 60 1.Chapter 5 — Preliminary Considerations Section 2 — Structure Type Figure 5-8.Absolute minimum depth for intermediate spans 2. 050 110 4.Chapter 5 — Preliminary Considerations Section 2 — Structure Type Figure 5-9. Prestressed – Precast.) Explanatory Notes for Figure 5-9 Approximate Superstructure Depth (ft) * Beam Type Span (ft) U40 U54 100 4. Expensive compared to prestressed I-beams Bridge Design Manual 5-18 TxDOT 12/2001 .210 * For skews up to 30° Advantages 1. More difficult to transport 2. U-Beam Spans (see following explanatory notes) (Online users can click here to view this illustration in PDF. Easier to cast than box beams Disadvantages 1.050 120 5.210 130 5. Can be aesthetically pleasing 2. 300 12.000 highway alignment to optimize 240 9.Can be erected with minimum interference beneath structure Disadvantages 1. Continuous Box Girder Units (see following explanatory notes) (Online users can click here to view this illustration in PDF.Awkward for flaring roadways 3.333 Structure type should be carefully 180 7.Aesthetically pleasing 2.Chapter 5 — Preliminary Considerations Section 2 — Structure Type Figure 5-10.Possibly uneconomical 2.May be difficult to fabricate and erect unless methods and procedure are carefully planned 4.) Explanatory Notes for Figure 5-10 Approximate Superstructure Depth (ft) Max Span (ft) Skew Approximately 0° ° 160 7.000 Advantages 1.600 constructibility. Segmental.Horizontal curvature limited to about 4° Bridge Design Manual 5-19 TxDOT 12/2001 .667 considered during the design of 200 8. Possibly uneconomical 2.583 highway alignment to optimize 6.Not practical for small projects 6.333 Advantages 1.Chapter 5 — Preliminary Considerations Section 2 — Structure Type Figure 5-11.917 constructibility.May be difficult to fabricate and erect unless methods and procedure are carefully planned 4.Long-term performance unproven 5.167 considered during the design of 6. 7.) Explanatory Notes for Figure 5-11 Max Span (ft) 80 100 120 140 160 Approximate Superstructure Depth (ft) Skew Approximately 0° ° 6.Awkward for flaring roadways 3. Segmental.Can be erected with minimum interference beneath structure Disadvantages 1.Aesthetically pleasing 2.Horizontal curvature limited to about 8° Bridge Design Manual 5-20 TxDOT 12/2001 .167 Structure type should be carefully 6. Simple Span Box Girder Units (see following explanatory notes) (Online users can click here to view this illustration in PDF. Expensive. except in comparison to prestressed concrete box beams 2.333 3.Chapter 5 — Preliminary Considerations Section 2 — Structure Type Figure 5-12.Can be aesthetically pleasing Disadvantages 1.) Explanatory Notes for Figure 5-12 Approximate Superstructure Depth (ft) * Beam Type W21 W24 W27 W30 2.833 Span (ft) W18 40 2. possibility of corrosion Bridge Design Manual 5-21 TxDOT 12/2001 .583 2.Painting is unreliable.Easier connections and shaping to unusual geometry 2. Steel. I-Beam Units (see following explanatory notes) (Online users can click here to view this illustration in PDF.833 3.583 3.333 50 60 70 80 90 100 * For skews up to 70° W33 W36 Advantages 1.083 3. ) Explanatory Notes for Figure 5-13 Approximate Superstructure Depth (ft) Alignment End Span (ft) Tangent 5° Curve ° 100 4. Usually the best choice for spans over 145 ft. Steel. Can be curved or cut to any geometry 3.750 10° Curve ° 4.500 4.000 4. Can be aesthetically pleasing Disadvantages 1.250 5.750 260 7.000 140 4. Lighter than concrete superstructures 4.250 Advantages 1.250 8.750 180 5.500 6.Weathering steel stains supports and rusts under continuous moisture or salt exposure and may not reach desired appearance in extremely dry climate Bridge Design Manual 5-22 TxDOT 12/2001 .Chapter 5 — Preliminary Considerations Section 2 — Structure Type Figure 5-13.000 6.500 7.750 6. continuous Plate Girder Units (see following explanatory notes) (Online users can click here to view this illustration in PDF.000 220 6.Painting is unreliable 3.Expensive 2.500 7. 2. 688 25 6.Chapter 5 — Preliminary Considerations Section 2 — Structure Type Figure 5-14.496 Concrete Girder 1.Large splices 4. 180 Contact the TxDOT Bridge Division. Continuous Trapezoidal Girder Units (see following explanatory notes) (Online users can click here to view this illustration in PDF. alone or with concrete U-Beam approaches 2. Steel.Corrosion possibilities less than for I-Girders Disadvantages 1.172 Pan Form Girder Timber Prestressed Concrete Beam Prestressed Concrete Box Beam 3.Aesthetically pleasing.Heavier sections to transport and erect 5.Erection stresses less than for I-Girders 3. 220 Bridge Design Section for information. 260 Advantages 1.Two-girder units are considered fracture critical Superstructure Type Number of Types of Superstructure and Quantity Breakdown of Bridges On-System Bridges in Texas (FY 2000) Concrete Slab 3.) Explanatory Notes for Figure 5-14 Approximate Superstructure Depth (ft) Alignment End Span (ft) Tangent 5° Curve 10° Curve ° ° 100 This table is under review and will be 140 completed at a later date.Very expensive 2.532 691 Bridge Design Manual 5-23 TxDOT 12/2001 .Weathering steel stains supports and rusts under continuous moisture or salt exposure and may not reach desired appearance in extremely dry climate 3. Drilling and founding conditions are good except for districts in the coastal plain.640 804 57 29 47 2 9 133 19. concrete pilings are always prestressed. it is often feasible to construct drilled shaft foundations even in very weak soils. and they develop the required resistance more consistently when tipped in firm material. aesthetics is an important factor to consider. They are easier to drive than concrete pilings. If the roadway width and span lengths are not too great. rectangular-shaped columns replace slender circular columns. Corrosion possibilities make steel less desirable for trestle piling. forming a trestle pile bent. a line of piling may be extended into the cap. Concrete pilings are used extensively in the coastal areas and occasionally in other locations. wide. For short bents on stream crossings. it may be advantageous to use single column bents.) 2. A discussion on aesthetics can be found in Section 4 of this chapter. Caps shaped like an inverted tee are often used to reduce the amount of cap and number of columns visible beneath the superstructure where fewer. ♦ ♦ Spread footing foundations are seldom used and should never be used in an erodable streambed. They may compete economically in trestle pile bents even if the soil is suitable for drilled shafts.Chapter 5 — Preliminary Considerations Steel I-Beam Steel Plate Girder Steel Truss Concrete U-Beam Segmental Concrete Box Girder Cable Stayed Movable Other Types Total Bridge Class Culverts (Culverts > 20 ft. Currently. Steel H Pilings are used sparingly. Foundation Drilled shafts are favored for bridge foundations in most areas of the state. Round or square columns usually rest on single drilled shafts or a footing that caps a group of piling.996 Section 2 — Structure Type Substructure Most structures with round columns and rectangular caps are satisfactory for bridges in rural and some urban areas. 5-24 TxDOT 12/2001 ♦ Bridge Design Manual . especially multilevel interchanges.325 12. Using the slurry displacement method. for bridges in urban areas. However. Single columns usually rest on a footing that caps a group of drilled shafts or piling. 200 Timber Piling 4.000 Steel H Piling 11. Foundation Types and Usage in Texas (calendar year 1999) Foundation Type Quantity (ft. Table 5.300 Concrete Piling 282.Chapter 5 — Preliminary Considerations Section 2 — Structure Type ♦ Occasionally.4 contains a summary of the quantities of foundation types let to contract by TxDOT during the fiscal year ending December 1999.) Drilled Shafts 466. which will indicate relative usage.500 Bridge Design Manual 5-25 TxDOT 12/2001 . some application will be found for timber piling. such as for dolphins or fender systems. good competition among the contractors. Structures that can be constructed rapidly might be justified if detour time can be minimized. The planners should continue to emphasize economy. Many things must be considered in planning a highway project. This has been attributed to consistency of details.Chapter 5 — Preliminary Considerations Section 3 — Economics Section 3 Economics Economics is not always the only basis for selecting structure type. Bridge Design Manual 5-26 TxDOT 12/2001 . and reasonable labor costs. Texas has long been favored with low bridge costs compared to the national average. Environmental considerations could justify the extra cost of especially aesthetic structures. Hydraulics and/or gradeline constraints may call for extra-thin superstructures. The team approach is recommended to determine the best style for the site. consider a remote and lengthy water crossing where the future adjacent architecture may be influenced by that of the new bridge. The phrase that is most often used currently is context sensitive design. Bridge Design Manual 5-27 TxDOT 12/2001 . The engineer is equally obligated to consider the aesthetic impact of the bridge at its proposed location. there is societal pressure to design a bridge that more harmoniously blends into and complements the particular site with attention given to the local architecture styles and culture. engineers must make the final decisions. A rural bridge today might be the focal point of a residential or resort area in 50 years. but as a bridge is an engineered structure. Bridges. architects. In consideration of their anticipated structural life of 75 years or more. The key to a successful aesthetic project is to build a unique and noteworthy structure — without breaking the bank. by virtue of their size alone. As an example. and cost. landscape architects. artists. Increasingly.Chapter 5 — Preliminary Considerations Section 4 — Aesthetics Section 4 Aesthetics It is the responsibility of the bridge designer to ensure that the structure meets the requirements for safety. and other interested parties. The design team should take into account the input of bridge engineers experienced and accomplished in aesthetics. Other valuable input has come from the public meetings process. Often. however. which simply points to a site-specific design integrating both function and aesthetics. neighbors. and landowners are asked to participate and give their opinions. a utilitarian and plain structure efficiently satisfies the functional need. the owner/engineer must take the responsibility for the final aesthetic decisions. durability. Again. are often works of public art. there are other design opportunities to propose a bridge that is unique to a site that otherwise has no nearby architecture from which to be inspired. While a particular design may take into account the local architecture close to the project. Aesthetic decisions made at the time of the design must also take into account the anticipated adjoining land use during the life of the structure (generally taken as 75 to 100 years). they leave a lasting testimonial to the spirit and the priorities of those whose decisions shaped their look. where users. Bridge Design Manual 5-28 TxDOT 12/2001 .Chapter 5 — Preliminary Considerations Section 5 — Corrosion Problems Section 5 Corrosion Problems Information for this section will be added later. For information. contact the TxDOT Bridge Division’s Construction Section. A variation of the 500 series. T6 (MOD).000 46 T6.200 51 T502 Retrofit 4. It is called the Single Slope Traffic Rail (SSTR) and is 36 in.900 77 Pedestrian and Bicycle 20.300 10 T101 and T101 (MOD) 13. Type T202 is a 27 in. high open concrete railing worthy of consideration. having a straight slope on the traffic side.8: Bridge Railing Usage on 12 Monthly Lettings Ending December 1999 Railing Type Quantity Number of (L. Table 5. there might be a different set of designs in each district. Improved visibility and aesthetics make this rail a popular choice in urban areas. The 500 series railings are 32 in. NCHRP Report 350 covers these requirements. A summary of bridge railings let to contract in Texas during the year ending December 1999 is shown in Table 5.8. tall.000 437 A new set of crash test requirements for bridge rails have been imposed by the FHWA.000 4 SSTR 169.) Jobs T501 and T501 (MOD) 388. Testing and updated standard development is currently ongoing. and Retrofit 30.500 36 Combination 48.900 8 T202 and T202 (MOD) 43.100 39 Miscellaneous 39. Type T4(A) is similar to T4(S) but with an aluminum top rail. The openings facilitate snow removal and provide Bridge Design Manual 5-29 TxDOT 12/2001 . has been developed.900 13 T502 and T502 (MOD) 131.800 34 T201 and T201 (MOD) 10. If the Bridge Design Section standards were not promoted so aggressively.100 53 Total 914.200 66 T501 Retrofit 15.F.Chapter 5 — Preliminary Considerations Section 6 — Railing Section 6 Railing There have always been many and various opinions about how a bridge railing should look or how strong it should be. Type T4(S) is a combination concrete parapet and steel top rail with an overall height of 33 in. high concrete walls with the safety shape face. Visibility over the railing by vehicle occupants is good. This rail has been successfully crash tested under NCHRP criteria. 113-2. “Influence of Casting Position and of Shear on the Strength of Lapped Splices. The standards also contain retrofit details. 113-4. refer to the Texas Bridge Railing Manual. and others.. pedestrian. “Factors Affecting Splice Development Length. and 113-5F. Type T201 is a 27 in. Bridge Design Manual 5-30 TxDOT 12/2001 . and 1543F. This rail is currently being modified with a wider top beam.E. and 242-3F. 1975. Reports 242-1. 1968 to 1971. and others. Standards for rails currently used in Texas are available on the TxDOT web site. The standard designation for the new version will be T203. and others. J. combination traffic and pedestrian. high strong steel post railing system that continues the approach guardrail continuously across the structure. Type T101 is a 27 in.” Breen.” Furlong. Reports 154-1. high tested weak steel post system with strong tubular W-beam rail member. CTR. CTR. as well as enhancing driver comfort. 113-3. CFHR.o. C. 242-2. For a more complete discussion on bridge railing.” Orangun. Designers should choose a railing from the available TxDOT standards and consider that there could be a configuration more suitable than the safety shape for a given condition. Reports 113-1. and bicycle railings that are suitable for most conditions with occasional modifications. 154-2. Type T6 is a 27 in. high vertical wall that meets NCHRP Report 350 criteria. R.W. 1 “Splices and Anchorage of Reinforcing Bars. 1981. The new configuration has passed crash tests for compliance with NCHRP Report 350.Chapter 5 — Preliminary Considerations Section 6 — Railing relief from overtopping floods. ......................................................6-13 Section 4 — Design Methods ...................................................................6-16 Section 5 — Design Philosophy .....................................................................................................................6-2 Section 2 — Loading ..................................................................................6-18 Bridge Design Manual 6-1 TxDOT 12/2001 ..........................6-4 Section 3 — Load Distribution ............Chapter 6 General Design Controls Contents: Section 1 — Specifications ........................................................................................................................... researchers. 1 was issued originally on May 24. Careful consideration should be given to structures where THD Supplement No. The membership is divided among approximately 20 technical committees. consider suggestions for change from other sources. These committees continually monitor their specification areas. and the issue date. this time with 17 items. It contained 18 items that had approval dates between October 14.Chapter 6 — General Design Controls Section 1 — Specifications Section 1 Specifications AASHTO Specifications The Standard Specifications for Highway Bridges1 adopted by the American Association of State Highway and Transportation Officials (AASHTO) controls the design of bridges and culverts under highway traffic. A supplement2 to the 1944 AASHO Design Specifications entitled THD Supplement No. 1 called for reduced axle loads in the design of concrete slabs. AASHTO Specifications are proposed. Guam. 1946. FHWA has been known to enforce its own specifications on the states in sensitive areas but currently appears to be following AASHTO-approved specifications and manuals. they are generally incorporated into the regular specifications. After a few years of use and necessary revisions. tentative or interim revisions have been published annually that carry the full force of the specifications. and present any needed revisions to the full subcommittee for consideration and possible approval for AASHTO publication. In the past 40 years. Many structures designed during the era will not have an acceptable rating and are usually replaced rather than rehabilitated or widened. which is composed of the 50 state bridge engineers and representatives from the Federal Highway Administration. These specifications were first published in 1931. Separate specifications and manuals have also been published for unusual types of structures or particular areas of bridge management. They are often in the position of strongly advocating specification revisions. The items pertaining to live load did not appear to change during this time frame. Mariana Islands. Puerto Rico. usually perceived to respond to national safety concerns. 1945. FHWA and Industry Participation The Federal Highway Administration (FHWA) sits in review of state practice involving the use of federal funds. 1 is referenced in the plan notes. Bridge Design Manual 6-2 TxDOT 12/2001 . 1944. Industry proposals are usually based on the latest research in the area. discussed. or organizations representing industries that supply the various bridge materials or components. each responsible for a certain area of the specifications. THD Supplement No. and approved by the AASHTO Highway Subcommittee on Bridges and Structures. The supplement was revised and reissued on June 13. They have been revised and republished approximately every four years since then. Revisions may be suggested by users of the specifications. and seven Canadian provinces. Guide specifications have been published that have the status of suggested or trial specifications. These structures will be discussed in Chapter 9. reinforcing steel. Sections 19. Such departures are based on proven experience or local research. The FHWA directly funds research that often finds its way into the specification. still remain in the specifications. AREMA Specifications Design of structures to carry railroad traffic is controlled by the American Railway Engineering and Maintenance-of-Way Association (AREMA). Research sponsored by AASHTO through the National Cooperative Highway Research Program (NCHRP) has also furnished valuable background. Texas conducts a Cooperative Highway Research Program with state universities using these funds. structural steel. Lately. prestressing systems. and 21 of this manual. This commentary is found in the “Load and Resistance Factor Specification.Chapter 6 — General Design Controls Section 1 — Specifications Industry participation in maintenance of the AASHTO Specifications is very important. Section 5 of this manual. some railroad companies have expanded interpretations or provisions that must be followed for structures supporting their trains. there are often compromises worked out during committee deliberations. culvert pipe. Manual for Railway Engineering. Bridge Design Manual 6-3 TxDOT 12/2001 . 20. and aluminum is the basis for much of the current specifications. Recent Changes Although research drives many of the specifications. These exceptions will be addressed in Chapter 9. timber. Research sponsored by organizations representing suppliers and fabricators of concrete products. Wind-Sensitive Structures Wind-sensitive structures are subject to the AASHTO Specifications for Structural Supports for Highway Signs. Luminaires. Some of these areas will be identified in the following sections of the manual. through an NCHRP project. and Traffic Signals. The rationale behind many provisions is lost due to lack of written commentary. the AASHTO Specifications have been completely rewritten using current knowledge and specification logic. Some of the early provisions. reflecting the wisdom of dominant bridge engineers of that time. Research sponsored by individual states using Highway Planning and Research funds has been useful. A complete commentary is provided in the new version as a historical record of specification changes.3 Additionally. While generally bound to compliance with the AASHTO Specifications. Texas design practice departs from it in a few areas.” which has the status of a guide specification in Texas. formerly AASHO) has not seen fit to require this. The trucking industry continually seeks to raise the legal load and size limits. The effects of a 36 ton HS20 design load are generally a little more severe than the current 40 ton legal 18-wheeler because of the number and spacing of the rear axles. AASHTO Standard Specifications for Highway Bridges Early Texas specifications required structures to safely carry 125 pounds per square foot as a live load or a 20 ton roller. Texas has used HS25 for some bridges in the Texas-Mexico border area where heavier loads due to international truck travel are encountered. whichever required the greater strength. The loading of bridges and structures associated with bridges. and H20 truck loadings. This section provides some additional information concerning the Texas Department of Transportation (TxDOT) policy for each of these specifications. loads on bridges are thoroughly discussed by the American Society of Civil Engineers4 (ASCE). Although not directly applicable to Texas bridge design. A military loading for interstate highways was introduced by the Bureau of Public Roads in 1956. Dead load is simply the weight of the structure. as discussed in Chapter 3 of this manual.Chapter 6 — General Design Controls Section 2 — Loading Section 2 Loading Overview Most short span bridges can be adequately designed using only dead and live loads. and equivalent lane loadings that remain in effect today. Live load is whatever the governing specification requires for service loads to be resisted by the structure throughout its life. The 1941 third edition added the current group of H-S truck trailer loads with an equivalent lane loading heavier than for H loads. HS25 is also being Bridge Design Manual 6-4 TxDOT 12/2001 . truck trains. The 1944 version made the equivalent lane load the same for H and H-S loading. Luminaires. Highway loads have increased in size and frequency during the past 50 years. A few states design for an HS25 loading. but the design load has remained virtually the same. The 1931 American Association of State Highway Officials (AASHO) specifications established the H10. H15. Additionally. but the American Association of State Highway and Transportation Officials (AASHTO. such as sign supports. this reference can be used as further guidance. is discussed in several mandatory specifications. and Traffic Signals The loading criteria presented in each of these specifications are mandatory for the appropriate structures covered by each. The most commonly applicable specifications include the following: ♦ ♦ ♦ AASHTO Standard Specifications for Highway Bridges AREMA Specifications AASHTO Standard Specifications for Structural Supports for Highway Signs. Centrifugal forces due to live load may be treated as shown in Figure 6-3. for the HS20 loading and also for the military loading. Longitudinal Force. above the ground. There has been research and statistical analysis directed toward a realistic mix of vehicle loads for various types of bridges. Texas generally uses only the live loadings prescribed by the AASHTO Specifications. the following guidelines should be followed: ♦ ♦ ♦ Specified design live loads are placed in each traffic lane as necessary to cause maximum stress. revised loadings are occasionally negotiated for long span bridges. For continuous spans. Longitudinal forces due to live load are thoroughly described in AASHTO Specifications. the lane loading shall be continuous or discontinuous as to produce a maximum value. there may be instances where alternative loadings are considered. Application of Live Loads. The 1931 specification required bridges to resist a wind load of 30 pounds per square foot on 1½ times the area as seen in elevations. If justified. and does not apply to deck slabs and direct traffic box culverts. The military loading only controls span lengths up to 37 ft. In addition to dead loads and live loads. applied as shown in Figure 6-1 and Figure 6-2. Refer to AASHTO’s Standard Specifications for Highway Bridges for additional information concerning continuous spans.Chapter 6 — General Design Controls Section 2 — Loading considered for new bridges on North American Free Trade Agreement (NAFTA) truck corridors. This practice is recognized by AASHTO. Only one design truck per lane is placed in a span or unit. Application of Other Loads. Live loads shall be multiplied by an impact factor to increase the effects of the live load to account for effects due to vibration and impact per AASHTO Specifications. Equivalent lane loads are placed in spans as necessary to produce maximum stress. As discussed above. When applying live load. plus all girders in excess of two in the cross section. Wind loads must be considered but will seldom control the design of grade separation or stream crossing structures less than 25 ft. The origin of this loading is lost in antiquity. The load was changed to 50 pounds per square foot on 1½ times the area in 1953 and then to the current 50 pounds per square Bridge Design Manual 6-5 TxDOT 12/2001 . Because of this. the effects of impact shall be transferred from superstructure to substructure but shall not be included in loads transferred to structural elements below the ground line for the analysis of those structural elements. Bridges on all highway systems are currently being designed. These deliberations are very complex but it is reported that the AASHTO lane loading may be unrealistically severe for long span bridges. at the minimum. and a second concentrated load is used to produce maximum negative moment. Wind Load. Impact Due to Live Loads. other areas in Texas may use HS25 upon approval from the Bridge Division. the following AASHTO Specifications loads are common to bridges: ♦ ♦ ♦ Centrifugal Force. Some states require their bridge designs to safely carry a family of overload truck configurations permitted over their highways. For the analysis of the structure. and Traffic Signals5 contains a more refined treatment of wind loads. Trusses and arches are designed for 75 pounds per square foot. and Rib Shortening. Long span structures may justify more sophisticated analyses. Sidewalk loading shall be applied as described in the specifications. Curb Loading. as defined in the specifications. Curb loading shall be applied as described in the specifications. Earth Pressure. Use an equivalent hydrostatic pressure of 40 pounds per cubic foot unless more exact determinations are justified. the forces are applied according to Figure 6-4 . Stream Current. Earthquake Motions. 3269. Buoyancy. TxDOT does not design for earthquakes. shrinkage. Railing Loading. All concrete box girders should undergo such an analysis. Seven combinations of the above loads (Groups I through VI and X in the AASHTO Specificaitons) must be considered in the design of bridges.Chapter 6 — General Design Controls Section 2 — Loading foot on the area as seen in elevation in 1957. If significant drift is expected. Ice pressure does not occur in Texas. but streetcars are becoming popular again. Shrinkage. Other loads mentioned in the AASHTO Specifications are treated as follows. At the present time. Luminaires. but the trend is toward crash testing to verify railing details. Bridge Design Manual 6-6 TxDOT 12/2001 . Temperature. Buoyancy is important for cofferdams but is seldom a factor in ordinary bridge design. The AASHTO Standard Specifications for Structural Supports for Highway Signs. Wind on the live load is also covered on this figure. Current railing standards are designed to AASHTO requirements. Sidewalk Loading. ♦ ♦ ♦ ♦ ♦ Load combinations.6 If the structure is considered sensitive to wind. but they are actually internal deformations that can result in stress redistribution. There is an excellent treatise on wind with a large bibliography in the ASCE Transactions. Shrinkage is seldom treated analytically. Stream current should be considered but rarely controls the design. are considered in the design of substructure for continuous units. Paper No. and creep have been observed to have a significant effect on large concrete box girders. including wind tunnel tests. Curbs are seldom used on bridges. Designing for drift loads is highly speculative. Temperature deformations. Ice Pressure. Electric railway loads are a holdover from early specifications. Rib shortening is a secondary effect that should be considered in the design of arches. Research has verified some of the parameters and methods available to analyze their effects. Temperature. These factors are listed in the combination of loads. wall or webbed piers should be used and careful attention given to span lengths and skew angle. ♦ ♦ ♦ ♦ ♦ Electric Railway Loads. to investigate the dynamic performance of the design. When applying the live load to the design. remember the following: ♦ Reduce live load effects by 10 percent if three lanes are loaded. H20 Truck. 6-7 TxDOT 12/2001 Bridge Design Manual . and Alternate Military Live Loads (see following explanatory notes) (Online users can click here to view this illustration in PDF. AASHTO HS20 Truck.Chapter 6 — General Design Controls Section 2 — Loading Figure 6-1.1 shows equivalent lane loading for HS20 and H20 trucks.) Explanatory Notes for Figure 6-1 Figure 6. S. represents heavy military vehicles. Regarding 0. Regarding 18k for moment and 26k for shear notes: An additional concentrated load is used in the design of negative moment regions for continuous spans. the uniform load is placed in spans only as necessary to produce maximum stress. developed by the FHWA in 1956.Chapter 6 — General Design Controls Section 2 — Loading ♦ ♦ Reduce live load effects by 25 percent if four or more lanes are loaded. or any highway bridge that may carry heavy truck traffic. Alternate military loading. Interstate Highway System.640 klf notes: In the design of continuous bridges. All bridges on the U. whichever produces the greatest stresses. are to be designed using HS20 or the alternate military loading. Bridge Design Manual 6-8 TxDOT 12/2001 . Increase for impact per AASHTO Specifications. 3P20 = 20. The live load is placed at critical locations. Beam Design. increase 30 percent for impact if applicable (1.Chapter 6 — General Design Controls Section 2 — Loading Figure 6-2. The live load is distributed to the stringers assuming the slab is simply supported at each beam. and Bent Design (see following explanatory notes) (Online users can click here to view this illustration in PDF.3P20 = 20. controlling between truck and land load. Live load reaction per land. Bent Design. 0 in. Slab Design. Applying Live Load on the Structure for Slab. Wheel load (P20) shall be increased 30 percent for impact (1. P20 = 16k.3P20 = 20.8k) for substructure elements above the ground line. Specific slab design moments and distribution widths are specific by AASHTO.8k). and using combinations of loaded lanes as to produce the maximum stresses. from face of rail when designing cantilever. Uniform load (w) is not applicable and is ignored. Wheel load (P20) shall be increased 30 percent for impact (1. Bridge Design Manual 6-9 TxDOT 12/2001 . Use live load distribution factors specified by AASHTO.) Explanatory Notes for Figure 6. Beam. The load on one rear wheel of HS20 or H20 truck.2 LL = 2P20 + 10w.8k). Exterior P20 shall be placed 1 ft. w= LL − 2P20 10 The uniform load portion of LL (k/ft). increase for impact if applicable. Application of Centrifugal Force (CF) (see following explanatory notes) (Online users can click here to view this illustration in PDF.) Explanatory Notes for Figure 6-3 Centrifugal Force (CF) = RF (n)(C)LLTL) Where: RF = Reduction in load intensity factor. kips (k) The direction of CF is radial. If the bent is skewed. if no superelevation is present D = Degree of curvature along the baseline To account for the effects of superelevation. per AASHTO N = Number of loaded lanes C = Centrifugal force in percent of live load LLTL = Live load due to truck load without impact. The centrifugal force in percent of live load shall be calculated by the following: C = 0.0000117S2D Where: S = Design speed in mph. applicable only if n ≥ 3. truck speed (S) may be taken as the following: S= 85950 (e + 0. the radial force shall be resolved into parallel and perpendicular components.Chapter 6 — General Design Controls Section 2 — Loading Figure 6-3.15) where e = superelevation rate D Bridge Design Manual 6-10 TxDOT 12/2001 . Application of Wind Loads.Chapter 6 — General Design Controls Section 2 — Loading Figure 6-4. WL (see following explanatory notes) (Online users can click here to view this illustration in PDF. applied simultaneously with 40 plf longitudinal and resolved into components parallel and perpendicular to the bent. Bridge Design Manual 6-11 TxDOT 12/2001 . applied simultaneously with 12 psf longitudinal and resolved into components parallel and perpendicular to the bent. Uplift (WUP) is 20 psf of deck and sidewalk plan area applied at the windward ¼ point of the transverse superstructure width. Wind on live load (WL) is 100 plf transverse. WSUB.) Explanatory Notes for Figure 6-4 ♦ ♦ ♦ ♦ Wind on superstructure (WSUP) is 50 psf transverse. Wind on the substructure (WSUB) is 40 psf transverse. including WSUP. applied simultaneously with 40 psf longitudinal and resolved into components parallel and perpendicular to the bent. WUP. Chapter 6 — General Design Controls Section 2 — Loading AREMA Specifications The American Railway Engineering and Maintenance-of-Way Association specifications for loading are strict. and Traffic Signals Used for the design of sign supports and poles. Refer to the AREMA specifications for loads and methods of application. Luminaires. Bridge Design Manual 6-12 TxDOT 12/2001 . AASHTO Standard Specifications of Structural Supports for Highway Signs. and Traffic Signals for loads and methods of application. but can be followed without any undue expense. Luminaires. Refer to the AASHTO Standard Specifications for Structural Supports for Highway Signs. these specifications frame a different type of structural design for which wind speed. ice load. and shape factor are important considerations. The conservatism of this approach may account for some of the reserve strength regularly observed when redundant stringer bridges are load tested. ♦ Transverse Beams (Floorbeams). Load Distribution Treatment of wheel load distribution to the various bridge components in the AASHTO Specifications is as follows: ♦ Longitudinal Beams (Stringers). No transverse distribution of wheel loads is allowed unless a sophisticated analysis is used. Similarly. In order to simplify the design procedure. Computerized grid systems and finite element programs can come close to reality. These two. has been used for many years to determine the portion of a wheel load to be supported by steel or prestressed concrete stringers under a concrete slab. Occasionally. so developed. Composite dead loads are distributed equally to all stringers except for extraordinary conditions of deck width or ratio of overhang to beam spacing. The factor S/5. such as span aspect ratio.Chapter 6 — General Design Controls Section 3 — Load Distribution Section 3 Load Distribution Overview Truck wheel loads are delivered to a flexible support through compressible tires. the number of variables was reduced to a minimum consistent with safety and reasonable economy. Live load is distributed to all types of outside beams assuming the deck to act as a simple cantilever span supported by the outside and the first inside stringer. Other variables. For the few cases where floorbeams have been used without stringers on highway bridges.or three-dimensional problems are reduced to one dimension through various empirical distribution factors given in the AASHTO Specifications. special conditions will justify the use of a discrete element grid and plate solution. Distribution factors given in the specifications are used almost exclusively. the distribution factor for moment is used also for shear. experience has shown that concrete slab spans and slabs on stringers will invariable support much more load than predicted by empirical analysis. are not considered except for occasional special bridges. which makes it very difficult to define the area of the bridge deck significantly influenced. skew angle and relative stiffness between stringer and slab. it has appeared proper to calculate reactions assuming the deck slab to act as a continuous beam supported by the floorbeams. according to the judgment of the AASHTO Highway Subcommittee on Bridges and Structures. but they are complicated to apply and are limited by mesh or element size and by the accuracy with which the mechanical properties of the composite materials can be modeled. For simplicity of calculation and because there is no significant difference.5. These distribution factors have been derived from research involving physical testing and/or computerized parameter studies. Bridge Design Manual 6-13 TxDOT 12/2001 . Calculate deformations due to temperature change of 70 degrees and convert to forces according to the stiffness of the fixed bents. ♦ ♦ For skews up to 30 degrees. Spread Box Girders. Loads are distributed according to the AASHTO Specifications. Other Structure Types. Timber Flooring. ♦ Concrete Slabs . distribution of the wheel load is built into a formula for moment. number of lanes. ♦ ♦ ♦ ♦ ♦ Horizontal Loads Horizontal loads on the superstructure distribute to the substructure according to a complicated interaction of bearing and bent stiffness. and span length. Concrete Slabs . TxDOT designs are standardized according to the requirements of the current AASHTO Specifications. Precast Concrete Beams Used in Multibeam Decks (Box Beams). Timber is not used in new structures. The approximate formula for moment is not used. and Glued Laminated Timber Decks. For continuous steel units. The specifications are followed closely. main reinforcing is parallel to traffic. Steel Grid Floors. and no additional edge beam strength is needed for usual railing conditions. For this component. This design is also standardized. overall bridge width. Divide the load between transverse and longitudinal spans according to the formulas for slabs supported on four sides. The simplified values for K shown in the specifications are usually used for final designs.Reinforced Parallel to Traffic (Slab Spans). Composite Wood-Concrete Members. The latest standard designs and current special designs comply with the current AASHTO Specifications. the following method will usually be sufficiently accurate: ♦ ♦ ♦ Apply transverse loads times the average adjacent span length.Reinforced Both Ways. The distribution factor is a function of box width. Slab Overhang Design. Apply longitudinal loads times the unit length to the fixed bents according to their relative stiffness. See Chapter 7 for distribution factors for other structure types not listed here. This type of construction is seldom used in Texas. The specifications are followed closely.Reinforced Perpendicular to Traffic (Slab on Stringers).Chapter 6 — General Design Controls Section 3 — Load Distribution ♦ Concrete Slabs . Span length of slabs on prestressed concrete stringers may be taken as the clear distance between flanges and adjusted to the flange quarter points for steel stringers. Use the appropriate load distribution in each direction. This type of construction is seldom used in Texas. For skews greater than 30 degrees. reinforcing is perpendicular to the bents and edge beam strength is provided and reinforced parallel to traffic. 6-14 TxDOT 12/2001 Bridge Design Manual . Forces due to temperature deformations may be ignored except for bearing design. deformations may be based on 40 degree temperature change. if its consideration is desirable. fixity is superficial and all bearings are approximately the same stiffness.Chapter 6 — General Design Controls Section 3 — Load Distribution ♦ ♦ Centrifugal force is based on the truck load reaction to each bent. Bearing stiffness may be based on a shear modulus of 175 psi. the maximum longitudinal force may be taken as 0. ♦ Bridge Design Manual 6-15 TxDOT 12/2001 . It will usually be sufficiently accurate to distribute horizontal loads in the following manner: ♦ ♦ ♦ Apply transverse and longitudinal loads times the average adjacent span length. If temperature consideration is desirable. The concentrated live load for longitudinal force would be located at each bent.10 times the dead load reaction for rocker shoes and polytetra fluoroethylene (PTFE) sliding bearings. Centrifugal force is based on the truck load reaction to each bent. For prestressed concrete beam spans and units on elastomeric bearings. For a complete discussion on bearing pad design. Friction in expansion bearings can usually be ignored but. refer to the paragraph titled “Elastomeric Bearings” in the Chapter 9 discussion of Design Recommendations for Bearings. Serviceability aspects. and crack control. AASHTO Specifications The AASHTO Specifications allow Service Load Design and Strength Design alternatively for reinforced concrete. Calculated stresses are compared to specified allowables that have been scaled down from the tested strength of the materials by a factor judged to provide a suitable margin of safety. A third method is proposed in the Load and Resistance Factor Design Specification. and railing. This method has more numerous and more accurate load and resistance factors based on probabilistic theory and reliability indices and should produce even more uniform and realistic safety factors between different types of bridges. such as deflection. Service Load only is indicated for timber. must be determined by Service Load Analysis. Strength Design is recommended for reinforced concrete bent caps and columns. and structural steel. fatigue. the same service loads are distributed empirically and the external forces on each member determined by elastic analysis. and soil-reinforced concrete structures. The Texas Bridge Design Section generally recommends Service Load Design for reinforced concrete slabs. traffic signals. Bridge Design Manual 6-16 TxDOT 12/2001 . The Strength Design Method produces a more uniform factor of safety against overload between structures of different type and span length. Load and Resistance Factor Design. Concrete strength and amount of prestressing is usually determined by Service Load Analysis. It is. Prestressed concrete design uses a combination of the two methods. structural steel. Strength Design also tends to produce more flexible structures. In Service Load Design. supports for signs. luminaires. of course.Chapter 6 — General Design Controls Section 4 — Design Methods Section 4 Design Methods Overview Under the current AASHTO Specifications. loads of the magnitude anticipated during the life of the structure are distributed empirically and each member is analyzed assuming completely elastic performance. soil-corrugated metal structures. These factored forces are compared to the ultimate strength of the member scaled down by a factor reflecting the possibility and consequences of construction deficiencies. reinforced concrete footings. which accounts for its slow acceptance by some states. In Strength Design. two basic design methods are allowed — Service Load Design and Strength Design. Those member forces are increased by factors judged to provide a suitable margin of safety against overloading. but Strength Design is used for shear and the ultimate flexure check. Service Load. Load Factor (Strength Design). more complicated. Bridge Design Manual 6-17 TxDOT 12/2001 . Moments. AASHTO Standard Specifications for Structural Supports for Highway Signs. and Traffic Signals Design methods are different under these specifications. must be considered in some structures. Although allowable stresses appear to be service load values. and torsions are determined by elastic analysis. Shape factors effect calculated stress.Chapter 6 — General Design Controls Section 4 — Design Methods AREMA Specifications Design methods under the AREMA Specifications are the same as for AASHTO. they are derived from load factor considerations. Wind induced oscillations. which produce fatigue stresses. shears. depending on geographical location and height of the structure above the ground. although most of the structures are determinate. Determination of wind forces is more complicated. Luminaires. Pan-formed concrete girders were developed in the late 1940s because so many short span stream crossings were being constructed uneconomically with steel beams or shored concrete girders. Design Considerations The Bridge Design Section has performed all types of design in-house except for cablestayed and suspension bridges. There have been designs that looked good on paper but were virtually impossible to properly construct. Texas has very few of these bridges. The most important part of the job is design and plan preparation for multitudes of conventional bridges that usually have some variation in geometry that prohibits the use of straight standard details. The more advanced structure types have as yet required only a small portion of the overall effort. Bridge Design Manual 6-18 TxDOT 12/2001 .000 bridges not counting box culverts (see the "Superstructure and Quantity Breakdown of On-System Bridges in Texas" table in Chapter 5). Construction experience is a valuable asset. The beams themselves still contain the standard shapes developed in the beginning. Designers need to consider how to build the components. These standards have undergone several changes in roadway width but are still used very economically in considerable numbers today. At least 90 percent of these bridges were designed in-house. Constructibility is important. Many of these standard groups covered a multiplicity of roadway widths. New shapes have been added in concrete and steel for which standard span and bent details have not been fully developed. and the accessories required to complete the span are covered on standard details. spans and skews and could require many original drawings. Framing dimensions and elevations must be accurate to avoid expensive field correction. Design engineers are primarily responsible for geometric accuracy. New structure types were developed to fill specific needs. An early resistance was developed to changes in roadway width and design specifications that would cause wholesale standard revisions. Geometry is considered an important part of bridge design. In the short and medium span categories the State owns approximately 20. Fewer plans are assembled from standard prestressed drawings today because bridge geometry has become so complicated and variable that most details must be specially prepared. After 50 years of use. Precast pretensioned beams were developed in the 1950s for medium span stream crossings and grade separations because steel beams were becoming expensive and sometimes slow on delivery.Chapter 6 — General Design Controls Section 5 — Design Philosophy Section 5 Design Philosophy Design Evolution Long span bridges excite the imagination and bring notoriety to the owner. it was necessary to maintain an extensive set of standard detail sheets that could be reproduced and used in the project plans. box culverts have been completely redesigned by the load factor method and new standard drawings prepared. To keep up with the demand for bridge plans. concrete overlay. Plan quantity items are concrete. Foundation quantities should be summarized from lengths shown on the plans and not repeated in the design notes. A number of computer programs are available. box beams. longhand methods may still be desirable in some areas. However. Design Notes Design notes are the documentation for structural adequacy and accuracy of pay quantities for each bridge. prestressed concrete beams. structural steel. Reinforcing steel.” The design office is the Office of Record responsible for the accuracy of pay quantities. A Texas design memorandum issued over 25 years ago is quoted below. which agree with the quantities shown on the plans. but the lesson is sometimes slow to be learned. offer the only realistic solution to a problem. Others. sealed expansion joints. the construction specifications state that “adequate calculations have been made in accordance with Article 9. but complete. reinforced concrete slab.Chapter 6 — General Design Controls Section 5 — Design Philosophy Details are more critical in design. should be billed on the plans but need not be included in the design notes. although complicated. These notes are kept on file for a reasonable period after construction of the bridge. Engineers and technicians should recognize and carefully evaluate untested details. Quantity calculations should be emphasized. Unfortunately. and easily followed record of all the essential features of the final design of each structure. Failure to provide for proper stress flow at discontinuities has often caused local distress and sometimes mortal injury to a system. and railing. During that time there may be many more bridges designed with the same problem. For “Plan Quantity” items. whether pay item or not. tee beams and slab beams. It is often necessary to refer to these notes because of changes or questions which arise during the construction period. It is imperative that accurate calculations. Virtually every bridge designer has access to a personal computer that can run most of the programs immediately. If properly prepared and assembled. Experience is a good teacher. Design engineers are expected to develop engineering judgment to recognize the degree of design complexity and accuracy justified by the type and size of structural element under consideration. It is as appropriate now as it was then. clear. The condition of the design notes reflects the attitude of the designer and checker. Some are so complicated as to be useful in very special investigations only. maintenance problems tend to occur several years after the structure is built. “Our design notes should consist of a concise.1. Others are very useful and save time during design and production. prestressing. retaining wall. Bridge Design Manual 6-19 TxDOT 12/2001 . these calculations are of great value as a guide and time saver in preparing designs of other similar structures. be found in the design note file. It takes a good designer to anticipate maintenance problems and spend just enough of the taxpayers’ money to prevent or delay problems. Maintenance Considerations The bottom line on bridge design is maintenance. preformed joint seal. It is usually much more expensive to repair a bridge than it was to build it. and the superstructure should be shown. A line diagram will suffice. these should be listed. In checking calculations. and design loading. the wind load is assumed and dead load generally is approximate only. save some bridge designer a week or more of computations. 1 on top followed in order by Sheet Nos. Outside Girders.’ etc. trial designs and comparative designs are not to be included in the design folder as finally filed. the design notes should be prepared and arranged so clearly that any reasonably experienced bridge designer can follow them and can obtain all the essential information he may need without consulting the [person] who made them and without wasting a lot of time. Superstructure calculations shall be placed in front of substructure calculations. Remember that structural design is not an exact science. Follow the original calculations and check them through. curb or sidewalk widths and heights.. The first sheet of calculations on any superstructure unit should show by sketch.” Bridge Design Manual 6-20 TxDOT 12/2001 . two years hence. a layout of that unit.’ ‘Summary of Shears. DON’T SPLIT HAIRS. In checking calculations. eliminate the original set and include the second set as a portion of the final calculations. instead of reverse order. The following essential features are to be observed in preparing. Preliminary designs.. with Sheet No. or at least check the results..Chapter 6 — General Design Controls Section 5 — Design Philosophy To properly serve the above purposes. don’t carry through corrections that are so minor in amount as to have no real effect on the structure. Calculation sheets shall be placed in file in usual sequence. Quantity calculations shall be placed at the bottom of the file. These headings should be supplemented by explanatory notes wherever necessary to clarify the portion of structure under consideration.e. should be freely used.. The strengths of the materials employed vary widely. Supplement the above rules with good judgment and plenty of common sense. In those few instances where original calculations are so poorly made that a new set must be prepared. Center Girder. i. If any deviations are to be made from the standard design specifications. 3. properly dimensioned. The live load is assumed. checking and filing design calculations: The headings at the top of each sheet are to be completely filled in and each sheet is to be numbered. The first sheet of calculations of any substructure unit should show an appropriate sketch or diagram of the unit. Appropriate headings and subheadings such as ‘Live Load Moments. The extra ten minutes you spend in making your calculation sheet clear and complete may save the checker an hour. the load combinations being used. Look for 20 percent errors. and may. not 1 percent error. giving number of spans and length (c-c bearing) of each span. The first sheet of calculations should always list clearly such governing features as roadway widths. etc. or the method of analysis being employed. 2. do not make up a separate set of design calculations. Part 2.” Proceedings of the American Society of Civil Engineers. 1945.Chapter 6 — General Design Controls 1 Section 5 — Design Philosophy “Standard Specifications for Highway Bridges. 3269. 6 “Wind Forces on Structures. 3 “Manual for Railway Engineering (Fixed Properties). 1981. 1)".” AASHTO. 2 "Supplement No. 1961.” AREA 1988-89. 5 “Standard Specifications for Structural Supports for Highway Signs. 1985.” J. Chmn. Bridge Design Manual 6-21 TxDOT 12/2001 .. Paper No. ST 7. American Society of Civil Engineers. Volume 126. Sixteenth Edition (1996). Public Roads Administration and Texas Highway Department. 1 to 1944 AASHO Design Specifications for Texas Bridges (THD No.” American Association of State Highway and Transportation Officials (AASHTO). 4 “Recommended Design Loads for Bridges. Briggs.M. Final Report. Transactions. Luminaires and Traffic Signals. Volume 107. .7-45 Section 14 — Prestressed Cast-in-Place Box Girder Spans ............................7-2 Section 2 — Two-Way Deck Slabs on Stringers ....................................................................................................................................7-112 Section 27 — Trapezoidal Box Girders ...................7-36 Section 9 — Prestressed Concrete Deck Slabs ..7-66 Section 21 — Prestressed Simple I-Beam Spans....................7-40 Section 11 — Prestressed Simple Slab and Girder Spans ........................................................................................................................................7-27 Section 7 — Continuous Concrete Girder Spans................................................................................................................7-47 Section 15 — Prestressed Segmental Box Girder Spans ........................................................................................7-16 Section 4 — Continuous Concrete Slab Spans .............................7-22 Section 6 — Concrete Pan Form Slab and Girder Spans..............................................................................................................................7-108 Section 26 — Steel Plate Girder Spans........7-34 Section 8 — Concrete Box Girder Spans..........................................................................................................................................................................................................................................................................................................................................................7-72 Section 22 — Prestressed Cantilever/Drop-In I-Beam Spans ..........................................7-19 Section 5 — Simple Concrete Girder Spans ...................................................7-53 Section 17 — Prestressed AASHTO/PCI Box Beam Spans...........Chapter 7 — Superstructure Design Section 1 — One-Way Deck Slabs on Stringers Chapter 7 Superstructure Design Contents: Section 1 — One-Way Deck Slabs on Stringers...........................................7-101 Section 25 — Rolled Steel I-Beam Spans ..7-63 Section 19 — Prestressed Single Tee Beam Spans..................................................................................................................................................7-43 Section 12 — Prestressed Pan Form Slab and Girder Spans ......7-129 Bridge Design Manual 7-1 TxDOT 12/2001 ................7-49 Section 16 — Prestressed TxDOT Box Beam Spans .......................................................................................7-44 Section 13 — Prestressed Continuous Concrete Girder Spans....7-38 Section 10 — Prestressed Continuous Slab Spans..................................................................................7-65 Section 20 — Prestressed Double Tee Beam Spans ............7-88 Section 23 — Prestressed Continuous for Live Load I-Beam Spans .......................................7-61 Section 18 — Prestressed Slab Beam Spans.................7-95 Section 24 — Prestressed U-Beam Spans..........7-14 Section 3 — Simple Concrete Slab Spans ...................... apart. However. Later.1 issued in 1945.000-pound axle or two 16.000 psi in the 1988 American Association of State Highway and Transportation Officials (AASHTO) Interim Specification. This limit kept deck slabs from getting thinner as the concrete strength increased. Beginning with Supplement No. 1 to 1944 American Association of State Highway Officials (AASHO) Design Specifications for Texas Bridges. and precast concrete beams. the design load for concrete deck slabs was one 24. A few experimental bridges using the method were constructed.500 psi and back to 4.600 psi. steel. Bridge Design Manual 7-2 TxDOT 12/2001 . most states limited the calculated service load stress. Class S concrete was required to have six and one-half sacks of cement and a strength of 4. The Federal Highway Administration (FHWA) accepted this method for phasing into general use by 1991. 4 ft. and a required strength of 3.000 psi to 4. to 1.000-pound axles. construction of this type of deck was abandoned.000 psi until 1974.Chapter 7 — Superstructure Design Section 1 — One-Way Deck Slabs on Stringers Section 1 One-Way Deck Slabs on Stringers Background From the beginning of Texas Highway Department history.000-pound axle. which has six sacks of cement per cubic yard. there have been reinforced concrete bridge decks. in response to the 1988 AASHTO provisions. Design Stress Allowable stresses in the concrete have changed from 800 psi for live load in 1931 to 0. Design Specifications Design specifications have undergone an evolution from none in the beginning through several empirical formulas for distribution of wheel loads to the completely empirical design method specified in the Ontario Highway Bridge Design Code. entrained air. using the empirical wheel load distribution. and the details were just more comfortable with the standard method. The potential savings in reinforcing steel were not significant when concrete panels were used. This was changed after the 1957 AASHO Specification to one 32. Although the allowable design stress in the concrete rose to 1. after a few tentative trials. placed monolithically with concrete girder spans. when Class S concrete was introduced. They were used for the entire superstructure as slab spans and culverts.200 psi. Beginning in 1980 the Texas Department of Transportation (TxDOT) sponsored an extensive research project investigating bridge decks designed according to the Ontario empirical method. AASHTO-required concrete strengths have increased from 3.4 f 'c today. The required 28-day concrete strength for concrete used in bridge decks in Texas was 3.800 psi. low water/cement ratio. and constructed on top of timber.000 psi. Wheel Formula E= 1931 16 k 0. Probably not used in design by any state.6S+2. which Texas had already adopted for nominal cover.000 1 1 1935 16 k 0. Expressed in terms of span length. The 1977 Specification required 2 in.4S+3.7(S+1. should be the absolute minimum cover.000 1½ 1 2 1973 16 k 8S/(S+2) 1.75 1.000 1 1 1941 16 k 0.500 psi concrete.200 20.200 20.200 24.5 900 16.000 20. until the requirement increased to 1 1/2 in. 2 in.000 2 1 1983 16 k 8S/(S+2) 1.000 18..800 24. Construction section required 4.800 24. Reinf.Chapter 7 — Superstructure Design Section 1 — One-Way Deck Slabs on Stringers Concrete Cover Required concrete cover to the top mat of reinforcing steel remained at 1 in. Bridge Design Manual 7-3 TxDOT 12/2001 . but some authorities believe that.000 1 1 1953 12 k 0.600 24. Texas practice is outlined in the second following table. 2. in the 1961 Specification. including AASHTO. Most construction specifications.4S+3.000 1½ 1 3 1977 16 k 8S/(S+2) 1.4S+3.000 1 1 1961 16 k 8S/(S+2) 1. Chronology of AASHTO Specification Requirements for Concrete Slabs Reinforced Perpendicular to Traffic Allowable Stress Concrete Cover 1 Conc.75 1. for spans over 7 ft.000 1 1 1944 12 k 0.000 2 1 1993 to 16 k 8S/(S+2) 1.000 2 1 Current 1. A chronology of dominant AASHTO effects on the design of deck slabs reinforced perpendicular to traffic is given in the table below. AASHTO Design Distribution (psi) (psi) (in) (in) Spec. allow a 1/4 in. Probably not used by most states.200 20. for top slab reinforcement. tolerance on position of reinforcing steel. Top Bott.25) 800 16.75 1. 3.000 1 1 1959 (T) 16 k 8S/(S+2) 1. 1955 12 k 0. Based on design clear spacing of 7 ft. often directly above the main reinforcing where a plane of weakness existed. slabs only. For constructibility reasons. TxDOT currently recommends the use of 8 in.1875 6. no direct formula for distribution width was given. of contributing flange).95 6.1875 6. 4 Allowable Concrete Slab Design Stress Cover for 8 ft.1875 1.75 5 in.75 1. The deck slab is designed as a beam in flexure supported by the stringers. Prestressed Concrete Panels Beginning in 1963.000 1 . 1961 16 k 8S/(S+2) 1. higher reinforcing steel stresses were allowed to justify #5 at 6 in.50 6 in. Cracking usually initiated transversely.200 20.40 7. Reinf. slabs may sometimes be used if it can be shown that the span can be designed using fewer beams as the result.25 in. 1996 16 k 8S/(S+2) 1. supporting the weight of the cast-in-place top half of the slab.75 1. (12 in. field and laboratory tests consistently identified failure in punching shear. and 7. cracking would be expected perpendicular to the main reinforcing or longitudinal to the bridge. Furthermore. Design practice from 1919 to 1946 cannot be documented.8 factor allowed for slabs continuous across 3 or more supports. 32 32 2.40 7.40 6.000 1. If the slab behaved like a beam.200 20. one layer is placed closest to the top of the slab. Top Bott. and Bridge Design Manual 7-4 TxDOT 12/2001 . Prestressed concrete panels span between stringers.50 6. This design results in primary reinforcing oriented transversely to the length of the bridge. 1968 16 k 8S/(S+2) 1. P15(S+ 2) P20(S+2) or .95 6. Design Methods Basic design methods were last revised in the 1959 Interim Specification. The allowable stress requirement used is the same as for prestressed concrete panel decks.50 in.000 1.50 in. and the other layer is placed closest to the bottom of the slab. Beginning in 1961.000 2 1.200 20. Texas developed a deck construction method whereby about half of the slab could be precast. 1960 16 k 8S/(S+2) 1.1875 6.8125 1.25 5 in. It was simply included in the direct formula for design moment per foot.40 8 6 in. 7.000 18. 4.000 1. Reinf.25 6. E (ft) Slab Year Design Distribution (psi) (psi) (in) (in) Thick. 1. To maximize design strength. 3.4S+3. From 1996 to current.75 in. This reinforcing is placed in two layers within the slab.1875 1.1875 6. This has not been observed to occur.25 6.75 5.1875 1. Early attempts to cantilever the panels across the outside beam to support the overhang encountered too many construction problems.000 2 1.600 24.200 20. in the top mat.Chapter 7 — Superstructure Design Section 1 — One-Way Deck Slabs on Stringers Chronology of TxDOT Design Requirements for Concrete Deck Slabs on Stringers 2 3. Span 1 1 Top Conc.4S+3. Wheel Formula #5 at (in) E= 1946 12 k 0. at a level of six or more times the design wheel load. with a 0. However. not flexure. Some of the causes suggested were: ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ Decks too thin Insufficient concrete cover over reinforcing Insufficient temperature reinforcing Slabs composite with steel beams Slabs not composite with steel beams Steel beams too limber Dirty concrete aggregates Reactive concrete aggregates Insufficient cement in concrete Water-cement ratio too high Chemical admixtures Ready-mix concrete Excessive concrete placement temperature Incomplete consolidation of concrete Slow finishing methods Lack of uniform curing 7-5 TxDOT 12/2001 Bridge Design Manual . The use of prestressed concrete panels between beams on prestressed concrete beam bridges gained slowly in popularity. A majority of the contractors prefer panel decks in spite of the shortcomings because a convenient and safe working surface is provided very quickly. Details have been modified continually for compatibility with construction conditions and manufacturing practice. When the bid item for reinforced concrete slab began to be measured by the square foot with the specification allowing either removable forms.Chapter 7 — Superstructure Design Section 1 — One-Way Deck Slabs on Stringers the concept was abandoned. Speculation was rampant as to the causes of cracking. the use of panels escalated. at the contractor’s option. Maintenance Issues The maintenance problem with bridge decks is deterioration under exposure to weather and traffic (see Chapter 5. or precast prestressed concrete panels. and finally portions of deck falling to the ground under and over traffic. Texas became aware of the severity of this problem after a period of extensive construction of the thinnest decks ever allowed by AASHTO. The method is subject to grading problems associated with geometric conditions and prestressed beam camber. scaling. Texas invested a considerable amount of time and money in research2 relative to prestressed concrete panels. Section 5:Corrosion Protection). Excess cast-in-place concrete is usually required because of inability to grade the panels properly. stay-in-place metal deck forms. delamination. Design of the beams was the same as for simple spans. to #4 at 9 in.Chapter 7 — Superstructure Design Section 1 — One-Way Deck Slabs on Stringers ♦ ♦ ♦ ♦ Insufficient long-term curing No protective coating provided Concrete too young when traffic allowed De-icing salts Most of these potential causes probably had some influence on the problem.) In the middle 1970s. the Bridge Design Section began experimenting with continuity of the deck slab over interior bents. Bridge Design Manual 7-6 TxDOT 12/2001 . but their size is considered acceptable. Recently there have been reports of longitudinal cracking in some Texas panel decks. and most of them were addressed by the various corrective measures taken in the ensuing years. Initially. space between beam ends across the center of cap. Texas panel decks experienced no longitudinal cracking or delamination. This justifies emphasis of the required method of construction to prevent this cracking as shown in Figure 7. Panel decks have been used in virtually every area of the state. from Cameron to Grayson counties. It was their practice to put the bedding strips at the edge of the panel. No grout could enter to provide a firm bearing for live load. Other states had experienced deck deterioration caused by the use of precast panels. Cracks can still be found. Deck Continuity for Simple Spans Background. Prestressed beams were constructed with a 4 in. including where de-icing salts are applied to the bridge deck. Determined to alleviate the deck joint problems with prestressed beam spans. Figure 7-1. but disenchanted with full continuity for live load. Prestressed Concrete Panel on Bedding Strips (Online users can click here to view this illustration in PDF. We looked at eight structures. without finding a single longitudinal crack. the design engineer and the construction engineer from Florida came to Texas to find out why their panel decks were cracking longitudinally and ours were not. This cracking led to changing the design standard’s longitudinal top reinforcing from #4 at 12 in. They had experienced several significant failures. This change was later adopted by an AASHTO code provision. Transverse cracking over joints between panels occurred.1. In some areas a single crack would split apart and come together again leaving portions of the deck encircled by the crack. the ends of the beams often cracked severely. Design in this case was by trial and error. Construction was simplified. Irregular cracking patterns were observed. In the mid 1980s this practice was changed after surveying several bridge decks constructed with continuous placement. Current reasoning is that this crack can be no worse than a leaky expansion joint and only slightly worse than a transverse shrinkage crack within the span. and only slotted holes were specified at the interior bents where the slab was continuous. The continuous slab joint performed fairly well through all this. Design Issues. No diaphragms were required under the continuous slab at the beam ends. Expansion joint hardware was minimized. Durability concerns for these regions of the deck led the Bridge Design Section to require forced construction joints at the centerline of all interior bents for a number of years. Eventually tight round holes in the beams were eliminated entirely. Maintenance Issues. contractors proposed a plastic crack forming strip be placed in the wet concrete combined with continuous placement. Success on the first bridges led to an escalation of such designs. Dowels projecting from the substructure into the ends of the outside beams had been used for years on simple spans and were then used on some early continuous deck only projects. Optional construction joints were given at centerline of bent and at midspan for spans in excess of 100 ft. There is always a slab crack in continuous slabs over the bent. On severely skewed structures where continuous placement was used without a controlled joint. referred to as a controlled joint. slab continuity was achieved with a single layer of dowel bars between adjacent spans. From beneath the structure. Although a slotted hole was specified. the crack is noticeable to casual observation in Bridge Design Manual 7-7 TxDOT 12/2001 . The cracks are usually small and have required little maintenance to date. As a compromise. The remedy was to eliminate dowels entirely for the ends of units and to change the standard distance from centerline of bent to the dowels from 7 1/2 in. was used successfully on a number of projects and was made standard practice in the early 1990s. and expansion provisions were insured. based on experience. particularly at skewed bents. This method. and gradually the dowels gave way to continuous top and bottom reinforcing mats through the joint. The continuous slab across the joint between adjacent beams cannot be justified by rigorous design methods. Slab reinforcing was less complicated on skewed bridges because variable length skewed end reinforcing was not required at the continuous span ends.Chapter 7 — Superstructure Design Section 1 — One-Way Deck Slabs on Stringers On the earlier bridges. Maintenance problems have been confined to the bridges with insufficient provision for expansion at the ends of long units. This caused the beam to bear directly on the dowel. Construction Issues. the crack is more noticeable as it scallops across the roadway. the slab concrete could be placed continuously across the centerline of bent. to 8 1/2 in. Originally. Maximum continuous unit length criteria was established. A number of structures still bear the scars of this era. some of which connected too many spans without proper provision for movement at the ends of the continuous section. and thermal shrinkage stresses would break the end of the beam. at the contractor’s option. Beam ends cracked before dowels were removed. The assumption was that the beam was often erected with the dowel at the back of the slot. Bridge Design Manual 7-8 TxDOT 12/2001 . It tends to be irregular in path and leaks to some degree. It is preferable that the width of slab overhangs be less than. and construction appear to have solved the most critical concrete bridge deck problems. Prestressed concrete panels are the preferred method of constructing decks on prestressed concrete I-beams and U-beams and are used occasionally on steel beams and girders. Two. Lap splices shall be designed according to AASHTO Specification requirements with the following provision: Required lap splice lengths shall be increased by a factor of 1. The use of 7 1/2 in. Slab overhangs are designed according to the AASHTO Specification with the further provision that a grid and plate analysis3 or finite element analysis may be used to evaluate design moments. Designs are standardized according to the AASHTO empirical moment method. with prestressed concrete panels (PCPs) as the contractor’s option. thick slab. making their construction more difficult in some situations. thick slabs may be justified only if the span can be designed with fewer beams as the result. A slab construction joint or controlled joints over the bent and parallel thereto as used in current practice made this crack straight and less noticeable. but efforts are still underway to improve the durability of bridge decks. However. Most bridges are designed with a cast-in-place (CIP) slab. Truss bars are no longer used for reinforcing CIP slabs. Current Status Concrete bridge decks are used on most of Texas’ bridges. and sometimes four spans are strung together to form units. Deck continuity over interior bents is often used for simple span structures to reduce the number of joints. three. an 8 in. thick (minimum) slab overhang shall be used regardless of the slab thickness between beams. one half of the beam spacing. at the future beam span adjacent to the stage construction joint. Design Recommendations All current Texas bridge standards use an 8 in. For constructibility. PCP decks are not allowed under the following circumstances: ♦ ♦ ♦ Curved steel girder bridges Bridge widenings.Chapter 7 — Superstructure Design Section 1 — One-Way Deck Slabs on Stringers the overhanging portion of the slab. at the future beam span adjacent to the existing structure. Extensive formwork is required for CIP slabs.5 when using epoxycoated reinforcing for all reinforcing steel in the slab. thick slab should be used for the majority of the decks in Texas. or equal to. Stage construction. materials. an 8 in. or 7 3/4 in. To properly support the bridge railing. All of the advancements in design. may warrant the use of a more detailed analysis and design. A #5 (Grade 60) bar placed between each bar T (temperature steel) in the top layer is usually sufficient. Cast-In-Place (CIP) The majority of the bridge decks in Texas shall use the TxDOT standard CIP slab design. The design meets the AASHTO Specification requirements with the following provisions: ♦ ♦ ♦ ♦ Service load design is used for conservatism. The modular ratio (n) is taken as 8. Live loads are greater than HS20. No change in the design of slabs is made when permanent metal deck forms (PMDF) are used. Thickened slab ends are often used with steel stringers as well (see TxDOT Bridge Detailing Manual). Widths of slab overhangs used are greater than that specified by the TxDOT standard cast-in-place slab design. The additional steel should run the length of the negative moment region and be developed into the positive moment regions.Chapter 7 — Superstructure Design Section 1 — One-Way Deck Slabs on Stringers A minimum 1 percent reinforcement ratio (longitudinally) shall be used in the slab at the negative moment region of continuous steel units. All bridge deck designs must meet the AASHTO requirements with the above provisions. as standard practice with the conventional type end diaphragm as the option. Unusual conditions such as these. A thickened slab end shall be used for prestressed beam spans. Bridge Design Manual 7-9 TxDOT 12/2001 .600 psi.000 psi. and deck continuity for simple spans are provided below. The TxDOT standard CIP slab design may be used without further analysis except under highly unusual conditions. ♦ ♦ IBTS standard sheet: Thickened Slab End Details for Prestressed Concrete I-Beams UBMS standard sheet: Miscellaneous Slab Details for Prestressed Concrete U-Beams Additional recommendations for cast-in-place decks. respectively. Such conditions may include the following: ♦ ♦ ♦ Clear span beam spacings used are greater than that specified by the TxDOT standard cast-in-place slab design. which is shown in Figure 7-2. prestressed concrete panel decks. Calculated concrete stress (fc) shall not exceed 1. and/or others. Thickened slab end standards details are as follows for prestressed I-beams and prestressed U-beams. Calculated stress (fs) in the transverse reinforcing steel shall not exceed 24. 7. TxDOT has developed a standard cast-in-place slab design. 1.) Explanatory Notes for Figure 7-2 To promote consistency. Use #5 bars at 9 in.5 ft.896 ft.5 in.75 ft. 8. Use #4 bars at 9 in. TxDOT Standard Cast-in-Place Slab Design (See following explanatory notes. This design may be used without further analysis for the majority of bridge slab designs in Texas if the design provisions below are followed: ♦ ♦ ♦ ♦ Concrete strength f 'c = 4. Criteria in table below shall not be exceeded: Slab Thickness TxDOT Standard Cast-in-Place Criteria * Max. top (bars A) and bottom (bars B) for all slab thicknesses.75 in. Clear Span Beam Flange to Face Beam Flange to Face of Beam Flange Rail at Midspan Rail at End of Unit to Beam Flange 7. 8. Spacing may be increased to 9. * Values shown are for concrete beams. Overhand * Max. NA NA 7. Adjust to 0.000 psi Use Grade 60 steel for all reinforcing.25 point of flange for steel beams.295 ft. (maximum) for temperature steel in top layer (bars T). Bridge Design Manual 7-10 TxDOT 12/2001 . Design guidelines are as follows: ♦ ♦ ♦ Use #5 straight bars at 6 in.Chapter 7 — Superstructure Design Section 1 — One-Way Deck Slabs on Stringers Figure 7-2. Online users can click here to view this illustration in PDF. for flared beam situations.00 ft. (maximum) for distribution steel in bottom layer (bars D). Overhand * Max. NA NA 8 in. 4. Normal maximum beam spacing should be limited to 9 ft.686 ft. Chapter 7 — Superstructure Design Section 1 — One-Way Deck Slabs on Stringers Prestressed Concrete Panels (PCP) The TxDOT standard detail sheets for prestressed concrete panels on prestressed concrete beams and structural steel beams are available for insertion into the bridge plans only when prestressed concrete panels are allowed on the project. PCP (U) standard sheet.375 in. This is justified by successful load testing.000 psi.1 kip. but they are intended to follow AASHTO with the following provisions: ♦ ♦ Service load design is used but ultimate strength is checked at mid-span. Reinforcing steel #4 (Grade 60) at 6 in. spacing may be substituted for strands in panels 5 ft. The composite PCP/CIP slab prestressed cross section resists live load positive moments.7 )(270) − 45 = 144 ksi Design is based on the use of 3/8 in. so the design is based on the amount that can be developed. Ultimate flexural capacity at mid-span is based on the following: Panel Length 2fse fsu = + 2D 3 Using 0. for use on structural steel beams or girders. The TxDOT standard detail sheets for prestressed concrete panels are as follows: ♦ ♦ ♦ PCP (C) standard sheet. The problem here is that panels are generally not wide enough to develop larger strands. stressed to 16. PCP (S) standard sheet. ft = 424 psi). width or less. for use on prestressed concrete I-beam spans. for use on prestressed concrete U-beam spans. f 'c ♦ ♦ ♦ ♦ ♦ ♦ Bridge Design Manual 7-11 TxDOT 12/2001 .5 ft. Calculated stress (fs) in the transverse reinforcing steel does not exceed 24. rather than the full development. Transverse reinforcing in the CIP portion is designed for the negative live load moment.. 270 kip strand.000 psi. The designs represented on these details are highly standardized. Calculated tensile stress in the bottom of panel does not exceed 6 (where f 'c = 5.-0 in. The panel alone supports dead load. strands and fse = (0. Distribution reinforcing is not required. width or less and shall be required in panels 3. Very short panels must have mild steel reinforcing instead of prestressing strand to prevent splitting. are detailed on the IBMS standard. Therefore. Deck Continuity for Simple Spans. Unusual conditions such as these. See TxDOT Standard IBMS. The capacity of the standard bearings as detailed on the IBB sheet must be checked according to the latest design procedures. long end spans are beneficial to the performance of the unit. Controlled joints. Slabs should be detailed with construction joints or optional controlled joints at the bents. a chamfer line should be required in the bottom of the overhanging slab. which would be factitious for this type of construction. and extend along the bentline to a line at a 2 ft. Standardized systems such as open armor joints. or the end of the inverted tee cap. a saw cut will intercept the random crack. unit lengths. ♦ ♦ ♦ ♦ Bridge Design Manual 7-12 TxDOT 12/2001 . sealed expansion joints have adequate capacity for short or moderate length structures with two expansion joints. sealed armor joints. or 4 in. therefore. At each interior bent. may warrant the use of a more detailed analysis and design. in length.) The slip capacity of the bearing is significantly controlled by the dead load reaction of the beam in the end span. increasing the possibility of spalling in this area. Rather than giving design controls. The random cracking may not be apparent at first. (See Bearings in Chapter 9. A deck expansion joint should be provided at abutments or no more than one bent removed from the abutments. Live loads are greater than HS20. recommendation will be made regarding details that appear to function acceptably: ♦ The total length of continuous deck slab is controlled by the capacity of the joint and by the ability of the end bearing to move without slip. Under no circumstances should the contractor be allowed to saw the joint after the concrete has set as a substitute for the other methods. Section 11. Widths of slab overhangs used are greater than that specified by the TxDOT standard cast-in-place slab design. Such conditions may include the following: ♦ ♦ ♦ Clear span beam spacings used are greater than that specified by the TxDOT standard cast-in-place slab design. long bridges have multiple units that expand into each other. this is not common practice and can rarely be justified with standard bearings. The chamfer line should begin at the outside face of the top flange.Chapter 7 — Superstructure Design Section 1 — One-Way Deck Slabs on Stringers All designs must meet these requirements and should conform to the TxDOT standard detail sheets. but it will show up later. This will usually contain the crack and render it less conspicuous from below. However. The TxDOT standard detail sheets are to be used without further analysis except under highly unusual conditions. which use a plastic crack former. Although some units have been designed to upward of 400 ft. and the capacity of the joint becomes a significant factor in selecting joint location and. and/or others. parallel offset then extend normally to the edge and vertically up the outside face of the slab. ♦ ♦ All other details and the design of the deck slab and beams should be the same as for simple spans. Top longitudinal bars should be spaced the same in adjacent spans and simply continue across the joint. Dowels projecting from the substructure into slots in the beam ends should be omitted at ends of units in most cases. There have been a few bridges built on extreme grades where dowels were used at the ends of short units.Chapter 7 — Superstructure Design Section 1 — One-Way Deck Slabs on Stringers ♦ At bents where adjacent spans have different beam spacing. Bridge Design Manual 7-13 TxDOT 12/2001 . Thickened slab ends must be used at expansion joints (see Figure 7-20). into the other span. bottom longitudinal bars. should be extended a minimum of 2 ft. from the span with the smallest beam spacing. No slab thickening is required at the interior bents where the slab is continuously reinforced. Calculate longitudinal moments according to influence lines for continuity and the AASHTO distribution formula for slabs reinforced parallel to traffic.5 will result in very little savings in slab depth over a one-way slab. After placement. consequently. the slab was observed to lift off the top of the girder flange by as much as 0. the AASHTO provisions of “Slabs Supported on Four Sides” were applied. maximum economy could be achieved by using fewer girders and no stringers.Chapter 7 — Superstructure Design Section 2 — Two-Way Deck Slabs on Stringers Section 2 Two-Way Deck Slabs on Stringers Background This slab design was not used early and has not been used often for Texas’ bridges. If one span is three times the other. Current Status The two-way slab is a viable design for spans in which girder spacing in excess of 15 ft. Design Recommendations Two-way slabs must bear longitudinally on the girder and transversely on the floor beams. is economical. Experience has been favorable except for a few construction problems. This was also attributed to shrinkage. This resulted in several such bridges being constructed with a channel span in excess of 300 ft. or influence surface charts5 could be used for analysis. There have been less than 20 bridges constructed with this type of deck in Texas. It was found that minimum steel weight and. the wheel load is shared equally by the two spans. One problem was excess slab cracking where the positive moment shear connectors stopped. This occurrence was attributed to shrinkage in the long negative moment placement. A ratio above 1. If girders and floor beams are spaced equally. and it became impossible for a while to get a permit for a bridge without navigational clearances. Worth. Plans were underway to make the river navigable as far up as Ft. virtually all of the load is carried by the short span.02 ft. To minimize slab thickness. Multiply by the appropriate span factor. and the slab later crept down to bear on the flange. 7-14 TxDOT 12/2001 Bridge Design Manual . Calculate transverse live load moments by the AASHTO empirical method. The first known use was in 1967 on a steel plate girder unit across the Trinity River. Multiply by the appropriate span factor. but it will usually suffice to design as follows: ♦ ♦ ♦ Calculate the percentage of load carried by each span according to the AASHTO formulas. Another problem occurred on a unit that had shear connectors in the center span and none in the end spans. A grid and plate computer program4 finite element analysis. 000 psi. a minimum 1 percent of longitudinal slab steel should be provided.5 or less. Calculate overhang moments according to AASHTO.600 psi and fs = 24.Chapter 7 — Superstructure Design Section 2 — Two-Way Deck Slabs on Stringers ♦ ♦ ♦ ♦ Specification requirements for distribution steel may be ignored if the span ratio is 1. Bridge Design Manual 7-15 TxDOT 12/2001 . In negative moment regions of continuous steel units. Select slab depth and reinforcing based on fc = 1. Substructure costs are excessive due to the limited distance between supports. Many bridges were constructed on the farm-to-market system according to these standards and have performed well under heavier than design loads. no railing was used. however.25 ft. When the curbs are removed. Nominal 25 ft. so a full-size slab bridge was constructed in Henderson County and load tested to verify the Illinois method. curbs were 18 in. Widening of H10 designs has been done but is impractical. slab spans were being constructed according to the third generation of Bridge Division standard details. slab spans were 25 ft. Concession was made to constructibility by adjusting span lengths for different skews so that the same length of bar joist could be used to support the forms. History In 1944.5 ft. Deterioration has not been a significant problem because slab spans are thicker and are located predominantly in rural areas which are not salted. These standards are no longer used. the thin slab will no longer satisfy the original design loads when analyzed by ordinary procedures. Hydraulically. AASHTO Specifications for slab spans have changed over the years. By strengthening the broken slab edge and analyzing with a grid and plate computer program. Testing had been on models only. it has been possible to widen the H15 designs. 25. Creation of a “farm-to-market” highway system was imminent. resulted in thinner slabs being required. The design span length and main reinforcing steel was parallel to traffic up to 15 degree skew but perpendicular to the bents for 30 and 45 degree skews. but not so drastically as for slabs on stringers. slab spans can become a nice solution for low headroom stream crossings where occasional flood inundation is expected. a new series of slab spans for all highways was developed. Economically. The Bridge Division became interested in experimental work being conducted on slab spans at the University of Illinois. Unfortunately. for 45 degree skew. and the Highway Department wanted to build these roads as economically as possible. center to center of bent for 0 and 15 degree skew. The latter method saves concrete but causes problems when widening is required.Chapter 7 — Superstructure Design Section 3 — Simple Concrete Slab Spans Section 3 Simple Concrete Slab Spans Background This is one of the earliest superstructure types designed to cross Texas creeks and backwaters. slab spans are better than box culverts. were developed around the concept. consequently. A design procedure was reported in their Bulletin 346 that recognized the contribution of the curbs in carrying load and. The tests revealed no significant problem and a new series of standard details. many of them have required widening because of traffic. high. Around 1948. and the slabs were unusually thin. called FS Slabs. With open railing. for 30 degree skew. Design loads were H10 and H15. Current policy requires widened structures to support at least H20 design load at service load stresses. they have never been desirable. Bridge Design Manual 7-16 TxDOT 12/2001 . and 26. Service load analysis allowing fc = 1. Use design span lengths perpendicular to bents for skews over 15 degrees. Widening of FS slabs is discouraged.06 x Effective Span for both cases. widths. This is especially evident at fixed ends. place the main reinforcing and longitudinal temperature steel parallel to traffic. between quarter points of the bent cap width. place the main reinforcing and longitudinal temperature reinforcing perpendicular to the bents and calculate effective span length accordingly.600 psi in concrete and fs = 24. along the centerline of bent. and is prohibited for H10 designs. For skews through 30 degrees. Calculate wheel load distribution as 4 + .000 psi in steel is recommended. load factor design is also acceptable. exceeds about 40 ft. However. Skewed spans tend to work themselves out of line laterally because of closed expansion joints over a smooth bearing surface. which are restrained from movement by dowels into the cap. Use design span length parallel to roadway for skews up to 15 degrees. 7-17 TxDOT 12/2001 ♦ ♦ ♦ ♦ ♦ ♦ Bridge Design Manual . A few structures have been constructed with simple slabs precast in about 8 ft. shrinkage cracking perpendicular to the bent can be alarming. This tendency can be prevented with a substantial shear key in the bearing surface.Chapter 7 — Superstructure Design Section 3 — Simple Concrete Slab Spans Design Issues There have been two significant performance problems observed with concrete slab spans. Current Status Most new simple concrete slab bridges currently being constructed are project specific designs conforming to Bridge Design Section or district design practice. Provide distribution reinforcing according to the AASHTO Specification. Place transverse reinforcing parallel to the skew. but this is not encouraged. Design Recommendations For new simple concrete slab span designs. Use live load moments from the AASHTO Specification6 or calculated from military loading. Precast reinforced concrete slabs are discouraged. For skews over 30 degrees. Where the width of a slab span. It can be minimized by placing all of the dowels in the center portion of the cap and lubricating the outside portions. but divide the required area of main reinforcing by the square of the cosine of the skew angle.. Place transverse reinforcement parallel to the skew. Effective span should be no more than 20 times total slab thickness to control long-term dead load deflection. the following practice is recommended: ♦ ♦ Class S concrete is required. Chapter 7 — Superstructure Design Section 3 — Simple Concrete Slab Spans ♦ The edge beam live load moment of 0.) Bridge Design Manual 7-18 TxDOT 12/2001 . This has been the practice and has shown to be acceptable through experience. Figure 7-3. the reinforcing required by the primary slab design will usually be sufficient for the edge beam. For skews through 30 degrees.1 PS may be applied to a slab width of 4 ft. Reinforced Concrete Slab Spans – Typical Details (Online users can click here to view this illustration in PDF. ♦ Typical bearing and joint details are shown in Figure 7-3. across the face of the cap at the ends of units. Hooked bars may be required. For skews over 30 degrees (main reinforcing perpendicular to the bents). separate reinforcing parallel to the edge must be provided. Check embedment length of bottom reinforcement steel. Chapter 7 — Superstructure Design Section 4 — Continuous Concrete Slab Spans Section 4 Continuous Concrete Slab Spans Background The use of continuous concrete slab units began around 1936. Spans were from 20 to 30 ft. Some of the early designs required bent caps to be placed monolithically with the slab, a practice that was later discouraged because of construction difficulties. Later designs extended the cap stirrups around large transverse bars in the slab, effectively deepening the cap for live load. Interior spans up to 40 ft. were occasionally constructed with constant depth slabs. The use of variable depth slabs began in the 1950s. Interior spans up to 60 ft. were feasible. Many were constructed on interstate and primary main lanes. One district designed a few interstate crossover structures as slabs with a parabolic soffit. These were attractive bridges with open concrete railing. The longer spans became unpopular because of long-term deflections. If the riding surface was constructed according to calculated short-term deflections, it had to be overlaid eventually to restore the riding quality. If cambered to account for long-term deflections, the bridge would give a rough ride during its early life. Neither situation was considered desirable. Another objection to continuous slabs was the amount of cracking that occurred in the negative moment area, especially when curbs and concrete railing were provided. Curbs were usually not considered in the design but they try to help anyway. Of course, reinforced concrete is supposed to crack in a tensile zone, but nobody seems to like it when it actually happens. Creep in the concrete, which caused the deflection problem, also aggravated the cracking. Current Status Variable depth reinforced concrete slabs are no longer recommended. Constant depth reinforced concrete slabs are a logical solution for many stream crossing problems, but they are relatively expensive. Because of fewer deck joints, continuous units are often preferred over simple spans. Continuous unit lengths to 150 ft. are used with the ends connected securely to the abutments. Unit lengths of 200 ft. may be used with both ends free to expand. Design Recommendations For new continuous concrete slab span designs, the following practice is recommended: ♦ Do not design continuous reinforced concrete slabs for skews over 30 degrees. Bridge Design Manual 7-19 TxDOT 12/2001 Chapter 7 — Superstructure Design Section 4 — Continuous Concrete Slab Spans ♦ For constructibility, it is desirable that all span lengths (face to face of bent) be equal for bridges of two spans and bridges of four spans or greater. Bridges of three spans shall be proportioned in 1.0-1.2-1.0 span lengths to balance the unit. Place the longitudinal reinforcing parallel to traffic. Use design span lengths parallel to traffic. Place transverse reinforcing parallel to skew. Effective supports may be taken at the centerline of interior bents and at the quarter point of end bent caps. Calculate wheel load distribution as 4 + .06 x Effective Span (7.0 ft. maximum). Use of interior span length is usually close enough for end spans also. Composite moments and reactions may be calculated from constant moment of inertia influence lines, or computer programs, assuming knife edge supports. Truck loading will control both positive moment and negative moment for span lengths below 30 ft. Military loading must be investigated as well. Design negative moment may be taken at the face of bent cap. Service load analysis allowing fc = 1,600 psi in concrete and fs = 24,000 psi in steel is recommended. However, Load Factor design is also acceptable. Provide distribution reinforcing according to the AASHTO Specification. Effective span should be no more than 23 times total slab thickness to control long-term dead load deflection. This has been the practice and has shown to be acceptable through experience. Check embedment length of bottom reinforcement steel, across the face of the cap at the ends of units. Hooked bars may be required. ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ Typical bearing and joint conditions are shown on Figure 7.4. Bridge Design Manual 7-20 TxDOT 12/2001 Chapter 7 — Superstructure Design Section 4 — Continuous Concrete Slab Spans Figure 7-4. Continuous Reinforced Concrete Slab Units – Typical Details (Online users can click here to view this illustration in PDF.) Bridge Design Manual 7-21 TxDOT 12/2001 Chapter 7 — Superstructure Design Section 5 — Simple Concrete Girder Spans Section 5 Simple Concrete Girder Spans Background Concrete girder spans have been constructed for Texas highway bridges since the creation of the Department. The earliest standard drawings were adapted from Bureau of Public Roads designs in 1918. Several series of standard drawings followed until the last issue for the end spans of interstate crossovers in 1956. For many years, cast-in-place concrete girder spans were more popular than steel I-beam spans. Many bridges were thus constructed in the 1920s and 1930s. Clear span lengths ranged from 16 ft. to 40 ft. with the majority being 26 ft. Most of these have now been widened because their roadway widths were too restrictive for the safety of modern traffic. In the 1940s the use of concrete girders faded in favor of steel I-beam spans. Precast reinforced concrete girders were used on a few projects to widen existing cast-in-place concrete girder spans. Design parameters are tabulated in Figure 7-3. Typical standard details are shown in Figure 7-5, Figure 7-6, and Figure 7-7. Construction Issues Construction problems were primarily due to the complication of forms and falsework. In the earlier years, forms were supported by falsework resting on the ground throughout the span. Later, contractors began to use steel joists, which could span from bent to bent for the average span. Concrete had six sacks of cement per cubic yard and no air entrainment agents, retarders, or super water reducers. Concrete was mixed on the job in batches considerably smaller than one cubic yard. Many of these bridges exhibit excellent concrete work and have been very durable. Maintenance Issues Very little deck maintenance has been required on the old concrete girders. When deck deterioration became serious on Texas bridges, one district engineer, who was conducting his own investigation to determine the cause, enjoyed demonstrating the durability of old concrete girders. There were several locations in the district where 1930 vintage concrete girders had been paralleled by pan form girders cast in the 1950s using five-sack concrete with air entrainment. Deterioration had begun on the newer bridge, while the old bridge was still sound. He was arguing for a return to six-sack concrete without chemical admixtures. Current specifications require six sacks of cement per cubic yard, but air entrainment is still used. The roadway surface of concrete girder spans tended to be rough because of excess deflections due to creep of the concrete. This problem could usually be remedied by an asphalt overlay. Bridge Design Manual 7-22 TxDOT 12/2001 Chapter 7 — Superstructure Design Section 5 — Simple Concrete Girder Spans Malfunctioning bearings caused numerous maintenance problems on concrete girders. Frozen bearings often caused girder ends to break off. The solution was usually to transfer the reaction to a pedestal constructed under the end diaphragms. Current Status Simple concrete girder spans are no longer used. Design Recommendations If there should be an appropriate occasion to use simple concrete girders, the following recommendations are offered: ♦ ♦ ♦ ♦ ♦ Limit spans to 45 ft. with a maximum span-to-depth ratio of 14. Distribute live load laterally as specified for concrete deck on concrete T-beams. Use service load design, Grade 60 reinforcing, and 4,000 psi concrete. Shear stress carried by concrete alone should be taken as 60 psi. Long-term deflections should be calculated for the gross section assuming a modulus of elasticity of 1,500,000 psi. This value is based on the creep of the concrete. Experience and observation has shown this value to be adequate. Table 7.3: Chronology of Concrete Girder Standards Allowable Stress (psi) Series Year Designed Live Load Steel Concrete 1 G1. . . 1918 20T Roller 16,000 650 1 G263. . . 1920 Typ. 15T Truck 16,000 650 DG-5. . . 1928 2-15T Trucks 16,000 650 G-24-28.5. . . 1933 2-15T Trucks 16,000 650 GL-22. . . 1937 H10 16,000 650 G-2-24. . . 1938 2-15T Trucks 16,000 900 G-28H. . . 1951 H20 20,000 1,000 G-28HS. . . 1951 H20S16 20,000 1,000 2 G-26(4)-35. . . 1956 H20 20,000 1,200 1. Adapted from Bureau of Public Roads and Rural Engineering Designs. 2. For Interstate Highway Crossovers. Bridge Design Manual 7-23 TxDOT 12/2001 Chapter 7 — Superstructure Design Section 5 — Simple Concrete Girder Spans Figure 7-5. Evolution of Concrete Girder Standards – 1918, 1920, and 1928 (Online users can click here to view this illustration in PDF.) Bridge Design Manual 7-24 TxDOT 12/2001 Chapter 7 — Superstructure Design Section 5 — Simple Concrete Girder Spans Figure 7-6. Figure Evolution of Concrete Girder Standards – 1933, 1937, and 1938 (Online users can click here to view this illustration in PDF.) Bridge Design Manual 7-25 TxDOT 12/2001 Chapter 7 — Superstructure Design Section 5 — Simple Concrete Girder Spans Figure 7-7. Evolution of Concrete Girder Standards – 1951, 1956, and 1956 (Online users can click here to view this illustration in PDF.) Bridge Design Manual 7-26 TxDOT 12/2001 In the last few years. The original span length was 30 ft. Each skew increased the span length by 10 inches: 9 in. Standard details have usually been available for pan form girder spans and substructure to fit the geometric requirements of most stream crossings. and Figure 7-10. a metal pan was provided to form the concrete and support the weight in flexure without intermediate support. it is impossible to change the depth or spacing of girders without retiring considerable contractor investment in forms. Pan form spans were more of a development than a design. With the faying joint surfaces 36 in. five roadway widths. prestressed concrete beams have overtaken them.688 pan form girder bridges had been constructed on the Texas highway systems by 1999. since the distance face to face of caps had to remain the same to allow form removal. The terminology depicts the modular steel forms required for the cast-in-place reinforced concrete spans. In 1956 a design was introduced for 40 ft. for the bolt spacing and an extra inch to facilitate form removal. as well as those previous and others that followed. and someone discovered the possibility of accommodating skew by offsetting adjacent form barrels. This allows the district to prepare a complete set of plans for a project with only a foundation review by the Bridge Design Section. Forms and falsework are combined in a sturdy reusable package. centers. Pan form girders have been the most economical method for constructing a highway bridge for most of their existence. Figure 7-9. approximately 3. span. the basic span length became 30 ft. Many bridges have been constructed to those standards. When assembled.Chapter 7 — Superstructure Design Section 6 — Concrete Pan Form Slab and Girder Spans Section 6 Concrete Pan Form Slab and Girder Spans Background Pan form girders were developed in the late 1940s in anticipation of a need for low-cost bridges on a farm-to-market road system soon to be funded. The evolution of pan form girder standard details is depicted in Figure 7-8. Bridge Design Manual 7-27 TxDOT 12/2001 . It was soon discovered that trestle piling would seldom fit inside a 20 in. wide caps and no skew. wide cap. and. This accounts for the unusual skew angles covered by the standard details. In the early 1960s. Other span lengths are not considered appropriate. Cap width was changed to 24 in. The interesting part was the development and design of the forms themselves. Longer spans would have excessive long-term deflection and shorter spans can be handled more effectively with concrete slab spans.-4 in.25. apart at the center of girders and bolts on 9 in. and the five skews. an offset of one bolt spacing produces a skew angle whose tangent is 0. and supported from the bent caps. standard drawings were distributed for superstructure and substructure for both span ranges. for 20 in. Once standardized. however. bolted together. In spite of their maintenance problems. ) Bridge Design Manual 7-28 TxDOT 12/2001 . Evolution of Concrete Pan Form Slab and Girder Standards – 1948. and 1952 (Online users can click here to view this illustration in PDF.Chapter 7 — Superstructure Design Section 6 — Concrete Pan Form Slab and Girder Spans Figure 7-8. 1950. and 1964 (Online users can click here to view this illustration in PDF. 1956.) Bridge Design Manual 7-29 TxDOT 12/2001 . 1959.Chapter 7 — Superstructure Design Section 6 — Concrete Pan Form Slab and Girder Spans Figure 7-9. Evolution of Concrete Pan Form Slab and Girder Standards. 1991. Designs in the 1960s appear to have deducted the flexure and shear strengths of the diaphragms from the calculated moments and shears in the cap. roadway. Bridge Design Manual 7-30 TxDOT 12/2001 . Design and detailing became onerous when numerous combinations of span.) The design was created by ingenuity and input from contractors. Evolution of Concrete Pan Form Slab and Girder Standards – 1966.Chapter 7 — Superstructure Design Section 6 — Concrete Pan Form Slab and Girder Spans Figure 7-10. and skew with two different types of foundation had to be produced for standard series. The AASHTO Specification did not exactly anticipate this type of structure. and 2001 (Online users can click here to view this illustration in PDF. it became rough after three years. Maintenance Problems Unfortunately. Long-term deflection of the girders was also a factor. If the surface was cambered to account for this. in a few years.75 in. Later. These measures Bridge Design Manual 7-31 TxDOT 12/2001 .-9. Also the large end diaphragms doubled the strength of the caps against live load.75 in. in the 2001 standard design detail sheets. the bent caps were allowed to extend past the outside girder face. Construction Problems Construction problems were worked out through experience. This trapped the outside form so that it had to be cut out around the bent cap to allow removal.. which was difficult to construct (Figure 7-10).Chapter 7 — Superstructure Design Section 6 — Concrete Pan Form Slab and Girder Spans The concrete thickness at the crown between girders was 3. Careful attention had to be given to the form support member and its connection to the cap to prevent rotation under load. the FHWA insisted that the section be thickened to 4. especially when the 1960 standards required some deck slab overhang past the outside girder face. large slivers are broken from the cap sides. Over the years widths of slab overhangs have varied from nothing in the beginning (Figure 7-8) to a maximum of 1 ft. the joint growth has shoved the spans outward across the cap. thick in the beginning.-1. When skews were introduced.5 in.25 in. The cracking still occurs in the thicker section but everyone is accustomed to it now.5 in. and the slab overhang widened to a maximum of 1 ft. the movement can eventually damage the cap. In the early 1960s small longitudinal cracks were noticed through this thin section for most of the length of some of the outside barrels. The major problem occurs when dirt enters the joint over each bent and cannot get out because of the cap underneath. which caused extra depth of the girders and a sag in the bottom of the stem. Even without dowels. the bridge gave a rough ride when it was opened to traffic. No adjustments were made to the design except to convert the flexural reinforcing in the girders to Grade 60 at the ratio of 20/24 times Grade 40 area. Deflection of the forms due to the weight of concrete was significant. reinforcing fixed deck joints across the top. The greater the skew. the slab overhang depth was increased from 6 in.. This occurrence allows the joint to open but not to close so that. Attempts have been made to alleviate this problem by eliminating fixity dowels on both sides of the cap. Outside girder forms were hard to hold in line. due to concerns about railing anchorage. This caused a stir among contractors who owned forms that had to be modified. Model tests and field studies7 in 1969 indicated the distribution of wheel loads to each girder to be about 25 percent less severe than assumed in the design. If the spans are fixed to the cap with reinforcing bar dowels. The forms could be cambered to counteract this. In the early 1990s the slab overhangs were reduced to a maximum of 7. In spite of Texas insistence that the cracks were harmless. Shear was not a significant problem. Neither was desirable. the further the center of form support was from the face of the cap. to 8 in. If not. maintenance problems have been quite extensive for pan form bridges. Trestle pile bents were limited in height because of the eccentric load caused by concrete placement on the side of the bent. The latest details are shown in Figure 7-11. and providing an opening beneath expansion joints for dirt to fall through. but the camber usually came out after several uses. Standard details Bridge Design Manual 7-32 TxDOT 12/2001 . There have been a few bridges with spans shoved almost off the cap and spans imbedded into the approach embankment because of joint growth. Pan Form Slab and Girder Joint Details (Online users can click here to view this illustration in PDF. The FHWA discourages pan form construction on interstate highways.Chapter 7 — Superstructure Design Section 6 — Concrete Pan Form Slab and Girder Spans have been successful to some degree. but there were already many pan form bridges with advanced symptoms before the corrective measures went into effect. Figure 7-11. Repair of the advanced cases is tedious.) Current Status Pan form girder spans will continue to be popular for constructing stream crossing bridges. and limited in width to 1 ft. to maintain the strength of the railing connection. In 2001. and reinforcing steel should be Grade 60. It appears that pan form bridges will continue to be a significant part of TxDOT construction. currently 4. Conventional bent design procedures are recommended. slab overhangs were increased in thickness to 8 in. uniform load CAP 18 analysis No allowance for diaphragm strength Load factor design fy = 60 ksi νu = 126 psi Service load reinforcing steel stress controlled by crack width (Z = 170) Dead load reinforcing steel stress < 22 ksi ♦ Bridge Design Manual 7-33 TxDOT 12/2001 .1 3/4 in. Design Recommendations The new standards are designed. Service load design should be used.8k concentrated loads and 10 ft.. Salient features of the design are as follows: ♦ ♦ ♦ ♦ ♦ Concrete should be Class S. basically. 8 in.Chapter 7 — Superstructure Design Section 6 — Concrete Pan Form Slab and Girder Spans were revised in 1991 to drastically reduce the amount of slab overhang past the outside girder face. Distribution of wheel loads should be taken as S/6. Standard design detail sheets that cover five roadway widths and contain all of the design improvements previously discussed are available on the TxDOT web site. The old methods and approximations have been abandoned. Research has indicated that the required bridge railing strength cannot be developed in the 6 in. • • • • • • • • Live load to deck as two 20. thick deck slab overhangs should be limited in width to 1 ft. according to the current AASHTO Specification.-1 3/4 in.0 according to the original design. thick slab overhang that previously was constructed.000 psi 28-day strength. Any nonstandard span should use the same girder design. This will provide adequate railing support and alleviate the construction problems associated with a wide overhang. Stirrup design should comply with the latest AASHTO Specification using 60 psi shear stress carried by the concrete. A variable-depth cast-in-place continuous reinforced concrete girder bridge. The Waco District constructed the majority of these bridges. This type of structure.000 psi concrete and Grade 60 reinforcing. Some of these were constructed on major highways in Austin. Long-term deflections are significant. Asphaltic concrete overlay can be applied if deflections become excessive. This economy was not as evident in other areas.000 psi.Chapter 7 — Superstructure Design Section 7 — Continuous Concrete Girder Spans Section 7 Continuous Concrete Girder Spans Background Except for a few early special structures. Maintenance Issues Maintenance problems have been few. using cast-in-place concrete superstructures for many bridges on the interstate and primary highway systems in the district. with parabolic girder soffits. cast-in-place bridges were fairly economical. Actual concrete strengths exceeded 5. The only serious problems with concrete girders have occurred where de-icing salts are used extensively. The structures are resistant to impact from overheight loads because of their mass. This type of structure was not used after 1968 because of the complication of falsework and forms. where concrete girder units were used primarily to satisfy aesthetic opinions. which increased costs and usually increased construction time. on 30 degree skew. built in the 1950s. Texas’ use of continuous reinforced concrete girders began in the 1950s and ended in the 1960s. The Bridge Design Section prepared two sets of standard designs for interstate crossovers in 1956. These were used for only the few concrete girder bridges in the Abilene and Bryan Districts.8 . have deteriorated severely. Possibly because of construction volume. and over interstate highways in the Abilene and Bryan Districts. The bridge had three girders with spans of 50-88-88-50 ft. Austin and Amarillo variable-depth concrete girder units were designed and detailed by one meticulous engineer who worked in both districts and the Bridge Design Section at various times. The research pronounced the performance of this structure completely satisfactory. but if initial camber is acceptable this effect can be controlled. It was load factor designed to H-15 loading using 3. These structures. was considered highly aesthetic by a few engineers. There appears to be no way to neutralize this problem other than complete removal and replacement of the bridge. Wichita Falls’ bridges were special designs by the Bridge Design Section. All such bridges in the Waco District were designed in the district bridge office. Bridge Design Manual 7-34 TxDOT 12/2001 . and Wichita Falls. Forming over traffic is especially hazardous. constructed in Hill County in the early 1960s was used for significant research on high-strength reinforcing steel. Amarillo. Long-term deflections should be calculated for the gross section assuming a modulus of elasticity of 1. currently 4. Experience and observation has shown this value to be adequate.000 psi. Service load stress should be based on crack control criteria (z = 130 for negative moment. the design should closely follow the AASHTO Specifications with the following considerations: ♦ ♦ ♦ ♦ ♦ ♦ Concrete should be Class S. Design Recommendations If there should be an appropriate occasion to use continuous reinforced concrete girders. Load factor design should be used.Chapter 7 — Superstructure Design Section 7 — Continuous Concrete Girder Spans Current Status Continuous reinforced concrete girders are no longer used. This value is based on the creep of the concrete. and reinforcing steel should be Grade 60.000 psi 28-day strength. Z = 170 for positive). Stirrup spacing should be calculated assuming the stress carried by concrete alone to be 126 psi.500. Lateral distribution of live load should be S/6.0. Bridge Design Manual 7-35 TxDOT 12/2001 . one of which was a continuous reinforced concrete box girder bridge. Bridge Design Manual 7-36 TxDOT 12/2001 . It had nine spans. cantilevered deck slabs. the longest of which was 93 ft. The cost was $25 per square foot compared to prestressed beam bridges at $16.Chapter 7 — Superstructure Design Section 8 — Concrete Box Girder Spans Section 8 Concrete Box Girder Spans Background In 1958 a continuous cast-in-place box girder bridge (80-95-80 ft. The high cost of these structures has resulted in very few continuous cast-in-place concrete box girder designs of the type shown in Figure 7-12. Design Recommendations No recommendations are considered necessary. The sides were sloped. and there was a sharp horizontal curve at each end of the bridge. wide spine beam containing three 3 ft. In 1962 concrete box girders were used in a railway service road structure in Dallas. round voids and with 9 ft. Shortly thereafter the Preston and Pearl Street Underpass was constructed over IH 20 with a 14 ft.) was constructed on SH 240 over the Wichita River in Wichita County. The cost was $65 per square foot compared to $16 per square foot for prestressed beam bridges. In 1974 a controversial freeway project in San Antonio was continued with numerous aesthetic and environmental embellishments. Current Status Cast-in-place mild steel reinforced concrete box girders are not recommended except for very special conditions. All of these were mild steel reinforced. in 1979. Similar sloping web box girders have been used sparingly to finish off the outside of precast box beam bridges to accommodate severely flared roadways. a small box girder unit (86 to 105 ft.) was constructed across a depressed section of Spur 366 in Dallas. This bridge has since been replaced due to heavy chloride damage. Then. ) Bridge Design Manual 7-37 TxDOT 12/2001 .Chapter 7 — Superstructure Design Section 8 — Concrete Box Girder Spans Figure 7-12. Cast-in-Place Concrete Box Girder – Typical Section (Online users can click here to view this illustration in PDF. The prestressed slab is still in good condition but the design has not been used since. It was considered impractical and obtrusive to provide bent caps. A large elevated highway project on IH 345 in Dallas.50 per square foot extra. The average bid price for structural steel was $0. This occurred in some of the conventionally reinforced bridge decks as well. Six 75 ft. constructed in the late 1960s. Strands were sensitive to misplacement due to foot traffic and wet concrete loads. Unfamiliarity of the contractor and state personnel with the system probably magnified the inherent problems. the bridges appeared to cost about $3. The benefits of composite action were sacrificed. Failure of the prestressing to close these cracks was considered significant. There was some diagonal cracking in the outer portions of the deck width adjacent to construction joints. Texas. Texas. Approximately 27 acres of this type slab were constructed on this elevated section as part of an attempt at aesthetics. support location was random. and the slab cost was about $4. a satisfactory design has not been developed. Unfortunately. had 10.9 Model testing was done and corrosion exposure specimens were evaluated. Delivery of the coated hardware was slow.Chapter 7 — Superstructure Design Section 9 — Prestressed Concrete Deck Slabs Section 9 Prestressed Concrete Deck Slabs Background For some time it has appeared desirable to prestress bridge deck slabs to counteract corrosion problems associated with cracking and lack of durability of the concrete. The random column spacing and wide girder spacing combined to create some unorthodox continuous units. Some were stressed only transversely and others stressed both transversely and longitudinally. Construction of the bridge decks was fraught with problems. prestressed concrete beam spans in an interstate highway river bridge in La Grange. were constructed with 80 ft. Because of a myriad of streets and utilities beneath the structure. and forming for blockouts was tedious.29 per pound. so widely spaced girders were supported directly on the columns. The slab rested on transverse floor beams a few in. Stressing was time consuming. A research project at the University of Texas was directed to the design and performance of transversely prestressed bridge decks. The consensus cause was shrinkage in the wide slabs. The report pronounced this a viable type of bridge deck with life cycle cost about 20 percent less than conventionally reinforced designs.00 per square foot. Concern about the ability of a conventional slab to conform gracefully to the anticipated warping led to the slab being supported by floor beams only and post-tensioned in both directions. above the girders. Compared to adjacent sections of this same highway.5 in. It was concluded that the possible advantages of the system were outweighed by the problems encountered during deck construction. wide prestressed decks. post-tensioned slabs on steel plate girder spans. Bridge Design Manual 7-38 TxDOT 12/2001 . If needed.10 Bridge Design Manual 7-39 TxDOT 12/2001 . Design Recommendations No specific design recommendations will be given here. general design information can be found in the report Application of Transverse Prestressing to Bridge Decks.Chapter 7 — Superstructure Design Section 9 — Prestressed Concrete Deck Slabs Current Status The Bridge Division does not expect prestressed concrete bridge decks to be used on Texas highway bridges in the foreseeable future except for very special situations. and the cast-in-place post-tensioned slab was relegated to special conditions where structure depth was critical or aesthetics were especially desirable. Center span-to-depth ratios as high as 50 have been achieved. Agreement between jack pressure and elongation is often beyond the specification allowable of 5 percent due to a difference between calculated and actual friction losses. This lightens the dead load and allows the absolute minimum superstructure depth. additional prestressing is required for stress control. and it is virtually impossible to prevent longitudinal shrinkage cracking above and below the voids. or substantial shear keys are provided. round voids have been formed in the positive moment zones using fiber tubes. a variable depth slab offset itself laterally about 1. constant-depth slabs have been used.Chapter 7 — Superstructure Design Section 10 — Prestressed Continuous Slab Spans Section 10 Prestressed Continuous Slab Spans Background Prestressing in Texas began in the mid 1950s and soon became accepted as an effective procedure to increase concrete span lengths and control deflections. Span-to-depth ratios as high as 40 have been achieved. Most of the designs have had variable slab thickness created by a parabolic soffit each side of the interior columns. Prestressing for zero dead load deflections is usually impractical. Emphasis was placed on precast pretensioned beams. Concrete consolidation beneath the voids is difficult. Fiber tube voids are subject to crushing or floating during concrete placement.5 ft. due to stressing across a severely skewed construction joint. Since then. Bridge Design Manual 7-40 TxDOT 12/2001 . Constant-depth prestressed concrete slabs can also be designed without voids. In order to lighten dead load. Separate interior bent caps are usually unnecessary since the slab itself can be strengthened transversely to span between columns. Severely skewed bents have been used. Where superstructure depth is critical close to the interior supports. but this tends to ruin the aesthetic quality of variable-depth slabs. Various configurations of prestressed concrete slab spans have been used. In one embarrassing incident. Deteriorating tubes can create methane gas which can actually rupture the concrete. any construction joints in prestressed slabs have been normal to the tendons. Because of the weight. Stressing of post-tensioning tendons creates initial problems on all types of structures. Construction Problems Most of the problems with these slabs have occurred during construction. Stresses due to primary and secondary effects of prestressing can be calculated accurately using computer program BMCOL51.75 f 's. ♦ ♦ ♦ ♦ ♦ ♦ Skews greater than 45 degrees should be avoided. Losses after seating may be taken as 33 ksi. jacking stress = 0. This should suffice for most units.11 Assume knife edge supports continuous for the width of the design section. Primary prestressing tendons. if used. The stress at anchorage after seating shall not exceed 0. Transverse reinforcing bars and prestressing tendons.70 f 's. 7-41 TxDOT 12/2001 ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ Bridge Design Manual .Chapter 7 — Superstructure Design Section 10 — Prestressed Continuous Slab Spans Current Status Continuous prestressed concrete slab units continue to be acceptable solutions to aesthetic or critical depth problems. friction coefficient = 0. Show the required values on the plans.12 or other rational methods. however. Ignore shear in the longitudinal design of the spans. Anchorage zone reinforcing against bursting may be evaluated using CTR Research Report 208-3F.25. longitudinal reinforcing bars. they are rarely used. Transverse design at interior bents may use prestressing or mild steel reinforcing. and design span length should be parallel to traffic. and wobble factor = 0. Shear should be considered.. Trucks and lanes of live load may be distributed over 14 ft. Design Recommendations The following items should be considered in the design of continuous prestressed concrete slab units.13 Ultimate flexural capacity should be checked at the maximum moment points only. Required final prestress force at the critical points should be shown on the plans. Compute moments and reactions using a beam analysis computer program such as Program B30. Tendon stress at the end of the seating loss zone need not be checked. of width for spans over 50 ft. Use negative moment at the centerline of interior bent for design.0002. Design of longitudinal prestressing tendons should be based on low-relaxation seven wire strand tendons with f 's = 270 ksi. Effective supports may be taken at the centerline of interior bents and the quarter points of end bent caps. The contractor has the option of selecting the size of strands and tendons and the type of anchorage. Losses may be calculated using anchor set = 0.625 in. Voided slabs are no longer recommended. should be parallel to the skew. Distribution steel requirements of the AASHTO Specification do not apply. Longitudinal reinforcing should be a minimum of #4 at 12 in. centers. Rules for measurement are in the construction specification. Figure 7-13. ♦ Typical bearing and joint conditions are shown in Figure 7-13. centers top and bottom.) Bridge Design Manual 7-42 TxDOT 12/2001 . Calculation of pay quantities of “Prestressing” should be shown on the plans to avoid confusion. Continuous Prestressed Concrete Slab Units – Typical Details (Online users can click here to view this illustration in PDF.Chapter 7 — Superstructure Design Section 10 — Prestressed Continuous Slab Spans ♦ Transverse mild steel should be a minimum of #4 at 6 in. Bridge Design Manual 7-43 TxDOT 12/2001 . prestressed concrete beam spans. One such installation formed the second prestressed bridge constructed on the Texas highway system. The danger of improper prestressing procedures was emphasized when a stressed bar. and erected by a large. pan forms lost the bidding competition to the monolithic spans. This two-mile-long bridge offered three superstructure alternates: pan form girders. Three spans on the east end of the bridge around elevation 15 ft. In 1954 the contract was awarded for a section of Dallas expressway that included the Pine Street Overpass containing three 58 ft. which could be classified as prestressed simple slab and girder spans. The spans were carried to the water by a gantry. Design Recommendations No recommendations are considered necessary. specially fabricated crane mounted on the barge.Chapter 7 — Superstructure Design Section 11 — Prestressed Simple Slab and Girder Spans Section 11 Prestressed Simple Slab and Girder Spans Background The use of prestressed simple-span concrete slab and girder spans has been sparing but notable. kinked by collapse of an anchorage shim. loaded to a barge. occurred in 1959 on the Lavaca Bay Causeway. In a rare occurrence. transported to their position in the bridge. The only other known use. barely missing a contractor’s employee. was launched into the work area. post-tensioned simple concrete slab and girder spans. were lost to Hurricane Carla before the bridge was opened to traffic. The monolithic spans were patterned after the Lake Pontchartrain Causeway in Louisiana. and monolithically cast pretensioned slab and beam spans. Current Status No designs of this type are anticipated. The contractor cast these spans in retractable metal forms near the bridge end. with one less girder and a slightly longer 60 ft. The contractor elected to cast the spans on the ground and lift them into place. They were replaced with prestressed concrete beam spans. span. Post-tensioning was accomplished with high-strength bars. slab spans and one 50-60-50 ft. large-diameter strand tendons were draped in each stem. It was constructed in 1952 on SH 60 across the San Bernard River between Austin and Wharton Counties. An approximate 2 ft. two sets of 30 ft. After the concrete reached a strength of 4. but needs mentioning for its historical significance. because of construction problems. The bridge still carries traffic. For the main span. The alternate won. pan forms were erected with a falsebent between. regular pan form girder spans and one 60 ft. Bridge Design Manual 7-44 TxDOT 12/2001 . the system will probably not be used again. Design Recommendations No recommendations are considered necessary.000 psi. a continuous post-tensioned bridge was constructed in the Waco District using pan form girders in the positive moment areas with solid slabs over the supports. continuous I-beam unit. The regular bid was for 24 25-ft. The stressing space was covered with a cast-in-place slab. prestressed pan girder span. The alternate bid was for 23 30-ft. Texas’ first prestressed highway span was post-tensioned pan form girders. Also. space was provided beyond one end for stressing the tendons. The bridge looks good but. Current Status No designs of this type are anticipated. Two coated. the tendons were stressed and the falsebent removed.Chapter 7 — Superstructure Design Section 12 — Prestressed Pan Form Slab and Girder Spans Section 12 Prestressed Pan Form Slab and Girder Spans Background This concept never caught on. Liberal stem widths should be provided to allow ample side cover to the post-tensioning ducts after allowance for construction tolerances. Positive moment prestressing should be aligned with the center of stems and anchored at the ends of unit. the following items are offered for consideration: ♦ ♦ ♦ ♦ Span-to-depth ratio should not exceed 38.Chapter 7 — Superstructure Design Section 13 — Prestressed Continuous Concrete Girder Spans Section 13 Prestressed Continuous Concrete Girder Spans Background Cast-in-place T-shaped girders are not desirable for long continuous spans because of large compressive stresses in the narrow girder stems at interior supports. This is shown as a ribbed slab in Figure 7-14. Span-to-depth ratios approaching 38 were achieved with spans up to 150 ft. Post-tensioning was provided in the T-girder stems with additional tendons in the slab portion anchored in the spaces between adjacent stems. ♦ Bridge Design Manual 7-45 TxDOT 12/2001 . Design Recommendations Although this type of construction is not anticipated in Texas. Current Status No further designs of this type are anticipated. For span lengths and depths within practical limits of the compressive stress. while the negative moment regions were the same depth but solid concrete slab. something else must be done to control compressive stresses over the support. mild steel reinforcing is usually sufficient to resist tensile stresses. Continuous prestressed concrete girders were used efficiently in crossover structures over a depressed freeway in Houston. Additional tendons required for negative moment should be anchored at the end of the solid slab portion in the space between girder stems. Length of the solid slab portion must be sufficient to maintain compressive stresses in the bottom of T-girders below 0.4 f 'c. Shear should be considered in the solid slab and in the T-girders. T-shaped girders were used in the positive moment regions of continuous units. For prestressing to be advantageous. ) Bridge Design Manual 7-46 TxDOT 12/2001 .Chapter 7 — Superstructure Design Section 13 — Prestressed Continuous Concrete Girder Spans Figure 7-14. Ribbed Slab Concept (Online users can click here to view this illustration in PDF. The steel design was selected by the contractor. The high cost of these structures has resulted in a very few continuous prestressed concrete box girder designs of the type shown in Figure 7-15. and type of deck reinforcing should be based on economic Bridge Design Manual 7-47 TxDOT 12/2001 .) in Belton. While California developed this design as its standard type structure. forming. Decisions regarding depth of box. number of webs. Design Recommendations The PTI Box Girder Manual14 and CALTRANS Design Practice15 are good references for the design of cast-in-place prestressed concrete box girders. Prestressed Concrete Box Girder – Typical Section (Online users can click here to view this illustration in PDF.Chapter 7 — Superstructure Design Section 14 — Prestressed Cast-in-Place Box Girder Spans Section 14 Prestressed Cast-in-Place Box Girder Spans Background In 1973 a cast-in-place prestressed concrete box girder (four spans at 155 ft.) Current Status Longitudinally prestressed cast-in-place box girders may have limited application. Complicated falsework. and extensive on-site construction time are the downfalls of this system. bottom width. Texas has not been able to make it an economically viable alternative. Preliminary design should be very thorough. Numerous provisions of the AASHTO Segmental Guide Specification16 also apply to this type of construction. Figure 7-15. Texas. was designed as an alternate bid against curved steel plate girders. Bridge Design Manual 7-48 TxDOT 12/2001 . ♦ ♦ ♦ Span-to-depth ratios should not exceed 20 for end spans nor 25 for interior spans. Final design should include allowances for creep and shrinkage. Field control by cylinder tests (Class H concrete) should be required. Suggestions based on limited design experience are given below. as they affect prestress losses and redistribution of moments in continuous structures. Prestressed concrete should be Class H with a maximum f 'c = 6.Chapter 7 — Superstructure Design Section 14 — Prestressed Cast-in-Place Box Girder Spans calculations.500 psi. but now an equilibrium-based plasticity model (strut-and-tie) as described in the standard specification may be more appropriate for sizing anchorage zone reinforcing. Anchorage zone reinforcing should be given careful attention. Web thickness should be sufficient to comfortably accommodate the prestressing tendons and withstand shear and torsion forces near the interior supports. Moment redistribution from progressive construction of continuous spans should also be considered.17 This was the generally accepted basis for consideration of lead-in stresses. There have been three types of segmental bridges built in Texas. It was a very modest 100-200-100 ft. See Figure 5-10 for general observations regarding balanced cantilever construction.. Cable-stayed bridges are discussed in Chapter 9.Chapter 7 — Superstructure Design Section 15 — Prestressed Segmental Box Girder Spans Section 15 Prestressed Segmental Box Girder Spans Background Concrete box girders can be built by connecting full cross-section cast-in-place or precast segments with longitudinal prestressing. the first precast segmental concrete box girder unit in the United States. Cast-in-place or precast segments are placed and post-tensioned in a symmetrical manner to balance the forces on the pier. again over the Intracoastal Canal. 183 (see below). with 150 ft. Span-by-span and balanced cantilever are best differentiated by their construction method and cable-stayed by its use of overhead cables to support the segments. spans possible. at High Island. cast-in-place closure is cast over the pier and tendons are made continuous between spans. Kennedy Memorial Causeway in Nueces County. It was to have been ♦ Bridge Design Manual 7-49 TxDOT 12/2001 . Epoxy is applied to match cast joints. a full-depth. Generally. This type of construction is characterized by the superstructure erection originating at the pier and progressing outward in both directions. Balanced Cantilever Balanced cantilever construction is suited for bridges with main spans between 250 and 800 ft. Side spans are typically 55 percent to 70 percent of the main span. segmental unit was designed as an alternate to a steel plate girder unit. Span-by-Span Construction Span-by-span construction is most economical in bridges with numerous spans (20 or more) ranging from 80 to 135 ft. Texas. Projects ♦ In 1973. Erection girders are lowered slightly to set the superstructure on permanent bearings. A construction cycle is begun by placing precast segments on erection girders and grading to proper alignment. Prestressing tendons were located in the concrete and anchored within the web of the box. See Figure 5-11 for general observations regarding span-by-span construction. Section 8 of this manual. Construction continues from all interior piers until the cantilevers connect to form a continuous superstructure. Texas completed construction of the John F.S. a 200-290-200 ft. unit over the Intracoastal Canal containing two lines of single-cell precast segments erected as balanced cantilevers. and longitudinal post-tensioning is installed to form a loadcarrying girder. a new simplified method has been used successfully for U. In 1975. however. 183 in Austin. only a thin (8 in. creep.Chapter 7 — Superstructure Design Section 15 — Prestressed Segmental Box Girder Spans erected as cantilevers with false bents in the end spans.S. The final geometry of the roadway is set in the casting operation. however. Sophisticated geometry programs are essential in the casting operation to account for horizontal and vertical curves. to 19 ft. and expansion joints were placed at approximately every third pier. superelevation. with a length of 350 ft. At the two interior piers of a unit. Sophisticated load history and timedependent computer programs are a virtual necessity. and external tendons. the first segmental contract was awarded in a massive rehabilitation program of downtown interstate freeways in San Antonio.000. This was big-time segmental. are used. the main span. The low bidder exercised the design option and has constructed a cable-stayed unit using segmental concrete box sections. camber. and errors from the Bridge Design Manual 7-50 TxDOT 12/2001 . Texas. was constructed incorporating segmental box sections in the design and was erected by balanced cantilever. Three subsequent contracts were for similar designs and details prepared by the Bridge Design Section. ♦ In 1984. A series of simple spans were built. and a width of 50 ft. Flexible neoprene pads allow the entire slab/bearing system to act as a hinge. steel plate girders were chosen by the contractor. Geometric design must also be considered in terms of the effects on segment casting and erection. The span-by-span portion is the first known project built entirely with semi-continuous joints over the piers (two test spans in the final San Antonio “Y” project were built this way). An elevated portion of U. except for the connections over IH 35. has recently been completed using single-cell segmental box sections erected span-by-span. An Intracoastal Canal bridge in Brazoria County near Surfside Beach. where the anchors and deviators are cast in concrete but most of the duct is outside the concrete in the void of the box. a 320-640-320 ft. Both internal tendons. While the approach spans utilized conventional AASHTO Type VI (Mod) Beams. born of an intense desire for aesthetics and minimum interference of construction with existing traffic. temperature. which were erected by balanced cantilever. Texas. balanced cantilever over the Neches River was designed as an alternate to a strutted steel plate girder unit or to a contractor-designed option. deflection. ♦ ♦ ♦ Design and Construction Issues Design is complicated.000 square feet of precast concrete box girder bridge. This semi-continuous joint reduces design and construction complexity with no increase in materials. Texas. there were about 3. on the skill of experienced casting yard personnel. In 1985.. with a maximum vertical clearance of 75 ft.-10 in. Eventually. Successful construction of precast segment projects depends. erected span-by-span. where the entire duct and anchors are encased in concrete. to a great extent. The effects of shrinkage. to the waterway below.. The type of segmental design was established by competitive bidding on the first three contracts on separate sets of details prepared by two different private engineering firms. and prestress loss must be analyzed in coordination with the erection sequence. The superstructure depth ranges from 8 ft. was constructed with cast-in-place segmental box sections.) slab is cast between the end segments. Wide boxes with sloping webs that extend to the edge of the roadway are also quite attractive but are more expensive and. In general. Design Recommendations It is highly recommended that the Bridge Design Section be consulted early in the planning stage.Chapter 7 — Superstructure Design Section 15 — Prestressed Segmental Box Girder Spans casting of the adjacent segment. Structural design is controlled by the AASHTO Guide Specification for Design and Construction of Segmental Concrete Bridges. Problems have been occasional and unusually minor. Aesthetics are in the eye of the beholder. The quality of the cast segments will determine the ease of fit during erection. with their narrow spine girder and wide overhanging wings. but it is generally conceded that the San Antonio elevated structures. The JFK Causeway Bridge in Corpus Christi. and the finish of segments will greatly affect the final appearance of the bridge. the performance of these bridges has been very good. The Bridge Design Section recommends the following: Bridge Design Manual 7-51 TxDOT 12/2001 . therefore. has performed extremely well for nearly 30 years.18 Analysis has been performed using the computer programs Bridge Designer II and ADAPT. Texas. Current Status This type of construction is considered appropriate for structures requiring the most pleasing appearance. Ramp entrance to or exit from the bridge is undesirable but can be accommodated. Similar span lengths are desirable for constructibility. The Bridge Design Section is capable of design and plan preparation for these bridges. Singlecolumn piers are often the substructure of choice because of their harmony with the spine and openness beneath the structure. The number of straddle and cantilever bents over lower roadways needs to be minimized for aesthetic reasons. Some cracking has been repaired in the anchorage zones of two segmental bridges designed with empirical methods. Geometric and highway design features can often be satisfactorily modified to greatly simplify construction of segmental concrete bridges. Overhead and mobilization expenses make them cost-prohibitive for short structures. enhance the appearance of the area. Location of bents and span lengths deserve careful consideration. It should also be considered for spans between 300 and 800 ft. The Austin structures also have the wide overhang feature and are further highlighted by special treatment of the substructure. very rare. There are acceptable adjustments that can be made to the customary ramp configurations that minimize complications of segmental construction. The current strut-and-tie method in the standard specifications has recently been used to successfully design anchorage zones. Bridge Design Manual 7-52 TxDOT 12/2001 .Chapter 7 — Superstructure Design Section 15 — Prestressed Segmental Box Girder Spans ♦ ♦ ♦ ♦ Epoxied joints A combination of internal and external tendons Provision for future installation of additional tendons Two-course asphalt surface treatment and asphaltic concrete overlay on the deck. Longitudinal cracks usually appeared in the ACP deck over each shear key. Shear key concrete was then placed between the ends of the beams and under the beams at the interior bents. wide by 1 in. These bridges experienced some rather significant problems. The bars were replaced with 1/2 in. boxes were designed but not distributed. the strands stressed.. Pan form spans. Other sizeable cracking was observed on a few bridges. and sought an economical system that would be durable in a salt environment. Deeper box beam sections were developed and distributed in a 1975 set of standard details. box beam. The Bridge Division chose to use a large cast-in-place concrete shear key with transverse reinforcing bars threaded through the boxes and bolted for lateral restraint. thick bearing pads. in the process. The Bridge Division prepared the plans and. deep box beam in the surrounding counties. The transverse post-tensioning strands were initially spaced at 25 ft. The Bridge Division revised the 20 in. Some transverse tendons corroded and failed under stress. and 34 in. which was covered by 2 in. the transverse strands threaded. and 40 in. as well as large diagonal cracks at the ends of the exterior beams. were included in the standards. The 1975 standards also included a detail for the bent closure concrete which essentially locked up each span. and the roadway surface sealed with a two-course asphalt surface treatment. box beam details and released a new set of 1984 standards in an attempt to solve these problems. The earlier county box beams had been joined together laterally by welding embedded steel attachments. were showing signs of severe deterioration. Box beam depths of 20 in. 28 in. The Corpus Christi District had assumed responsibility for constructing a park road along a barrier island. The box beams were erected side by side. The Corpus Christi District decided that box beams would be appropriate for bridges over the cuts and channels along the island. The Bridge Design Manual 7-53 TxDOT 12/2001 . the system was not brought to the attention of the state until the late 1960s. The large shear key was maintained by using the Type A prestressed I-beam shape in the sides of the additional depths. The closure pour at interior bents modified to allow for expansion joints. The beams were placed on 2 in. developed the TxDOT 20 in. seven wire strands since fabrication and construction tolerances in the hole location prevented a large bar from being threaded consistently through several boxes. A prestressed beam fabricator was marketing a 20 in.. The abutment backwall was cast flush with the ends of the beams with the backwall concrete placed under the beams as well. shear key and bent closure concrete placed. increments. the economical favorite. Although the first bridge constructed with these box beams was a Goliad County bridge in the late 1950s. Additional reinforcing was added to the box beams. of asphaltic concrete pavement (ACP). A box beam bridge in Fort Worth contained large longitudinal cracks in the bottom flanges of the boxes.Chapter 7 — Superstructure Design Section 16 — Prestressed TxDOT Box Beam Spans Section 16 Prestressed TxDOT Box Beam Spans Background The TxDOT box beams originated in the lower coastal counties in the late 1960s. Chapter 7 — Superstructure Design Section 16 — Prestressed TxDOT Box Beam Spans ends of the beams were notched to permit the installation of an armor joint or sealed expansion joint with a cast-in-place end block. have been constructed with 40 in. and includes span capabilities and approximate superstructure depths. The deck can be completed quicker than with beams and cast-in-place slab. The void in each box beam was initially formed with stay-in-place cardboard or styrofoam blocks. Ultimately. The beams still had a tendency to rock slightly on the pads. some fabricators continued to place concrete monolithically because of the stringent requirements for cold joint cleanliness. Spans of 118 ft. The Houston District experimented with a reinforced concrete slab on the boxes instead of the ACP overlay. and then was forced upward by buoyancy when both sides and the bottom were placed. The usual potential for honeycomb also existed. The combination of the concrete slab with the shear keys performed well. In addition. forming the void posed a significant fabrication problem. However. The box beams were placed on two elastomeric bearing pads at expansion joints. Construction Problems Fabrication has been the leading problem with prestressed box beams. a three-pad system was adopted for box beams. Figure 5-6 shows a typical section through a box beam bridge. thick. along with special problems at skewed box ends due to camber on release. This rocking was especially pronounced on heavily skewed bridges. The void tried to move laterally when the concrete was placed on one side. Despite these changes. box beams. the four-pad system proved to be somewhat problematic. However. TxDOT later Bridge Design Manual 7-54 TxDOT 12/2001 . They used slabs from 4 in. The void forms were extremely difficult to hold in position as the concrete placement proceeded. The cost of box beam spans is considerably more than for other prestressed beams. which promoted conflicts because of bar fabrication tolerances. TxDOT encouraged sequential concrete placement in 1983 by permitting cold joints between the slab and walls. but the advantages sometimes outweigh the additional cost. Houston also switched to a four-pad system. problems continued with some bridges. utilizing two elastomeric pads at each end of the box beam. which is currently the highest possible with precast members. allowing first-stage detour traffic to be restored to normal conditions sooner. The box beams on some bridges tended to rock transversely on the bearing pads. The longitudinal deck cracks continued to appear. Box beams have become popular for building bridges where speed of construction or minimum section depth are critical. The reinforcing details were intricate. Stage construction can often be expedited by using box beams. to 5 1/2 in. This often resulted in slab and/or wall thickness that was beyond the specified tolerances. Span-to-depth ratios of 30 can easily be attained with ACP. Attempts to circumvent the lateral movement by placing the concrete on each side simultaneously often resulted in air voids in the middle of the bottom slab with little or no slab thickness. both with and without the shear keys. A 1 in. Construction details. Every beam has a 12 in. Details and section properties for all available standard prestressed TxDOT box beams are shown in Figure 7-16. A three-pad system is currently used with box beams. Details for 20. and are covered with a seamless plastic sheath. In addition. Figure 7-18. Interior diaphragms are required in the exterior boxes at the location of the tendons. maximum spacing both transversely and longitudinally. along the length of the span. These sheets have not been released as statewide standards and therefore must be signed and sealed by a registered professional engineer. while the back station end sits on two smaller pads. The additional cost of box beam bridges is primarily in the fabrication of the beams. Current Status Standard drawings for box beams are not yet available. 28. reinforced with #5 bars at 12 in. Grade 1030 threaded bar. End diaphragms are required at the end of each span. All box beam bridges must have concrete shear keys. minimum thickness. wide by 7 in. The slab should have a 5 in. Void drain holes are installed at the corners of the bottom slab during fabrication. Transverse post-tensioning is required with ACP overlay. Figure 7-17. Box beams are fabricated using a two-stage monolithic casting.Chapter 7 — Superstructure Design Section 16 — Prestressed TxDOT Box Beam Spans revised the casting requirements to require a two-stage monolithic pour. and Figure 7-19. The tendons are either 1/2 in. the forward station end of the beam sits on a single elastomeric bearing pad. regardless of whether a reinforced concrete slab or an ACP overlay is used. deep block out at each end to facilitate the installation of the diaphragms. typically at the center of span (or at center of bearing in situations such as sag vertical curves). diameter PVC pipe sleeve is threaded through the tendon holes to protect the strands from possible corrosion and to allow for possible replacement of damaged tendons in the future. Grade 1860 strand or 5/8 in. The fixed slab joints in a continuous unit have a single diaphragm cast across both block outs. The slab should be Class S concrete. Design Recommendations ♦ The use of ACP overlay on box beam bridges is discouraged. cardboard void forms are no longer permitted. All interior voids must be formed with polystyrene. The strands are located at a maximum spacing of 10 ft. The bottom slab is cast in the first stage. and hold down details are also available. A cast-in-place reinforced concrete slab is recommended. and the sides and top are cast in the second stage while the slab concrete is still plastic. 34 and 40 in. rail anchorage details. ♦ Bridge Design Manual 7-55 TxDOT 12/2001 . deep box beams have been developed and are available from the Bridge Division. Typically. The box beams and gap sizes should be chosen so that the edge of the slab corresponds to the edge of the top flange of the exterior beams. The complexity of the geometry required to frame the bridge increases dramatically as the degree of curvature exceeds 1 degree or 2 degrees. Differential camber and torsion problems and the tendency of the box beams to rock on the pads become more pronounced as the skew angle increases. Beam hold downs should be used on water crossings when the superstructure could be subjected to pressure flow. boxes should be used as exterior beams when the roadway width requires a combination of both 4 ft. Stresses at the end of the beam are controlled by debonding strands. tall ear walls located at the ends of each abutment and interior bent cap.500 psi at 28 days. between boxes and a maximum gap size of 3 in. wide by 7 in. Live load should be distributed laterally according to the latest AASHTO Specification for concrete beams used in multi-beam bridges. boxes. Concrete strengths for box beams should be limited to 6.2 times the span length. or when the beam grade exceeds 6 percent. The maximum debonding length should be the lesser of one-half the span length minus the maximum development length as specified in the current AASHTO Specifications. Bridge skews exceeding 30 degrees should be avoided. increments. Prestressing strands are typically 1/2 in. 0. Bridge skew should be minimized as much as possible. diameter. The hold downs are typically placed at the center of the joint between the exterior beam and the first interior beam on both sides of the structure. Box beams are not appropriate for use on curved structures. or “fixed. The hold downs may be moved to the second interior joint for heavily skewed bridges 7-56 TxDOT 12/2001 ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ Bridge Design Manual .” interior bents is available from the Bridge Division. The use of box beams on flared structures should be avoided. Longitudinal restraint is provided at interior bents only. 5 ft. Rail dead load is typically distributed equally among the three exterior boxes when a span consists of six or more box beams. or 15 ft. Lateral restraint is provided by 12 in. The shear keys may also be cast integrally with the slab. The required concrete strengths are determined according to the limitations for concrete compression and tension stress given in the current AASHTO Specifications. and 5 ft. Dowels are no longer used with box beams. A 1/2 in. The box beam arrangement should allow a minimum gap of 1/2 in.500 psi at release and 8. No more than 75 percent of the strands should be debonded. Longitudinal restraint at interior bents should be provided only when a continuous unit exceeds four spans in length. A detail for the longitudinally restrained. Strands are typically debonded in 3 ft. 270 ksi low-relaxation strands. gap is provided between the ear wall and the outside edge of the exterior beam. The shear key and end diaphragm concrete should be Class S concrete. The end diaphragms must be cast integrally with the slab.Chapter 7 — Superstructure Design Section 16 — Prestressed TxDOT Box Beam Spans ♦ ♦ Slab overhangs should be avoided. Elevation points may be required as frequently as every joint in certain situations. and one at the center of each joint on either side of the middle beam. or are rotated to the average cross-slope of a span in a transition area. at the centerline of bearing.88 Bridge Design Manual 7-57 TxDOT 12/2001 .81 5B20 717. A minimum gap of 1 1/2 in. This may necessitate stepping the cap at some joints so that adjacent beams not only have a different slope but also sit at a different elevation.) Property Area (in2) Y Top (in) Y Bott (in) Beam Properties 4B20 591. Four elevations points should be provided for spans with an odd number of beams: one at the outside edge of each exterior beam. but either parallel the roadway surface when the cross-slope is constant.12 9. The pads sit directly on the top of the cap. Framing is more complicated in cross-slope transition areas and skewed bridges. Top of cap elevations should be provided at the points coinciding with the outer edge of the exterior boxes at the centerline of bearing. Elevations should also be provided at any intermediate points along the cap. where either a change in cap slope or change in cap elevation occurs. The orientation of the beams should minimize the variation in slab thickness both longitudinally and transversely along the span. ♦ A minimum of three elevation points are necessary for unskewed spans with an even number of box beams and a constant housetop slab profile: one at the outside edge of each of the exterior beams and a third point at the center of the middle joint.Chapter 7 — Superstructure Design Section 16 — Prestressed TxDOT Box Beam Spans (approximately 25 to 30 degrees).19 9. Since there are no bearing seat build-ups. ♦ Figure 7-16.8 10. is required at a joint where a hold down is located. Online users can click here to view this illustration in PDF. ♦ Bearing seats are not used with box beams. It is possible for the forward half of an interior bent cap to have a different elevation than the back half at some locations. Box beams are not vertical. 20-inch Prestressed Concrete TxDOT Box Beam Properties (See the following table of beam properties.8 10. the top of the cap must be sloped to match the rotation of the beams. Chapter 7 — Superstructure Design I (in4) Weight (lb/ft) 28.8 14.74 85.370 838 Bridge Design Manual 7-58 TxDOT 12/2001 .086 616 Section 16 — Prestressed TxDOT Box Beam Spans 35.26 13.8 14.38 13. 28-inch Prestressed Concrete TxDOT Box Beam Properties (See the following table of beam properties. Online users can click here to view this illustration in PDF.745 707 5B28 804.) Property Area (in2) Y Top (in) Y Bott (in) I (in4) Weight (lb/ft) Beam Properties 4B28 678.62 68.234 748 Figure 7-17. 8 17. 40-inch Prestressed Concrete TxDOT Box Beam Properties (See the following table of beam properties.161 963 Figure 7-19.) Bridge Design Manual 7-59 TxDOT 12/2001 .8 17.) Property Area (in2) Y Top (in) Y Bott (in) I (in4) Weight (lb/ft) Beam Properties 4B34 798.72 16. Online users can click here to view this illustration in PDF. Online users can click here to view this illustration in PDF. 34-inch Prestressed Concrete TxDOT Box Beam Properties (See the following table of beam properties.28 142.08 115.Chapter 7 — Superstructure Design Section 16 — Prestressed TxDOT Box Beam Spans Figure 7-18.655 832 5B34 924.92 16. 114 Bridge Design Manual 7-60 TxDOT 12/2001 .007 1.8 21.088 1069.36 18.300 1.93 215.07 18.63 18.159 983 1.69 176.31 18.37 180.64 219.8 21.8 21.Chapter 7 — Superstructure Design Section 16 — Prestressed TxDOT Box Beam Spans Beam Properties 4B40 Property Beam length 100ft or less Area (in2) Y Top (in) Y Bott (in) I (in4) Weight (lb/ft) Beam length over 100ft Area (in2) Y Top (in) Y Bott (in) I (in4) Weight (lb/ft) 5B40 918.8 21.044.607 957 943. and box beams. Lacking the large shear key recess.Chapter 7 — Superstructure Design Section 17 — Prestressed AASHTO/PCI Box Beam Spans Section 17 Prestressed AASHTO/PCI Box Beam Spans Background In 1962. with asphaltic concrete overlay. Standard details were prepared. For this reason. Bridge Design Manual 7-61 TxDOT 12/2001 . Relative costs are indeterminate because of many factors that influence contractor bid prices. Concern about performance of the small shear key delayed the use of these shapes with asphaltic concrete overlay. so the publication had little effect on Texas. 33. It also identified the old AASHTO/PCI shapes as probably being simpler to fabricate than the TxDOT boxes. AASHTO box beams are no longer used. for 27. they have a greater height of side surface to interfere with the adjacent box. The research resulted in simplified TxDOT box standards and double tee standards. deep AASHTO boxes in widths of 3 and 4 ft. it must be increased in height at the span ends to maintain good lines. However. AASHTO boxes are more susceptible to this lateral growth. Boxes cannot be placed side by side without some increase in the nominal out-to-out width. Adjustments will be necessary also at abutments where finished grade will be above the prescribed roadway grade by the amount of camber minus dead load deflection. piling. and problems with fabrication and construction were soon evident. a few bridges have been constructed using special details for the AASHTO boxes. Also.19 The Bridge Design Section had already developed shapes for I-beams and piling. The ultimate problem was cracking in the slab due to lack of adequate shear keys. If the railing rests on the top of the box. however. The construction of TxDOT shapes began in 1969. One bridge exhibited unsightly lines on the outside because of the inability of the deck and substructure to adapt to this lateral growth. Most of those bridges have been in Houston and all have had reinforced concrete overlay. these boxes remained popular for certain conditions.. and did not want to use slabs and box beams. AASHTO and the Prestressed Concrete Institute (PCI) published recommendations for standard shapes of prestressed concrete I-beams. box beam camber can result in the overlay being considerably thicker at the span ends than in the center. and 39 in. slabs. Current Status The use of AASHTO box beams is not recommended. Since 1983. but not issued. some box problems have been observed. Departmental research20 was conducted in an effort to develop an economical precast bridge. Construction Issues Construction experience is limited. Bridge Design Manual 7-62 TxDOT 12/2001 .Chapter 7 — Superstructure Design Section 17 — Prestressed AASHTO/PCI Box Beam Spans Design Recommendations Design would be similar to TxDOT box beams. Bridge Design Manual 7-63 TxDOT 12/2001 . Consideration was given to asphalt overlay or concrete slab. without voids. shear key or not. AASHTO and the Prestressed Concrete Institute published recommendations for standard shapes for prestressed slabs. section was later dropped in favor of a 5 ft. For more information on slab beams with shear keys contact the Bridge Design Section. wide section. Usage has grown over the years. especially for offsystem bridges. wide slabs. Round voids were indicated in the slab sections. It appears that slab beam structures are about the same cost as 20 in. overhang or not. and strand arrangement are shown for a Houston slab beam in Figure 7-20. None of these were constructed by TxDOT. type of bearing and lateral restraint. The Bridge Division has produced a limited number of custom projects with 4 and 5 ft. Most of these designs were for an asphalt overlay. along with I-beams and box beams. Current Status The Houston District went forward with simple rectangular sections without shear keys in 3 and 4 ft. They used shear keys and were post-tensioned together. widths. reinforcing. The 3 ft. Standards have been developed for these beams and are available from the Houston District. Dimensions. box beam bridges. These slabs could be cast on the same beds as the standard box beams without the problem of void movement. which is prevalent for the boxes. widths and with a cast-in-place slab that tied the sections together. In the 1980s the Houston District began experimenting with solid prestressed concrete slabs of 3 and 4 ft.Chapter 7 — Superstructure Design Section 18 — Prestressed Slab Beam Spans Section 18 Prestressed Slab Beam Spans Background In 1962. at the other end. Computer program PRSTRS1421 will design the slabs as non-standard.7. Design of the beams is according to the current AASHTO Specification. Houston District’s Prestressed Concrete Slab Beam Dimensions.) Design Recommendations (Houston’s Slab Beam Only) No shear key is required. from experience using beam width divided by 5. Slabs should be placed on two neoprene bearings at one end and one bearing. The slab is designed to be composite with the deck.5 is adequate for most designs.Chapter 7 — Superstructure Design Section 18 — Prestressed Slab Beam Spans Figure 7-20. Shear reinforcing in the slab beam is usually not required. with the following provisions: ♦ ♦ Slab beams should be Class H concrete of a strength required by the design. ♦ ♦ ♦ ♦ Bridge Design Manual 7-64 TxDOT 12/2001 . Reinforcing. #5 at 6 in. centered. Lateral restraint is not required. but a nominal amount of transverse reinforcing is provided on the standard. and Strand Pattern (Online users can click here to view this illustration in PDF. Separate problems are required for each width. strands should be used.5 in. Low-relaxation 0. Distribution of wheel loads can be according to the AASHTO Specifications for K=0. Class S concrete slab should be used. transversely and #4 longitudinal bars placed as shown on standards. Tie down reinforcing is required. however. A 5 in. with one mat of reinforcing steel. No longitudinal restraint is required. was marketed in Texas. No maintenance problems have been reported on the few bridges that were constructed. That is the total extent of Texas involvement with precast single tees for bridges. In the early 1960s the Lin Tee. The Bridge Division prepared a set of standard drawings in 1969 for single tees that were never used. The shape was offered as an alternate on a reservoir bridge in East Texas. They were also used to replace a bridge lost to flood on a farm-to-market road. Bridge Design Manual 7-65 TxDOT 12/2001 . which usually win bidding competition. Current Status Single tee beams are no longer used and are not recommended. Single tees. composed of Type A beams. were used for one project in Houston. but this type of construction never became desirable for Texas bridges.Chapter 7 — Superstructure Design Section 19 — Prestressed Single Tee Beam Spans Section 19 Prestressed Single Tee Beam Spans Background Precast single tee shapes are currently available from some fabricators. named after its famous originator. but was beaten by pan form girders. Two twin bridges on IH 10 near Van Horn were constructed in 1968 using these details with a reinforced concrete deck. One pedestrian underpass was constructed in Waco using this shape. The primary reason for the unpopularity of single tees was their instability during construction. with a slab section cast on top by the fabricator. Design Recommendations No design guidelines are needed. A series of standard designs and drawings were prepared in the El Paso District for precast single tee bridges. This method of bidding is no longer acceptable practice. another fabricator cast the job in a set of forms that are more adjustable. Plates welded to imbedded plates at 10 ft. the forms obtained by the fabricator could only be used in the range of 30 ft. The contractor did not have to commit to one type until after the letting. Meanwhile. a few more small bridges were constructed with double tees and reinforced concrete overlay. double tee beams with reinforced concrete overlay were selected over pan form girders and prestressed box beams. there was enough volume to justify the cost of forms for the double tees. This time. there were no double tee bridges constructed for awhile. Another shape was included in the standards at the request of a fabricator who owned those forms. configuration of beam flange connection shear key. Also. to reduce field-observed longitudinal deck cracking over flange Bridge Design Manual 7-66 TxDOT 12/2001 . but they went out of business before getting a bridge job. Unfortunately. The PCI shapes were adopted because of a better section for bridge loading. details of deck joint. consequently. Design Issues Design problems concern the choice of shape. although no fabricator in Texas had forms. TxDOT reduced this spacing to 5 ft. Standard drawings for precast prestressed concrete double tee spans with asphaltic concrete overlay were completed by the Bridge Design Section in 1985. in 1987. (In 1997. maximum spacing were used initially. Beam flange connection shear keys are subject to intuition rather than design.22 One fabricator sold double tees for several county bridges around Waco. depth. which covered all superstructure. On a long IH 37 river bridge widening project in 1984. Texas research in 1982 identified these shapes as possibly economical alternatives for short span bridges. spans.. Shortly thereafter.Chapter 7 — Superstructure Design Section 20 — Prestressed Double Tee Beam Spans Section 20 Prestressed Double Tee Beam Spans Background Double tee shapes were suggested by a PCI short span bridge publication in 1975. Efforts continued to foster competition for pan form girders. and type of overlay. The bid item was each bridge or each square foot of deck. Unfortunately for double tees. span length and 22 in. substructure. a similar design was the only alternative on another long river bridge widening in the same district. and efforts began to get them accepted for Texas highway bridges. one member of the family was usually pan form girders. and miscellaneous items. Finally. The concept was to offer a family of short span bridge types from which the contractor could select the most economical solution without the administrative complication of alternate bids. four small projects in the Corpus Christi District were constructed with double tee beams at the contractor’s option. which are hard to beat for 30 and 40 ft. The bridge had slight span length variations and flaring width sections which placed pan form girders at a disadvantage. Maintenance Issues Maintenance problems have now been observed in some bridges constructed by the 1980s-era standard details. Oklahoma has reported unsatisfactory performance of double tees with ACP overlay where de-icing salt is used. Diagonal stem cracking has been observed in some tee beams fabricated from the obsolete standards. The greater volume of Texas double tee usage has had reinforced concrete overlay. but the double tees are wider. The older flange shear connection between beams was found to be difficult to install and somewhat ineffective evidenced by the above-mentioned cracking in the riding surface. Total camber is theoretically slightly greater than corresponding depth box beams. One of the few problems concerns the fact that some fabricators have flexible flange bulkhead location methods. spans. Current details have improved constructibility in this area also.Chapter 7 — Superstructure Design Section 20 — Prestressed Double Tee Beam Spans joints. Fabrication Issues Fabrication problems. This caused some fabrication problems when metric width roadways and consequent hard metric converted flange widths were specified. In the 50 to 60 ft. Possible span-to-depth ratios are not as great as for box beams and only slightly greater than pan form girders for 30 and 40 ft. in the vicinity of the end diaphragm block-outs. which can cause variation in overlay depth and possible railing alignment problems. the earlier cracking will not cause severe problems since de-icing salt will not be used where most of these bridges have been constructed. Shear key installation is a potential problem because of placement tolerances and differential camber. along with additional improvements based on recently completed research should reduce or virtually eliminate this cracking. and 8 ft. which gives a potential for teetering. The sections are slightly heavier than corresponding depth box beams. span range. nominal widths. Bridge Design Manual 7-67 TxDOT 12/2001 . which Oklahoma now uses as standard practice. and. The first standards had asphaltic concrete overlay only. where others accommodate only the TxDOT 6. Construction Issues Construction problems have not been reported. This problem has been corrected in current details by reducing the block-out depth to be no deeper than the beam flange. Deck joints were detailed to control leakage better than the early box beam designs. but should be about the same as for box beams in the range of span and skew that have been used to date. to a lesser degree. 7. the depth of double tee spans is about equal to Type A beams with reinforced concrete decks. Hopefully. thus requiring fewer pieces per span.) Field performance will check the design enhancements. Cracking in the asphaltic concrete overlay. have been few. The 1997 connector spacing reduction. in the concrete decked versions has occurred. once the commitment is made to buy forms. There are four bearings under each section. but the shape is less stiff torsionally and better able to adjust. When asphaltic overlay must be used. Standard details for additional roadway widths will be produced and made available on the TxDOT website in the future. in length. Bearing pads for a range of span lengths are specified on current TxDOT standard sheets DTBMD-S and DTBMD-O. Standard shapes are recommended. is a “Type A” deck joint that provides an economical alternative to other methods for sealing joints at the ends of units up to 120 ft. Field observations of older structures indicated numerous instances of pad misplacement.. The maximum beam slope without special beam bearing considerations or end “pinning. overlay.) However. Standard shapes and section properties can be found in Figure 7-21. roadways in both an asphaltic concrete version as well as a Class S (Mod) concrete decked version.e. the designer should consider specifying a 2 ft. of course. It is of considerable importance. Currently detailed connection angles should be individually field fabricated for a snug fit. also. that the field inspection crews are made aware of the critical nature of the flange connections in insuring deck serviceability.Chapter 7 — Superstructure Design Section 20 — Prestressed Double Tee Beam Spans Current Status Standard details are currently available for 24 ft. but it will reduce maintenance costs and improve serviceability.5 percent. be careful to use a two-course surface treatment under the 2 in. adds a moderate amount of expense to the project. In this event. The construction time savings for the asphaltic deck option compared to the concrete deck option is more in terms of a few days rather than weeks under previous TxDOT criteria for strength and curing requirements. it is recommended that the concrete decked version be used to avoid serviceability problems associated with the increased deck cracking in the asphaltic version. for example) over all longitudinal joints to reduce deck cracking. Also detailed on these standards. Bridge Design Manual 7-68 TxDOT 12/2001 . width of joint reinforcing mesh (“Poly-Guard” or “Pave Prep” products. or “hanging out” from under stem edges or over cap edges. it should be noted that the asphaltic concrete deck option includes concrete diaphragms at the ends of units that are subject to curing and strength requirements similar to the concrete deck option. If the designer is considering the use of the asphaltic concrete version due mainly to a perceived construction time savings factor (i. There could conceivably be a situation where double tee beams with an asphalt overlay are desirable. assuming less concrete curing time for deck surface before traffic can be placed on it). Pad design criteria for max unit lengths to accommodate thermal expansion/contraction movement is specified on the previously mentioned standard sheets. (Class S (Mod) reflects a reduction in coarse aggregate size to improve consolidation in the flange shear key. This. Pad sizes from previous standards should be avoided to prevent problems associated with undersize bearing surface areas.” is 5. and quality welding employed. 5 26.5 13.79 24.) Beam Type Asphaltic Concrete Overlay 6T22 7T22 8T22 6T28 7T28 8T28 6T36 7T36 8T36 Reinforced Concrete Slab 6T21 7T21 8T21 6T27 7T27 8T27 6Y35 7T35 8T35 Width (ft) Double Tee Beam Section Properties Depth Yb Yt Area (in) (in) (in) (in2) I (in4) Weight (plf) 6.0 36.5 34.45 7.0 36. 22.21 6.286 56.19 15.0 22.052 26.00 6.5 26.79 11.00 6.Chapter 7 — Superstructure Design Section 20 — Prestressed Double Tee Beam Spans Figure 7-21.36 19.292 23.5 20.65 603 657 711 691 745 799 795 849 903 21.00 7.021 1.617 99.99 8.18 10.10 10.00 8.5 20.42.283 42.572 27.00 20.81 15.17 8.49 9.55 715 787 859 804 876 948 908 980 1.00 8.0 28.51 17.0 28.00 7.017 93.647 104.00 6.00 14. Double Tee Beams (See the following table of beam section properties. Online users can click here to view this illustration in PDF.90 25.51 18.08 11.325 89.71 23.88 6.096 6.585 745 820 895 837 912 987 946 1.00 8.81 6.881 46.0 36.577 54.00 8.85 6.0 14.00 7.00 7.50 6.32 23.00 8.99 18.0 22.75 11.62 14.965 109.00 7.64 8.33 17.00 22.110 51.25 24.00 8.0 28.86 19.140 22.5 34.159 628 684 740 720 776 833 828 885 941 Bridge Design Manual 7-69 TxDOT 12/2001 .511 44.937 29.5 26.942 84.5 34.14 8.00 7.21 11.51 8.00 6. 25 (for wheel load) has been adopted for beams with either reinforced concrete overlay or asphaltic concrete. for both deck option types. along with the following considerations: ♦ ♦ Reinforced concrete overlay should be 4. but design can easily be accomplished using the non-standard option and inputting the section properties as listed on the DTB-S or DTB-O standard sheets. a conservative live load lateral distribution factor (derived from recent research and in-house studies) of S/5. It may also be necessary to use this beam at interior locations such as when pedestrian sidewalks are specified and the traffic rails occur over interior beams. Fabricators of double tee beams in Texas currently do not drape strands. Debonding is not permitted in the bottom row of strands. Bridge Design Manual 7-70 TxDOT 12/2001 . Low-relaxation strands should be used. Basic design rules for other prestressed members apply also to double tees. and no more than 50 percent of the strands in one row or in the entire section may be debonded. to insure adequate rail impact strength.5 in. less than nominal width to allow for lateral growth.0 ft. Debonding occurs at 3 ft. fabricators should be advised to cast the beams 0. Full length debonding to accommodate production line considerations is not permitted. thick (min) with #5 at 6 in. transversely in the top layer and #4 at 9 in. but the magnitude of the problem will be limited by the relatively short span capability of the tees. Since the specifications are unclear. spacing. increments from the beam ends subject to maximum criteria in AASHTO. The distance from outside edge of deck slab to centerline of outside tee stem should not exceed 1. A total average stem width for both stems must be entered in the “non-standard” section properties area of the program input data to produce proper shear design. Unless the tee beam spacing can be adjusted. Possible slab edge treatments are shown in Figure 7-22.Chapter 7 — Superstructure Design Section 20 — Prestressed Double Tee Beam Spans Design Recommendations Standard double tee shapes are not resident within the PSTRS 14 program. Designs for standard span lengths from 30 to 60 ft.25 in. longitudinally in the bottom layer. ♦ ♦ ♦ ♦ Camber of double tees is significant. which are available on the TxDOT web site. wide beam on the outside of all bridge spans. This then makes it mandatory to place the 6 ft. are tabulated on the DTBSD-S or DTBSD-O standard sheets. Chapter 7 — Superstructure Design Section 20 — Prestressed Double Tee Beam Spans Figure 7-22.) Bridge Design Manual 7-71 TxDOT 12/2001 . Prestressed Concrete Double Tee Beams (Online users can click here to view this illustration in PDF. prestressed slab and girder span. to be constructed economically with prestressed beams. prestressed concrete I-beams. Texas’ first attempt at prestressing came in 1952 when two 30 ft. and the Sunshine Skyway Bridge in Tampa was contracted in 1951. The precast concrete industry was eager to help in the development. Bridge Design Manual 7-72 TxDOT 12/2001 . Tennessee.Chapter 7 — Superstructure Design Section 21 — Prestressed Simple I-Beam Spans Section 21 Prestressed Simple I-Beam Spans Background Nothing has been so beneficial to the economy and durability of Texas highway bridges as precast pretensioned concrete I-beams. Spans over 65 ft. and post-tensioned. standard shapes were being developed that were suitable for pretensioning or posttensioning. and California. The basic shapes of these early standard beams have remained the same through the years. It soon became obvious that steel could not compete economically. prestressed beam spans were offered to the contractors as alternates to continuous steel I-beam units. prestressing began in Europe. Like many good construction methods. William E. the standard details were fine-tuned to fit fabrication capabilities and a variety of geometric configurations. Prestressing development continued in Pennsylvania. Figure 7-23 gives a history of quantities and bid prices for prestressed I-beams since 1963. Highway construction was beginning to escalate. Of the bridges for which bids were taken during that period. Prestressed beams became the best choice for many crossover structures and stream crossings. along with the demonstrated ability of the prestressing industry to produce high-strength concrete. The arrival of prestressing was very timely. but the size and strength of prestressing strands increased and expertise in depressing strands developed. of 40 and 60 ft. Arrival of the AASHTO Type IV Beam.000 ft. The first significant prestressed bridge in the United States was the Walnut Lane Bridge in Philadelphia. heading toward the interstate boom that began in 1956. Steel beams were showing signs of unpredictable availability and escalating prices. For several years. were avoided for awhile. was an early champion of the method. using precast post-tensioned beams in the approach spans. It was contracted in May 1956 and contained 2. Dean. The Bridge Design Section chose to develop the capabilities of precast prestressed I-beams. designed under the supervision of Belgian Professor Gustave Mangel and contracted in 1949. The first significant beam bridge in Texas was over Corpus Christi Harbor. pan form spans were post-tensioned together to make a 60 ft. allowed spans of 130 ft. Cast-in-place post-tensioned bridges soon followed on the Dallas expressway. bridge engineer of the Florida State Road Department. precast on the job. approximately half contained prestressed concrete I-beams. The beams were of special shape. In the early years of interstate highway construction. Simultaneously. confidence in longer spans increased. 66. The AASHTO Type VI (Mod) was first offered on a Texas project in 1987. although there had been some previous usage in Texas. B. Details for AASHTO Type IV beams were prepared in 1968. Bridge Design Manual 7-73 TxDOT 12/2001 . but the other two have remained proprietary to Texas.Chapter 7 — Superstructure Design Section 21 — Prestressed Simple I-Beam Spans Figure 7-23.) Standard shapes were established in 1956. Types A. Only Types 54 and 72 have been used to any extent. Type A became the AASHTO Type I. 60. Bid Letting Statistics (Online users can click here to view this illustration in PDF. 54. and only in Texas. and C beams had larger bottom flanges to accommodate more strands. Types 36. 48. and 72 were developed simultaneously. 250K strands. While most of the beams are still sound. These beams were spread to a 10 in. The beams each had one straight pattern of 3/8 in. Beam shapes are the same as those established in 1956 and 1968. The Bridge Design Manual 7-74 TxDOT 12/2001 . Spans through 80 ft. bridges carried city streets over a series of railroad tracks. The 1971 standard spans and bents covered Type C and Type 54 beams with spans to 110 ft. 270K maximum strands. C. the Buena Vista and Commerce Street overpass in San Antonio. The beams now had two straight patterns each and an optional depressed pattern. The other end of the bridge was pretensioned. B. spans over the main tracks. Type 54M.. Many bridges were constructed to standard span and bent details. 250K strand with optional design parameters. It may be economical above 100 ft. and post-tensioned. The contractor for one end of a long ship-channel bridge in Houston cast Type 54 and Type 72 beams on the job and post-tensioned them. Consideration of this beam was recommended for stream crossing structures with spans between 100 ft. Until recently the bridges were still functioning but the decks are badly deteriorated. and Type IV beams in five roadway widths and three skews. Although theoretically unstable. fabricated of lightweight concrete. The Type C plus 6 in. The fabricator could design his own pattern in 1/2 in. One notable modified shape project. until they were issued in metric units in 1996. Standard spans and bents were not provided for AASHTO Type IV beams.. the Type 72 beam had been used for 140 ft. and span lengths were limited to 57. including 90 and 100 ft. spans. Modified Shapes Shapes have been modified occasionally for some special purpose..5 ft. if depth is unimportant. was used for a brief period for increased span length while matching the Type 54 for perceived aesthetics of the total bridge. the fat 54. was let in 1957 using Type C side forms. Economy was usually maintained by allowing standard side forms and a standard soffit. Type C depth was maintained throughout the bridge. web. and C beams in 1957. of depth was used on a few railroad underpasses. Span lengths were tailored to the geometric requirements for interstate highway crossovers at the time. Parallel 1. five roadway widths. the decks have now been replaced.Chapter 7 — Superstructure Design Section 21 — Prestressed Simple I-Beam Spans Standard Span and Bent Details Standard span and bent details were first developed for Types A. Span-to-depth ratio needed to be maximized because of congested conditions. Sometime between 1965 and 1968. Currently. A new series was distributed in 1960 that allowed only Type C beams and had spans to 65 ft. Post-tensioning was allowed for the standard beams for several years but was seldom used. end blocks were removed from pretensioned beams. B.. were covered in five different roadway widths. and 120 ft. An updated English unit standard followed in 1999. with more roadway widths. but many more required specially prepared details because of non-standard geometry. standards cover Types A. In 1965 the Type C beam design was shown as a depressed pattern using 7/16 in. and four skews.600 ft. A lightweight concrete deck was also constructed. After years of design using 20 percent loss and a lot of controversy. and shear threaten to complicate things more. Loss of prestress affects the bottom stress considerably. Because of the lateral instability the length was limited to 100 ft. The key item is stress in the bottom flange at mid-span. Some fabricators bend their own reinforcing to maintain control of the tolerances. The condition of the anchorage chucks appears to be the key to eliminating strand breakage. Some difficulty has been experienced verifying the properties of low relaxation strand. Calculated camber due to prestressing was about twice the amount that actually occurred. Design Issues Design problems have been few. Texas was reluctant to accept certification. but the structure was completed. There were problems with fit of abutting pieces and with gluing the joint. the stresses associated with this deflection will Bridge Design Manual 7-75 TxDOT 12/2001 .Chapter 7 — Superstructure Design Section 21 — Prestressed Simple I-Beam Spans beam was also adaptable to railroad underpasses in the 70 ft. One experimental bridge in Sherman. and external vibrators alone are not sufficient. Proper vibration of concrete is necessary to prevent honeycomb. Hold-down hardware must be released prior to release of strand tension to prevent damage as the beams move along the soffit. spans that were Type 72 beams match cast in three sections and post-tensioned together on the ground beneath the structure. contained two 150 ft. Concerns about strand fatigue. but they were usually solved through the perseverance of the fabricator. debonding strands. Depressing of strands into a trapezoidal pattern was developed through considerable effort and ingenuity by the fabricators. There have been few problems with prestressing strand. Larger span-to-depth ratios can be obtained with cast-in-place concrete units and continuous steel beams. Slab weight deflection left the beams with a slight sag. The complication of this method is considered to outweigh any economical advantage. it has not been possible to predict camber with any consistent accuracy. so a 30 minute relaxation test was devised to run on lot samples in the Materials and Tests Section laboratory. Texas. If the number of depressed strands and the angle of depression is great. span range. which can be lethal during fabrication because of the extreme length of stressed strand. Congestion of reinforcing steel in end regions was a problem until equitable details evolved from trial and error. the Materials and Tests Section. There has been some controversy regarding the accuracy of the test. but it appears adequate to distinguish between low relaxation and stress relieved strand. In the design stage. The specification allowable has gone from zero to about 400 psi tension and in 1986 was threatening to go back again. the specification was equipped with a complicated loss formula. Fabrication Issues Fabrication problems have been many. Bent bars are kept as small as possible with strict tolerances on bending radii. Shear considerations also became complicated in the 1981 interim specification. but economy and ease of plan preparation made prestressed beams the favorite. and the Bridge Design Section. Release of the hold-downs causes the beam to deflect upward. particular attention was directed to this problem. Strand wrapping itself can cause splitting if not properly done. Unless the beam is obviously beyond hope. and insufficient concrete strength. Dead weights help. Some horizontal end cracking has been observed in the face of bottom flanges. cracking. A system of selective strand wrapping near the ends was devised that effectively eliminates this problem. misplaced reinforcing steel. Periodically. deep with 16 in. Cracking without end blocks was seen to be very similar to transfer zone cracking in beams with end blocks. For some time. or weather. If the Materials and Tests Section inspectors reject a beam for non-conformance. considered acceptable after proper repair. Typical deficiencies are honeycomb. wide top and bottom flanges. Usually the lapse can be attributed to aggregate strength. the beam was later limited to 95 ft. the fabricator may request a structural review. Construction Issues Construction problems have occurred mostly with the Type 54 beam. by TxDOT policy. This beam also tends to excessive lateral deflection (sweep) after erection. However. even the good areas appear to fall into a slump in strength. adjustable external stiffening devices (hog rods) have been required for Type 54 beams over 96 ft. although some areas of the state have some difficulty. A system of structural review is in place that allows the fabricator some recourse on beams that are not fabricated in conformance to the plans and specifications. the deficiencies will be recorded and forwarded to the Bridge Construction Section for review. but there have been times when the reason remained unknown. creating a dangerous situation. Concrete strength is not generally a problem. When end blocks were eliminated. Bridge Design Manual 7-76 TxDOT 12/2001 . Another problem occasionally associated with upward deflection is corner cracking at the bottom ends of beams as they drag along the soffit during release. This problem has been solved by adding dead weight to the beams before release of hold-downs. Cracking in the transfer zone at the beam ends has always been a possibility. long. It had been observed that the beam becomes unstable at lengths greater than 102 ft. These beams are more susceptible to breakage during hauling and on several occasions have broken during the erection process. or rejected. or a design consultant.Chapter 7 — Superstructure Design Section 21 — Prestressed Simple I-Beam Spans cause cracking in the top of the beam over the hold-downs. The cracks are usually small and horizontal at the flange-to-web juncture or diagonal in the web such that they tend to close under installation conditions. and thin slick sheets inserted between beam and soffit before release have been used effectively to prevent further cracking. The beam is 54 in. which tends to lateral instability in long lengths.. Beams outside of the specification tolerance for sweep must be field straightened and held until the diaphragms and/or slab are placed. cement condition. will make a determination of adequacy based on comparison of the deficiency with detailed design requirements. The beams may be accepted as is. The Bridge Construction Section. Originally designed for spans up to 110 ft. It appears to be associated with a lack of reinforcing steel in that face and aggravated by prolonged storage on supports located some distance from the end of beam. Taping alone is insufficient. this is difficult to predict. Acceptance without repair usually requires a return of a little money to the state as recompense by the fabricator to justify acceptance of a beam in non-compliance with the specification. Bolted steel interior diaphragms have been allowed for some time and. deteriorated due to salt exposure. seem to indicate that even prestressed I-beams are not immune to destruction by salt water. Long-Term Performance Issues There has been no evidence observed in the field to indicate design deficiencies in flexure or shear. where the strands are thus exposed. A 25-yearold pretensioned county bridge close to salt water near the gulf coast was found to be approaching the threshold level of chloride 2 in. beneath the beam surface. Extreme camber and camber differential between adjacent beams has caused construction difficulties. Depending on how many strands are broken. but there was no evidence of strand rusting. Torsion in the outside beam due to deck slab placement in the overhang was a problem until a system of bracing was devised and required by the specifications. This. concrete would be cast directly on the cap. Thickened slab ends became standard in 1996. resulting in the patch cracking away from the beam end. End diaphragms are now of such configuration that they may be formed and placed with the deck slab. Damage is usually confined to concrete cover over the bars or strands. This became the bearing under load. Leakage of salt-laden water through the deck joints has caused deterioration of caps and columns.23 Girder impact damage is more severe with the use of interior diaphragms. Also the patch. being deck concrete. The patch was supposed to be insulated from the top of the cap but. Lately. involving a whiskey truck. the beam may be patched with gunite or dry packed mortar or removed and replaced. Highway crossover beams get hit by overheight loads frequently. interior diaphragms have often been omitted. lately. a flammable load will burn beneath a bridge. There is a bridge known to have existed for 15 years on an interstate highway with prestressed beams that have an inventory rating of H6 due to a recently discovered design error.Chapter 7 — Superstructure Design Section 21 — Prestressed Simple I-Beam Spans Diaphragms have always been a source of irritation to the contractors because of the expense of forming. two prestressed beams were replaced because of soft concrete in the web near the beam ends. has been noted. unless careful attention was given. and other observations. severe damage to beams has been noted in a long salt-water crossing at Galveston Bay. Laboratory tests Bridge Design Manual 7-77 TxDOT 12/2001 . In the late 1980s TxDOT discontinued the use of interior diaphragms except on unusually long and unstable beams. Occasionally. Speculation was that strand rusting would be just a matter of time. Embedded large mild steel anchor bars were rusting. Deck slabs have deteriorated and been replaced over beams that are still in good condition. Slight deterioration of the beams. A research project conducted by the Center for Highway Research (Report #158-1F) concluded interior diaphragms were not effective. but proven relief measures have yet to be found. No cracking or unusual deflection under load has been observed. Maintenance Issues Maintenance problems through 1990 have been few. in which case steel interior diaphragms are typically specified. short of the deck joint centerline and was completed by a concrete patch cast with the deck. After one such accident. The precast beam stopped about 6 in. The 1957 standards had a detail that was unsightly. but the high-strength prestressed concrete appears to be very durable. Greater strength requirements should be restricted to special projects. diameter is approved and may be accepted in optional designs. dead load deflection. What does stand out as a design deficiency is an inability to predict deflections.000 psi in workable batches of concrete. Low-relaxation strand is readily available and economical. Overlength permits are readily available for beams 150 ft. It should only be required for special projects.24 Beam camber. including widenings and railroad underpasses. This beam should only be used to widen existing bridges that used Type 54 beams. ♦ Type 54 beams are no longer recommended for use. The economy of the Type C and Type IV have made the Type 54 obsolete. The research also demonstrated that an uncracked beam would soon crack under repeated flexure to a stress near the modulus of rupture of the concrete. have been successfully transported. In rural areas. AASHTO Specifications have not been revised as a result of the research. To achieve this. Low relaxation strand of 0.6 in.500 psi at 28 days are feasible for usual designs. Subsequent extensive research at the University of Texas25 verified that fatigue failure is possible in the strands of a cracked pretensioned beam subject to repeated flexure. Competition is such that predictions of which fabricator will sell beams for a particular project are unreliable. through research. long Type IV beams may cause transportation problems. beam lengths. and longtime composite deflection are far from consistent with analysis methods currently employed. It now appears that prestressed concrete manufacturers can achieve strengths in excess of 12. has been developing ways to produce higher strength concrete for several years. but at a premium cost. Texas and other states have sponsored Bridge Design Manual 7-78 TxDOT 12/2001 . and shear fatigue. Concern over strand fatigue erupted in 1978 when a PCA testing project for the Louisiana DOT produced premature fatigue failures in full-size pretensioned beams. Quality beams are readily available from several competitive sources. Transportation costs are apparently not significant. The standard strand for prestressed beams is 0. ♦ ♦ ♦ ♦ ♦ ♦ TxDOT.5 in. This beam should only be used to widen existing bridges that used Type 72 beams. shear. additional prestressing is desirable. but they have yet to show up in the field. Field performance of prestressed beam spans has given no indication of this problem.Chapter 7 — Superstructure Design Section 21 — Prestressed Simple I-Beam Spans continue to identify weaknesses in flexural fatigue. Concrete strengths of 6. Type 72 beams are no longer recommended for use. long. They are suitable for most geometric conditions and structure types. Beams up to 150 ft. They clearly result in the most economical and durable bridge within their span capability. 270K low-relaxation. AASHTO Type VI (Mod) beams are now economically available in Texas. Transportation is not a deciding factor in urban areas up to 130 ft. To utilize this strength effectively. Current Status Prestressed concrete beams continue to be used in about 45 percent of Texas’ bridges.500 psi at release of prestress and 8. 602 260.4 22.4 360.000 psi to construct a 153 ft. diameter prestressing strands. Details and section properties for all available standard prestressed I-beams are shown in Figure 7-24.6 in. B.403 Wt/ Lf lbs 287 375 516 821 Bridge Design Manual 7-79 TxDOT 12/2001 . grid spacing. Standard Prestressed Concrete Beams – Types A. Online users can click here to view this illustration in PDF.09 24.9 788. and IV (See the following table of beam dimensions and section properties. Figure 7-24. Prestressing strand constraints are shown in Figure 7-27and Figure 7-28.3 494.658 43. span with Type IV beams.) Beam Type A in B in C in Beam Dimensions and Section Properties D E F G H W Yt Yb in in in in in in in in Area in2 I in4 A B C IV 12 12 14 20 16 18 22 26 5 6 7 8 28 34 40 54 5 5¾ 7½ 9 11 14 16 23 3 2¾ 3½ 6 4 5½ 6 8 6 6½ 7 8 15.39 19.93 17.25 12. The result has been approval by the FHWA to use 0.75 275. and Figure 7-26.6 in. C.07 22. Figure 7-25.91 29.177 82. A recent project in San Angelo utilized high strength concrete with a concrete strength of 14. Specific design conditions and experience will decide the economy or desirability of using this capability.61 14. diameter strands in a 2 in.Chapter 7 — Superstructure Design Section 21 — Prestressed Simple I-Beam Spans extensive research toward the use of 0. Online users can click here to view this illustration in PDF Beam Type A in B in C in D in Beam Dimensions and Section Properties E F G H W Yt Yb in in in in in in in Area in2 I in4 54 72 16 22 16 22 8 11 54 72 5 7½ 30 40 ½ 5 7½ 4 5½ 6 7 28.47 38.4 863.Chapter 7 — Superstructure Design Section 21 — Prestressed Simple I-Beam Spans Figure 7-25.53 33.060 Wt/ Lf lbs 514 899 Bridge Design Manual 7-80 TxDOT 12/2001 .73 493.27 52.4 164.022 532. Standard Prestressed Concrete Beams – Types 54 and 72 (See the following table of beam dimensions and section properties. 4 670.54 36. Standard Prestressed Concrete Beams – Type VI (Mod) (See the following table of beam dimensions and section properties.) A in B in C in Beam Dimensions and Section Properties for Type VI (MOD) Beams D E F G H J K W Yt Yb Area in in in in in in in in in in in2 I in4 40 26 8 72 10 42 3 5 4 13 6 35.Chapter 7 — Superstructure Design Section 21 — Prestressed Simple I-Beam Spans Figure 7-26. Online users can click here to view this illustration in PDF.351 Wt/ Lf lbs 980 Bridge Design Manual 7-81 TxDOT 12/2001 .46 940. Chapter 7 — Superstructure Design Section 21 — Prestressed Simple I-Beam Spans Figure 7-27. Standard Strand Information – Types A, B, C, 54, and 72 (Online users can click here to view this illustration in PDF.) Bridge Design Manual 7-82 TxDOT 12/2001 Chapter 7 — Superstructure Design Section 21 — Prestressed Simple I-Beam Spans Figure 7-28. Standard Strand Information – Types IV and VI (Mod) (Online users can click here to view this illustration in PDF.) Design Recommendations The various types of standard prestressed concrete beams and their recommended usage is as follows: ♦ Type A - Depth 28 in. Used primarily for widening old concrete and steel spans for compatibility with existing depth. Reasonable span limit is about 50 ft. Type B - Depth 34 in. Used for widening and for new structures where depth is important. Reasonable span limit is 65 ft. 7-83 TxDOT 12/2001 ♦ Bridge Design Manual Chapter 7 — Superstructure Design Section 21 — Prestressed Simple I-Beam Spans ♦ Type C - Depth 40 in. The dominant beam of the group. Over 6 million linear feet used since 1963. Suitable for widening, grade separations, stream crossings, pedestrian underpasses, and shortspan railroad underpasses. Economical span limit about 75 ft. Maximum recommended span limit 90 ft. Types 36, 42, 48, 60, and 66 Forms are not available for these beams. They should not be used. Type 54 - Depth 54 in. Historically the second most popular beam, over 3 million linear feet have been used since 1963. However, because of lateral instability, construction problems, and cost, this beam is now considered obsolete. Note: Preferably, this beam should not be used at all. However, it may be used for widening at existing Type 54 spans. The Type IV beam can also be used to widen existing Type 54 bridges and may prove more desirable. Type IV - Depth 54 in. Since 1986 this has been the dominant beam. Over one million linear feet were used 1986 through 1988. This is a tough stable beam, and it is recommended for span lengths up to 130 ft. Type 72 - Depth 72 in. Because of lateral instability, construction problems, and cost, this beam is now considered obsolete. Note: Preferably, this beam should not be used at all. However, it may be used for widening at existing Type 72 spans. The Type VI (Mod) beam can also be used to widen existing Type 72 bridges and may prove more desirable. Type VI (Mod) - Depth 72 in. This is a newcomer. The wide top flange improves lateral stability. The Type VI bottom flange allows a maximum number of strands. The beam has 175 ft. span capabilities but has been limited to 150 ft. due to handling constraints. This beam is recommended for spans greater than 130 ft. ♦ ♦ ♦ ♦ ♦ The suitability of prestressed concrete I-beams for railroad underpasses is severely limited by the requirement of 18 in. clearance between flanges imposed by one railroad company and a 12 in. requirement by others. Designers should check with the Bridge Design Section for the latest information. For grade separation structures the same beam depth should be used for the full length of structure, for aesthetic reasons. Stream crossing structures may have different types and sizes of beams if economy so dictates. Beam spacing should be optimized in each span. There is no significant advantage to maintaining a constant beam spacing for the full length of structure. Selection of the proper type of beam for a span is a matter of economics. Relative costs should be calculated using current average bid prices for beams and slab. Concrete strength maximums may be taken as 6,500 psi release and 8,500 psi design, but for large volume structures, verification with a prospective fabricator is advisable. High strength is not achieved without some extra cost, which will be unknown to the designer. Unfortunately for the fabricator, these costs are often lost in the bidding competition. Bridge Design Manual 7-84 TxDOT 12/2001 Chapter 7 — Superstructure Design Section 21 — Prestressed Simple I-Beam Spans Beams should be designed for 0.5 in. low-relaxation strands using the computer program PSTRS14. Optional design parameters for maximum top flange stress, bottom flange stress, and ultimate moment due to all design loads should be shown on the plans for each design. The fabricator will retain the option to use other strand arrangements, including straight strand patterns, stress relieved strand, or 0.6 in. diameter strand, provided the design parameters are satisfied by the prestress and concrete strength selected. Although PSTRS14 allows considerable liberty in selecting design controls, the following practice is recommended for standard designs: ♦ ♦ ♦ ♦ ♦ ♦ Strands should be added and depressed in the order shown on standard drawing IBNS. See Figure 7-27 and Figure 7-28. Hold-down points are shown on the standard to be 5 ft. or .05 span length (if greater) either side of mid-span. Fabricators are allowed -0, +2 ft. tolerance from this. Strand stress after seating of chucks will be 0.75 f 's for low-relaxation strands. Section properties given on the standard drawing and built-in to the program will be used. Section properties of the beam should not be increased to account for the transformed area of strands or mild steel. Composite section properties should be calculated assuming the beam and slab to have the same modulus of elasticity (for beams with f 'c < 7,500 psi) with no haunch between top of beam and bottom of slab. Note: The previous two items tend to compensate each other and simplify the design process. ♦ ♦ ♦ ♦ Live load distribution should be S/5.5 wheels for moment and for shear. The program will iterate to the required number of strands using loss calculations in accordance with the 1989 AASHTO Specifications. The primary control will be final stress in the bottom of beam at midspan. Tension in the amount of 6 f' c will be allowed. The required f 'c is calculated based on the following: a. The compressive stresses under all load combinations, except as stated in (b) and (c), shall not exceed 0.60 f 'c. b. The compressive stresses due to effective prestress plus permanent (dead) loads shall not exceed 0.40 f 'c. c. The compressive stresses due to live loads plus one-half of the sum of compressive stresses due to prestress and permanent (dead) loads shall not exceed 0.40 f 'c. Final stress at the bottom of the beam at the ends will not be checked. ♦ Release strength will usually be controlled by the compressive stress after release at the bottom center of beam. The effective strand stress after release will be 0.75 f 's-ES-0.50 CRs for low-relaxation strands. Bridge Design Manual 7-85 TxDOT 12/2001 Chapter 7 — Superstructure Design Section 21 — Prestressed Simple I-Beam Spans ♦ The end position of depressed strands will be as low as possible to prevent control of the release strength, or higher if necessary, to limit top tension to 7.5 f ′c . Occasionally, release strength will be controlled by end conditions when the depressed strands have been raised to their highest possible position. Required stirrup spacing is calculated for #3 Grade 60 bars according to the AASHTO 1989 Specification. Stirrup spacing according to ACI 318-89 is also shown but AASHTO governs. Spacings are given at the bearing, one-half the composite depth and the twentieth point of the span, where principal tension is likely to control the shear carried by the concrete. At the tenth points of span, shear at inclined cracking will likely be the concrete contribution, but usually the nominal maximum spacing will control. The required stirrup spacing at the bearing will always be satisfied by the splitting reinforcement in the transfer zone. The requirement at h/2 should be satisfied from the end of splitting reinforcement to the 1/20 point and so on. This method of shear consideration was adapted from equations introduced in ACI-31871. It may be complicated, but it is the official AASHTO method and, according to recent research, predicts shear capacity as well as other known methods. Horizontal shear between beam and slab may be ignored for standard beam details in highway bridges that are considered to be in compliance with the AASHTO horizontal shear specifications. Camber is calculated using an adaptation of the computer solution.26 This method has been observed to predict average camber within tolerable limits. More accurate methods may be justified for unusual conditions if the more important parameters affecting long-term camber can be controlled.27 Deflections due to slab weight and composite dead load are based on the input value of modulus of elasticity of the beam (5,000,000 psi is recommended). This should be shown on the plans although field experience indicates actual deflections are generally less than predicted. Use this deflection times 0.8 for calculating haunch depth. When precast concrete deck panels are allowed, the beam should be designed using the basic slab thickness, except in rare cases. This is considered justified by successful testing.28 When stay-in-place metal forms are allowed, the design slab thickness may be used for beam design. Additional dead load of concrete required because of the corrugations is not considered. Thickened slab ends should be detailed at the ends of each simple span or at the end of each unit. End diaphragms will usually be an option of the contractor. See Figure 7.29. Intermediate diaphragms are not required except for erection stability of Type VI (Mod) beams or other beam sizes stretched beyond their normal span limits. ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ Bridge Design Manual 7-86 TxDOT 12/2001 Chapter 7 — Superstructure Design Section 21 — Prestressed Simple I-Beam Spans Figure 7-29. Slab End Details – Prestressed Concrete Beam Spans (See following explanatory notes. Online users can click here to view this illustration in PDF.) Explanatory Notes for Figure 7-29 Many structures have been built successfully without a thickened slab end at inverted T bents. The stem of the inverted T is assumed to act as support for the slab edge. However, this remains a controversial issue. On several projects, particularly those with wide inverted T stems, a modified thickened slab end has been used. Its length is usually half of the stem plus 4 feet into the span. This tends to be a local preference issue and the designer is encouraged to contact the Bridge Design Section or the district bridge design office for recommendations. Bridge Design Manual 7-87 TxDOT 12/2001 Chapter 7 — Superstructure Design Section 22 — Prestressed Cantilever/Drop-In IBeam Spans Section 22 Prestressed Cantilever/Drop-In I-Beam Spans Background To increase the span length capability of prestressed concrete beams, a few bridges have been designed with sections extending past the interior bents to support a simple span. Thus the span capability of a particular beam type could be increased by the sum of the two cantilever lengths and the structure remain determinate. The first design used variable depth pretensioned girders as the cantilever member. They were to be cast and hauled upside down, righted during erection, and connected across the center by standard prestressed beams notched to fit the cantilever connection. This produced a 110 ft. span with Type C beams. It was offered as an alternate to a continuous steel girder unit on a Brazos River bridge, but was not selected for construction. The first unit constructed had cast-in-place reinforced concrete cantilevers with drop-in 75 ft. Type C beam spans. This project resulted in an 87.5-100-87.5 ft. unit across the Bosque River at Iredell. The next variation had 85 ft. cast-in-place concrete girders cantilevered past the interior bents with 90 ft. Type C beam drop-in. This was a 62-116-62 ft. interchange structure in Waco. An interesting and aesthetically pleasing variation was constructed on a long interstate elevated highway in Temple. Notched Type C beams were used, but the matching cantilever sections were cast-in-place bent caps, flush with the bottoms of beams. Every third bent was a drop-in situation with deck expansion joints. The two interior bents were cast around the beam ends while they were supported by falsework. The type that followed, and was used in several bridges around the state, consisted of standard shape pretensioned beams cantilevered across the interior bent and notched to support the same beam shape as a drop-in. The usual beam was Type IV and the longest span achieved was 165 ft. with a 135 ft. drop-in span. Later, cantilever/drop-in Type 72 beams were chosen by the contractor over 150 ft. simple spans using Type 72 or Type VI (Mod) beams, for a coastal pleasure boat channel crossing near Kemah. The most severe use of this type of bridge was the Cypresswood Drive Overpass, designed by others, in Houston. The spans were 113-188-123 ft. using Type VI (Mod) beams with a 140 ft. drop-in span. The system has also been used for widening continuous steel girders. Design Issues Design of these units is tedious since the cantilever section pretensioning must be located low to resist positive moments within the span and high to resist negative moment at the interior bent. The problem is compounded by the fabricator who, in order to check the work, Bridge Design Manual 7-88 TxDOT 12/2001 Chapter 7 — Superstructure Design Section 22 — Prestressed Cantilever/Drop-In IBeam Spans invariably wants to redesign the strand pattern, creating another task for the designer. The notched end has always been a concern of designers, since the detail is not subject to accurate rigorous analysis. A conservative detail was suggested by the Fort Worth District, successfully laboratory tested at the University of Texas,29 and became the official detail for the state. A sample of this detail is shown in Figure 7-30, Figure 7-31, and Figure 7-32. Early designs had deck expansion joints at the cantilever ends, but later units were made continuous, in the deck slab only, by continuing the longitudinal slab reinforcing through the joint. Longitudinal reinforcing over the interior bents was designed for negative live load moment. Figure 7-30. Example of Cantilevered I-Beam Details – Type IV Beam (Online users can click here to view this illustration in PDF.) Bridge Design Manual 7-89 TxDOT 12/2001 Chapter 7 — Superstructure Design Section 22 — Prestressed Cantilever/Drop-In IBeam Spans Figure 7-31. Example of Drop-In I-Beam Details – Type IV Beam (Online users can click here to view this illustration in PDF.) Bridge Design Manual 7-90 TxDOT 12/2001 . Chapter 7 — Superstructure Design Section 22 — Prestressed Cantilever/Drop-In IBeam Spans Figure 7-32. Embedment of reinforcing and steel plates used at the notched ends created difficult forming and concreting problems. Bridge Design Manual 7-91 TxDOT 12/2001 . The required number of strands was usually large and concrete strengths on the high side.) Fabrication Issues Fabrication was complicated by the need to deflect a group of strands downward in the end span and upward over the cantilever support. Example of Cantilever/Drop-In I-Beam Details – Bearing Seat and Strut Assembly (Online users can click here to view this illustration in PDF. where economy and avoidance of structural steel is more important than aesthetics. Bridge Design Manual 7-92 TxDOT 12/2001 . Prestressed concrete deck panels are usually not allowed in negative moment regions because of insufficient clearance for large-diameter longitudinal slab bars over the cantilever support. Current Status Cantilever/drop-in prestressed concrete beam units are no longer recommended. Various span arrangements that have been used are shown in Figure 7-33. Elastomeric bearings required adjustment to fit the erected slopes of distorted cantilever and drop-in bearing surfaces. the following information is provided if the need for cantilever /drop-in prestressed concrete beam units is unavoidable. and Type VI (Mod) beams may be used for spans between 150 and 190 ft. Type VI (Mod). Slab forming adjustment was also aggravated. and 150 ft. Problems continue with misalignment of the notch surfaces. but this can be mitigated with proper forethought. or structural steel spans should be more economical and are preferred from a long-term durability and maintenance standpoint. The use of HPC simple spans. Cantilever/drop-in prestressed concrete beam units using Type IV beams may be used for spans between 130 ft. However.Chapter 7 — Superstructure Design Section 22 — Prestressed Cantilever/Drop-In IBeam Spans Construction Issues Construction posed no particular problems except for one unit that developed pronounced deformation due to prestressing in the cantilever section. ) Design Recommendations If there is an appropriate occasion to use this structure type. Cantilever/Drop-In Precast Pretensioned Concrete Beam Spans (Online users can click here to view this illustration in PDF. the design should closely follow the AASHTO Specifications with the following considerations: ♦ ♦ Type IV beams or Type VI (Mod) beams should be used for grade separation structures.Chapter 7 — Superstructure Design Section 22 — Prestressed Cantilever/Drop-In IBeam Spans Figure 7-33. End blocks will be required on the notched ends of prestressed beams to provide space for bearings and notch reinforcement. 7-93 TxDOT 12/2001 Bridge Design Manual . Concrete strength for the cantilever beam will usually be controlled by compression in the bottom flange over the support. shears. similar to details shown in TxDOT Standard IBMS.500 psi. Shear effects should be investigated according to the AASHTO Specification. Camber in the cantilever section may be approximated using BMCOL51 and the multipliers in “A Rational Method for Estimating Camber and Deflection of Precast Prestresseed Members. and deflections due to the eccentricity of prestressing can also be approximated with BMCOL51 by inputting end moments and effective vertical forces at strand deflection points. Stress at tenth points of the span must be calculated to ensure adequate distribution of prestress. and Figure 7-32 and should conform to the requirements in “Optimum Design or Reinforcement for Notched Ends of Prestressed Concrete Girders. Computer program BMCOL5131 can be used to calculate moments. Grade 60 steel should be used. Cantilever beams should be considered pinned at the notched ends.5 wheels to each beam. and deflection due to external loads in the cantilever section. Deck expansion joints are not recommended at the notched ends. Live load distribution should be S/5.”32 and “Time Dependent Deflections of Pretensioned Beams. End bearings must be designed to accommodate the movement caused by temperature change. shears. Figure 7-31. Moments.”30 Standard elastomeric bearings may be used at the drop-in supports. Design concrete strength should generally be limited to 8.Chapter 7 — Superstructure Design Section 22 — Prestressed Cantilever/Drop-In IBeam Spans ♦ Notch reinforcement should be similar to the detail shown in Figure 7-30. should be required to control the crack. A deck construction joint and overhang chamfer.”33 ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ Bridge Design Manual 7-94 TxDOT 12/2001 . Interior bearings should be designed for the heavier-than-standard loads that will exist. Prestress losses may be hand calculated separately at each maximum moment location. Deck slab reinforcement over the cantilever support should be sized to help resist ultimate negative moment. the potential joint should be restrained by continuous longitudinal deck slab reinforcement. Instead. “Design of Continuous Highway Bridges with Precast. Computer program PSTRS 13 was developed on a research project documented in “Automated Design of Continuous Bridges with Precast Prestressed Concrete Beams. Design was more complicated than for simple spans because the unit was indeterminate for live load. Continuity was provided by mild steel reinforcing in the deck slab across the bent with the beams connected in the compressive zone through cast-in-place diaphragms. Prestressed Concrete Girders. the use of prestressed beams continuous for live load was being abandoned. To resist pullout of the beam bottom due to shrinkage. large reinforcing bars from each adjacent beam were extended past each other and bent upward in the diaphragm.Chapter 7 — Superstructure Design Section 23 — Prestressed Continuous for Live Load I-Beam Spans Section 23 Prestressed Continuous for Live Load I-Beam Spans Background Most problems with simple prestressed concrete beam spans are caused by leakage through the deck joints at the ends of each span. The Waco District has used live load continuity for several bridges in excess of 300 ft. Some units with inverted tee caps had the flange of the cap reinforced to carry beam weight.”35 but its treatment of creep and shrinkage was unsatisfactory. range of continuous length. Fabrication Issues Fabrication problems increased because of the extra hardware to be installed and allowed to protrude from the beam end. By the time this program was debugged and updated for production. Others required the flange to be supported on falsework until the beams were erected and stem cast and cured. Later designs had an inverted tee bent with the stem cast around the beam ends. This detail was considered necessary because of the limited distance between adjacent beam ends. With the stem width available. prestressing strands from each adjacent beam were sometimes extended for anchorage. This can be satisfactory if proper provision is made for expansion at the ends of the unit. in length with the bridge ends fixed by doweling the bridge slab to the approach slab and the approach slab to the abutment wing walls. and temperature-induced positive moment. In an attempt to circumvent this problem. The large-diameter bars had to be offset in adjacent beams to avoid fouling. continuous for live load designs were introduced in the early 1960s. Accurate determination of the effects of time-dependent deformations on the continuous unit was not possible with design methods available at the time. They are performing reasonably well. Improper provision for expansion will lead to distress. but it was complicated for production design and subject to the usual scatter of prestressed beam camber and deflection behavior. This was a source of error that often resulted in fouling of the bars going Bridge Design Manual 7-95 TxDOT 12/2001 .”34 provided a rational method for computing shrinkage and creep effects. The Bridge Design Section has designed a few such units in the 700 ft. creep. performance is not influenced by the presence or absence of a positive moment connection over interior bents. also. Design of precast prestressed concrete beam structures continuous for live load is not recommended. Details are shown in Figure 7-34. Beams subject to significant camber growth were allowed to rotate with respect to the diaphragm causing cosmetic spalling. Even after insulation of the interface was emphasized. which is currently recommended. which may lead to excessive camber growth. Try to avoid extreme camber after dead load deflection. a continuous for live load unit would be similar to a continuous deck only unit. nor does it occur on all regular caps. but several unsightly conditions exist because of inability to predict and accommodate time-dependent deformations of the beams. Construction Issues Construction problems involved the diaphragm between beams. Rectangular caps may be used with a large interior diaphragm for continuity connection. Segmental deck placement was more restrictive than for simple spans.Chapter 7 — Superstructure Design Section 23 — Prestressed Continuous for Live Load I-Beam Spans unnoticed until the beams were erected. Design Recommendations Although not recommended for new construction. It was soon evident that the contact surfaces of beam sides and diaphragm required insulation to prevent future beam end rotation from spalling the diaphragm. Inverted tee caps added a measure of awkwardness to the construction schedules. The project was delayed while the bars in one beam were cut off and drilled and grouted in the proper position. Current Status The Bridge Design Section has decided that the disadvantages outweigh the advantages of designing continuous for live load. 7-96 TxDOT 12/2001 ♦ Bridge Design Manual . One inverted tee unit has been given a concrete overlay because of severe slab cracking over the bent. This problem was not manifest with inverted tee caps. the following suggestions are given for consideration if there is an appropriate occasion to use this structure type: ♦ Use beams that are easily capable of the span length and beam spacing used. The cracking appeared to be related to restraint of the large-diameter continuity bars on the plastic deformations of the concrete as the placement progressed. several separation structures on a major highway bypass developed the symptoms before the project was opened to traffic. depending on the age of the beams when the slab is cast. because the stems could not be cast until the beams were erected. Recent research36 reported that anywhere between 0 and 100 percent continuity for live load may be obtained. Without a positive moment connection. Maintenance Issues Maintenance has not been extensive. ♦ ♦ ♦ ♦ ♦ ♦ Bridge Design Manual 7-97 TxDOT 12/2001 . Recommended details for positive moment reinforcing of both types is shown in Figure 7-36. Design without consideration of creep and shrinkage is recommended.Chapter 7 — Superstructure Design Section 23 — Prestressed Continuous for Live Load I-Beam Spans ♦ Inverted tee caps are recommended. More effective positive moment connection can be achieved by extending the large anchorage bars into the stem. Figure 7-35 shows recommended details. Concrete strength in the beam may be controlled by the top stress at mid-span or the bottom stress over the supports. The stress range in mild steel reinforcing shall not exceed 21 ksi. Aesthetics can be improved by casting the stem flush with the outside beam shape. Deck slab reinforcement over the negative moment regions should be sized to resist ultimate LL+I moment. Cap shoring should be securely in place until the stem is cast and cured. Grade 60 reinforcing steel and Class S (currently 4.000 psi) cast-in-place concrete may be used. Chapter 7 — Superstructure Design Section 23 — Prestressed Continuous for Live Load I-Beam Spans Figure 7-34. Prestressed Concrete Beams Continuous for Live Load – Rectangular Cap (Online users can click here to view this illustration in PDF.) Bridge Design Manual 7-98 TxDOT 12/2001 . Chapter 7 — Superstructure Design Section 23 — Prestressed Continuous for Live Load I-Beam Spans Figure 7-35.) Bridge Design Manual 7-99 TxDOT 12/2001 . Prestressed Concrete Beams Continuous for Live Load – Inverted Tee Cap (Online users can click here to view this illustration in PDF. Chapter 7 — Superstructure Design Section 23 — Prestressed Continuous for Live Load I-Beam Spans Figure 7-36. Prestressed Concrete Beams Continuous for Live Load – Positive Moment Connections (Online users can click here to view this illustration in PDF.) Bridge Design Manual 7-100 TxDOT 12/2001 . As a result of their input. the Louisiana Department of Transportation used an open-top trapezoidal beam that required a collapsible interior void form to fabricate. the Texas U-beam was shaped to allow removal of the interior tub form without the need for a collapsible form. which is 40 in. deep and 89 in. wide at the top. as well as from precast concrete beam fabricators. but to give districts the option of obtaining a different aesthetic bridge appearance with the economy and ease of precast construction.tx. Standard crosssection dimensions. In addition. and the U54 beam. Thus. the Bridge Division has seen an increase in the number of bridge projects in Texas using the U-beam. The underlying premise in the development of the U-beam was not to replace these concrete I-shapes. At the time. curves. Bridge Design Manual 7-101 TxDOT 12/2001 . Popularity of the beam has even progressed to other states such as Florida where the Florida Department of Transportation has developed an identical U-beam for use in that state. and the grid for possible strand locations for both the U40 and U54 are shown in Figure 7-37. special consideration must be given to issues such as the haunch of the slab across the top width of the U-beam and the framing of these beams in flaring sections. Because of the physical size of these beams. Much has been learned since the first U-beam project was constructed in Houston in 1993. the standard drawings contain details consistent with concrete I-beam construction. Even secondary issues such as drain details might prove to be difficult if not considered early in the design process. Current Status Complete sets of standard drawings for the Texas U-beam in English and metric units can be downloaded from the TxDOT web site (http://www. economic alternative to the much-used AASHTO Type IV and Texas Type C precast concrete I-beams.us/). which is 54 in.Chapter 7 — Superstructure Design Section 24 — Prestressed U-Beam Spans Section 24 Prestressed U-Beam Spans Background The Bridge Division began development of the Texas precast concrete U-beam in the mid1980s out of a desire to create an aesthetic. wide at the top. The result of these efforts generated details for two sizes of U-beams: the U40 beam. Preliminary bridge costs show U-beams to be a modest increase in cost over their concrete Ibeam counterparts. The concept for the shape of the Texas U-beam began with a preference for a trapezoidal shaped beam.state. reinforcing. Additional details for the beam were also generated with input from beam manufacturers as well as inhouse experience with concrete I-beam construction details. However. deep and 96 in. it appears that U-beams are a viable precast concrete beam option for bridge projects in which aesthetic issues are deemed important. A checklist to assist in the review of U-beam shop plans is also available from the Bridge Division. narrow spans.dot. or long. the Miscellaneous Slab Details for Inverted-Tee Bents (UBMST) standard sheets show overhang details using this configuration over the inverted-tee bent caps. for U54s. for U40s and 120 ft. past the bottom edge of the exterior U-beam. Instead. See the Miscellaneous Slab Details (UBMS) standard sheets for details on thickened slab ends. These span lengths provide for a more efficient use of the number of beams in a given span. In addition. show a table of estimated quantities with the total reinforcing steel based on 3. show a table of bar designations with sizes used in the slab as is currently done with I-beam structures.Chapter 7 — Superstructure Design Section 24 — Prestressed U-Beam Spans Design Recommendations With normal beam concrete strengths. for U54s. A full depth cast-in-place deck with permanent metal deck forms is the optional design and the Permanent Metal Deck Form (PMDF(U)) standard sheets have notes addressing the required slab reinforcing. This extension allows for a more defined break between cap and beams. In addition. Appendix A contains a table of U-beam spacings versus span lengths for help in the preliminary layout of U-beams. This quantity includes the extra slab steel required over inverted-tee bents and in thickened slab ends. Bridge Design Manual 7-102 TxDOT 12/2001 . the maximum span lengths are approximately 110 ft. use a 4:1 slope normal to the centerline of the bent. for U40s and 130 ft. It is also suggested to extend the ends of the inverted-tee bents about 6 in. Use thickened slab ends at all expansion joints with non-inverted tee bents. Do not show a detailed bill of reinforcing steel on production drawings. If inverted-tee caps are used and are sloped to match the sloping face of the U-beam.7 lbs/sf of bridge deck. Avoid trying to take into account the actual cross-slope of the U-beams framing into the bent as this potentially complicates construction of the bent cap. Slab details should show a cast-in-place slab with precast concrete panels since the standard drawings are set up to work with this option. However. especially since it is virtually impossible for the contractor to set the beams perfectly in line with the end of the cap. the recommended economical span length limit is 100 ft. 0 23.5 31.125 8.25 57.108 1. Standard Prestressed Concrete U-Beam Strand Pattern Constraints and Section Properties (See the following table of U-beam dimensions and section properties.375 17.9 183.58 22.30 979.5 27.125 11.0 403.Chapter 7 — Superstructure Design Section 24 — Prestressed U-Beam Spans Figure 7-37.36 1120.0 10.021 47.25 64. Online users can click here to view this illustration in PDF.167 Bridge Design Manual 7-103 TxDOT 12/2001 .) Beam Type U40 U54 C in 89 96 D in 40 54 U-Beam Dimensions and Section Properties I Weight E F G H J K Yt Yb Area in4 plf in in in in in in in in in2 33.66 16.020 1.5 30.875 20.5 24. the slope of the bottom face of the overhang will vary due to only the vertical curvature of the roadway surface and to the camber and dead load deflection of the exterior U-beam and. One-Way Deck Slabs on Stringers. have a more pleasing appearance. However. thick normal overhang or a sloped overhang where the 8 in.0. but are significantly more than those values from the AASHTO formulas when they apply.. For exterior U-beams. interior slab thickness should be used whenever possible. measured from the centerline of the bottom of the exterior U-beam to the edge of slab. Prestressing strands typically are 1/2 in. See Section 1. the minimum lldf value for an interior U-beam would be 1. use a live load distribution factor of 0.-9 in. (Sext = 1/2 × interior beam spacing + distance from centerline exterior beam to edge of slab) On live load distribution factor (lldf) equations: the interior U-beam formula. The standard overhang dimension is 6 ft.500 psi and f 'c = 8.Chapter 7 — Superstructure Design Section 24 — Prestressed U-Beam Spans Slabs ♦ As with prestressed I-beam bridges.-3 in. S/11. The minimum slab thickness will be 7 1/2 in. thereby. on a straight bridge slab edge.6 in.9 (S is the interior beam spacing measured between beam centerlines). For overhangs. in the transverse direction and at 9 in. Although 0. 0.9 × Sext/11. in the longitudinal direction. Virtually all projects use the continuous deck only concept. Consideration should be given to using a normal overhang when conditions are present that might make the sloped overhang unsightly and/or difficult and expensive to construct. were decided upon for use by the Bridge Division after studying several methods of lldf calculation including the formulas for spread boxes recommended by AASHTO. diameter 270 ksi low-relaxation strands. For overhangs in excess of 7 ft. an 8 in. diameter strand is available for use with the standard grid locations. Thinner slabs should be used only if fewer beams will be required or if sufficient reduction in beam concrete strengths can be obtained. use either an 8 in. Using this method. it should be used only when necessary. ♦ ♦ ♦ Beam Designs ♦ ♦ Normal concrete strengths for beams should be limited to f 'ci = 6.500 psi. These equations generate values 3 to 10 percent less than the lldf calculations made using simple beam distribution with the centerline of beam flange as the supports for a given beam spacing. and the exterior U-beam formula. dimension is applied at the edge of slab. the ♦ ♦ Bridge Design Manual 7-104 TxDOT 12/2001 . However. The cast-in-place portion of slab contains Grade 60 reinforcement with #5 bars spaced at 6 in. the outside web-to-bottom flange joint of the exterior U-beam needs to be checked for adequacy under construction loads.9 x Sext/11 per truck/lane with no minimum value. the slope of the bottom face of the overhang may vary significantly when used with curved slab edges primarily due to the overhang distance varying along the length of the exterior U-beam. For interior U-beams. use a live load distribution factor of S/11 per truck/lane with a minimum value of 0. For the sloped overhang condition. from midspan of the beam. Each diaphragm should be accounted for as a 2 kip load for U40s and 3 kip load for U54s on the non-composite section. This allows the bearing pads to taper in one direction. inverted-tee bents. Each U-beam has two interior diaphragms at a maximum average thickness of 13 in. the depth of slab haunch at the left and right top edges of the 7-105 TxDOT 12/2001 Bridge Design Manual . Bearing seats for U-beams are level perpendicular to the centerline of bent but slope uniformly between the left and right bearing seat elevations.28. 3. ♦ half-span length minus the maximum development length as specified in the 1996 AASHTO Standard Specifications for Highway Bridges. Stresses at the ends of the beam are controlled with the use of debonding. Section 9. The designer should include a Bearing Pad Taper Report sheet in the plans that summarizes bearing pad tapers to be used by the fabricator. ♦ ♦ Beam Framing ♦ U-beams are not vertical but are rotated to accommodate the average cross-slope of a given span. However. U-beams rest on a three bearing pad system with two pads on the back station beam end and one pad on the forward station beam end. Use 2/3 of the rail dead load on the exterior beam and 1/3 of the rail dead load on the adjacent interior beam.2 times the span length.9 for the interior beam represents the thought that some additional distribution of truck/lane loads is occurring between closely spaced U-beams. If the designer chooses to group beams.Chapter 7 — Superstructure Design Section 24 — Prestressed U-Beam Spans lower limit of 0. including overlay on the design of U-beams can significantly limit their ability to span the longer span lengths. ♦ ♦ ♦ Grouping of U-beam designs are at the discretion of the designer. However. Appendix A contains information on the calculation of bearing pad tapers for U-beams. ♦ Overlay should be included if the district plans to immediately overlay the structure after construction. The UBB sheets also show standard distances to centerline of bearings and ends of beams for abutments.0 ft. The maximum debonded length is the lesser of the following: 1. and conventional bents. As a result. 2. a general rule of thumb is to group beams with up to a four-strand difference. or it can be included at the discretion of the designer. and shall conform to details shown on the Beam End and Bearing Details (UBB) sheets. no exterior U-beam shall have less carrying capacity than that of an interior U-beam of equal length. The maximum amount of debonding is limited to 75 percent of the strands per row and per section. Bearing Conditions ♦ Bearing pads shall be designed according to current TxDOT criteria for size and thickness. Draped strands are not permitted in U-beams. 0. A left and right bearing seat elevation is given for each U-beam bearing seat location. They are located as close as 10 ft. or 15. and bearing pad taper reports for U-beams using the alternate method. Beam spacing at top of beam may vary due to crossslope of U-beams. formwork dimensions for the slab are simplified for construction. that is.Chapter 7 — Superstructure Design Section 24 — Prestressed U-Beam Spans beam may differ. When calculating these elevations for each beam seat. the Bridge Division is currently seeking alternative bridge geometry software that will frame U-beams using the preferred method. However. The information is tailored for use with RDS. Special attention should be given to these beams in calculating the haunch values. However. The criteria for locating slab dowels within units are similar to the method used for locating dowels within concrete I-beam units. the designer should call attention to the variable beam spacing at the top of the beams in the plans. the minimum deduct at the correct elevation point and the maximum deduct at the other elevation point. Bridge Design Manual 7-106 TxDOT 12/2001 . the minimum deduct and maximum deduct are each applied at diagonally opposite corners of a beam in plan view. bearing seat elevations. The RDS manual includes information on three framing options specifically written for U-beams: Options 20. thus. These framing options help the designer calculate accurate slab haunch values. Typically. from the centerline of bent.” The Bridge Division currently uses the Roadway Design System (RDS) program to frame U-beams. The alternate method for framing U-beam centerlines is at the bottom of the beam. Typically. The latest version of RDS frames U-beams using the alternate method mentioned above. but the principles behind the method remain the same. if this method is used. Use the same minimum haunch value for all U-beams in a given span when reasonable to do so. one dowel is placed at the centerline of every beam 1 ft. 21. See Appendix A for information on calculating U-beam slab haunches. This prevents the spacing at the top of the beam from varying due to cross-slope of the beam and. be careful to apply the appropriate deduct at that elevation point. ♦ The preferred method for framing U-beam centerlines is at the top of the beam. while beam spacings shown on the substructure details need to take into account the horizontal offset between the centerlines at the top and bottom of the beam. This method allows the U-beams to be framed as a vertical member whereby the beam spacings dimensioned on the span details and/or beam layouts match the beam spacings shown on the substructure details. Left and right bearing seat elevations are located at the intersection of the edges of bearing seats with the centerline bearings. Slab dowels only need to be placed on one side of the centerline of bent. Beam spacings shown on the span details should be noted as being at the top of the beam. and 22. A recommended construction note to include on the span details is “Beam spacing shown is measured at bottom of beam. These dowels are located at the top of the inverted-tee stem and are in a slotted pipe to allow for expansion and contraction of the unit. ♦ ♦ ♦ Restraining Superstructure Lateral Movement ♦ Slab dowels are used to provide lateral restraint when constructing U-beams with inverted-tee bents. Bituminous fiber material can be used as the bond breaker at the beam/shear key interface. The designer has discretion of the placement of shear keys between U-beams. They are designed as pedestals and are poured after the beams have been erected. ♦ Concrete shear keys are typically poured 5 in. above the bottom of the U-beam. consideration should be given to the transverse expansion of the slab. on abutment caps at the ends of simple spans on abutment caps at the end of units if the first interior bent does not have slab dowels or other lateral restraints on rectangular bent caps Shear keys are not required when using slab dowels with inverted-tee bents.Chapter 7 — Superstructure Design Section 24 — Prestressed U-Beam Spans ♦ Concrete shear keys (or some other form of lateral restraint) between U-beams are recommended for superelevated cross-sections on curves or on cross-sections sloping in one direction on straight roadways as follows: 1. from either side of the structure centerline. in locating the shear keys. particularly with wide structures. Bridge Design Manual 7-107 TxDOT 12/2001 . 3. 2. However. A suggested rule of thumb is to locate shear keys no further than 40 ft. Texas began to develop the capability of quality field welding of beam splices and framing members. By the early 1940s. They succumbed to rising steel prices and a new and vital prestressed beam industry in Texas. and apparently they were well motivated to carry out the wishes of the department. since about 188737 I-beams made of wrought iron were available for some 16 years previous. Continuous units were used extensively in the 1950s. The Bridge Division hired several welders. and flare could be handled with straight beams between splices. In the process. the district field personnel learned what to watch on future work. Span length capabilities were first increased by use of cantilever drop-in Type 3 span units. Almost any geometric combination of skew. who was influential in steel design and construction for many years. Standard drawings were maintained through the 1960s but their use was minimal. These Bridge Division welding inspectors were generally hard nosed by virtue of their former ability to excel in a competitive trade. welding contractors. and assigned one to each construction project on which field welding was required. Simple spans still retained popularity because of simpler construction. All of this plus a liberal use of radiographic testing (RT) can be credited for Texas’ long usage of field welding without significant fatigue or fracture problems. Some of the advantages of continuity could be obtained without the structure being statically indeterminate.Chapter 7 — Superstructure Design Section 25 — Rolled Steel I-Beam Spans Section 25 Rolled Steel I-Beam Spans Background Structural steel I-beams have been rolled in the U. Various types of structural steel specified by the Texas Highway Department and AASHTO with allowable design stress in flexural tension are shown in the "Chronology of AASHTO Specification Requirements. . The earliest standard drawings in the Texas Bridge Division file were prepared by the federal Bureau of Public Roads in 1917. Pennybacker. Hinges were notched beam seats with bearings first and pin and hangers later. curvature. instructed them on the basics of metallurgy. Bridge Design Manual 7-108 TxDOT 12/2001 . The development of standard I-beam details is outlined in "Chronology of Simple Steel IBeam Standards" table. in general. Heat curving or heat cambering of I-beams was seldom successful. . Then. and construction inspectors. The state produced its own standard drawing in 1919 based on designs by Percy V. ceased to be economical in the early 1960s. but reinforced concrete decks soon became prevalent. Their purpose was to observe the welders at work and do whatever was necessary to get good welds.S. With the simplified splice details allowed by welding. " table. I-beams. continuous units with riveted splices were being designed. The earliest simple I-beam spans had timber decks. a program of education and emphasis on weld quality was undertaken among designers. simple spans became a thing of the past. in the late 1940s. Under the guidance of Percy Pennybacker. 27 A242 (4) 24 A94 30 A8 18 1953 A7 22. 24. 50W 27 (1) Carbon (2) Silicon (3) Nickel (4) Low alloy (5) Weldable carbon ♦ Allowable compressive stress often blow 0. Gr. and steel is much easier to adapt to severe geometric constraints. A373 (5) 22.Chapter 7 — Superstructure Design Section 25 — Rolled Steel I-Beam Spans There has been a slight resurgence in the use of steel I-beam spans. Gr. 36 20 A709. Gr. 50 27 A709. Chronology of AASHTO Specification Requirements for Structural Steel I-Beam Era AASHTO ASTM ♦ Maximum Allowable Specification Specification Bending Stress (ksi) 1918 None 16 (THD) fy≥30 ksi 1926 A7 16 (THD) 1931 A7 16LL 24DL 1935 A7 18 1941 A7 (1) 18 1944 A7 18 A94 (2) 24 A8 (3) 30 18 1949 A7 22. Their cost is comparable to concrete box beams. 27 A242 24 A94 30 A8 1996 A709. 24. 27 A242 24 A94 30 A8 18 1957 A7.55 Fy Bridge Design Manual 7-109 TxDOT 12/2001 . 24. 5 IL-24. . .5 2 Is-28(20) 1960 H20 S 5. .000 16. according to the specification requirements for 7-110 TxDOT 12/2001 Bridge Design Manual .5 I-18.0 I-24.0 I-9. . . .000 16.000 18.000 16. 1932 2-15T Trucks S 4.0 2 Is-24(S16) 1960 H20S16 S 5. 1946 H20 S 5. . Simple spans units with continuous slabs are preferred.000 16.000 16.Chapter 7 — Superstructure Design Section 25 — Rolled Steel I-Beam Spans Chronology of Simple Steel I-Beam Standards Year Live Live Load Series Designed Load Distribution Factor 1 I-Beam bridges and 1919 15T Roller S culverts or 100 psf 5.5 I-32.5 IB-22. . Adapted from Bureau of Public Roads and Rural Engineering Designs 2.000 18.000 18. . 1929 15T Roller S or 100 psf 4.000 18. Design Recommendations The following suggestions are offered for the design of steel I-beam spans or units: ♦ ♦ ♦ ♦ Service load design should be used.5 IL-2. Cover plates should not be used.0 FI-9. Composite action should be assumed in dead load positive moment sections. 1946 H20 S 5.000 Current Status The use of rolled steel I-beams by TxDOT is usually because of section depth limitations. Welded cover plates are highly susceptible to fatigue problems. 1950 H10 S 5. . . or other geometric anomalies. 1930 15T Roller S or 100 psf 4. 1937 H15 S 4.000 18. . . For interstate highway crossovers Allowable Stress (psi) 16. severe skews. . Shear connection should be provided. 1937 1-10T Truck S 4. .5 1. . Field splices should be welded and optional bolted designs provided. at 24 in. These are local specification interpretations intended to simplify and standardize the design of shear connectors. should be from within the same rolling family.org. should be welded. maximum spacing of stud groups. if spliced. Beams. A check for ultimate strength is not required if service load design is used. Bridge Design Manual 7-111 TxDOT 12/2001 .000 cycles.steelbridge. if used. Recommendations of the Structural Steel Quality Council should be followed in the design and detailing of rolled I-beams and can be found at the web site www. at the span ends.Chapter 7 — Superstructure Design Section 25 — Rolled Steel I-Beam Spans “Fatigue” at 500. ♦ Beam splices should be minimized and. A nominal number of additional connectors should be spaced at 6 in. Originally. detailing. detail. The last of these on a highway bridge was constructed in the early 1950s. Details improved as fabrication and construction experience increased. Flange splices could be butt welded in the shop and field splices butt welded on the job. Closely Spaced IGirders” table . topped by a one-way deck slab. Flange angles were no longer needed since the web plate could be welded directly to the flange plates. Welded girders were used first on tangent alignment at about 8 ft. and fabrication of steel girders became much simpler when welding was accepted as a quality connection technique. and fabricate. spacing topped with a one-way deck slab. Design. Structural steel weights for typical tangent.Chapter 7 — Superstructure Design Section 26 — Steel Plate Girder Spans Section 26 Steel Plate Girder Spans Background Since the 1930s. Quasi-continuity was occasionally provided with pin and hanger joints. Bridge Design Manual 7-112 TxDOT 12/2001 . Various types of structural steel specified by TxDOT and AASHTO are shown in the "Chronology of AASHTO Specification Requirements" table . but usually full continuity was accomplished with riveted splices. closely spaced Igirder units are shown in the “Structural Steel Weights for Tangent. There were no more rivet spacings and splice plates to design. I-shaped plate girders have been used to span beyond the range of rolled beams. girders were fabricated by riveting flange angles to a web plate and adding cover plates top and bottom. The most common configuration was two girders connected by transverse floorbeams with rolled beam stringers parallel to the girders. A373 22. 25. 23. 25. A440. A440. A440. 24. 55 A514/517 20 1983 A36 27 A572. Gr. 23. A588 (9) 49. 25. A441 20 1969 A36 22 A441 > 4 in. A440. 27 A242. 27 A242. A588 23-36 A572 49.A. A709. 36 27 A709. 50(W) (11) N. 25. Gr. 36 27 A709. Gr. A441. A588 N. A441. 70(W) N. A514/517 20 1989 A709. A709. Gr. 50W Proposed Specifications A709. A373 (5) 22. A440. 27 A242. 24.A. A588 49.A. HPS70W 38 Bridge Design Manual 7-113 TxDOT 12/2001 . 100(W) 20 1996 A709. A441. 27 A242.Chapter 7 — Superstructure Design Section 26 — Steel Plate Girder Spans Chronology of AASHTO Specification Requirements for Structural Steel Plate Girders Era AASHTO ASTM ♦ Allowable Fbt Specification Specification (ksi) 18 1953 A7 (1) 22. 27 A242. 55 A514/517 (10) 20 1973 A36 22 A441 > 4 in. 24. 27 A242 24 A94 30 A8 18 1961 A7. Gr. 55 A514/517 (10) 20 1977 A36 22 A441 > 4 in. 23. A441 24 A94 (6) (7) 30 A8 20 1965 A36 (8) 22 A441 > 4 in. 50 27 A709. 27 A242 (2) 24 A94 (3) 30 A8 (4) 18 1957 A7. Gr. Gr. Gr. 23. Chapter 7 — Superstructure Design Section 26 — Steel Plate Girder Spans (1) Carbon (2) Low alloy (3) Silicon (4) Nickel (5) Weldable carbon (6) Low alloy rivet steel (7) Manganese vanadium low alloy (8) Weldable high yield carbon (9) Weathering low alloy (10) Quenched and tempered (11) Covers all and includes AASHTO Charpy impact requirements ♦ Allowable compressive stress often below 0.55 Fy Bridge Design Manual 7-114 TxDOT 12/2001 . with two-way slabs and no stringers were used on tangent girders spanning from 175 to 480 ft. with the additional complications of fabrication and erection. Bridge Design Manual 7-115 TxDOT 12/2001 . Structural analysis was difficult. Variable depth girders were almost always used with this system. Spacings from 12 to 27 ft. When geometric requirements began to extend span lengths and complicate framing. After some design studies for long span steel girders it became evident that steel weight could be saved by increasing girder spacing. Curved units were always heavier because of torsional effects and. Constant Depth I-Girders” table. Widely Spaced I-Girders” table. Closely Spaced I-Girders Span Lengths Deck Width Structural Steel Weight (ft) (ft) (lbs/ft2) 86-104-108 (1) 42 32 120-150-120 50 35 140-180-140 44 49 150-180-180-150 58 43 180-240-180 44 53 198-270-198 (2) 31 59 (1) Constant depth (2) Variable depth Weights are from actual designs. Structural steel weights for typical widely spaced I-Girder units are shown in the following “Structural Steel Weights for Tangent. curved I-girders became appropriate. There was no economical alternative. Structural steel weights for typical curved I-girders units are shown in the following “Structural Steel Weights for Curved. and some of the earlier designs were based on engineering judgment. since Texas had not developed a concrete box girder capability. as well as a few intracoastal canal bridges and crossings of the now defunct navigation channel of the Trinity River.Chapter 7 — Superstructure Design Section 26 — Steel Plate Girder Spans Structural Steel Weights for Tangent. Texas’ first hybrid girder was constructed in this manner. structure costs were quite high. Chapter 7 — Superstructure Design Section 26 — Steel Plate Girder Spans Structural Steel Weights for Curved. ingredients. Constant Depth I-Girders Span Lengths Deck Width Radius of Curve Structural Steel Weight (ft) (ft) (ft) (lbs/ft2) 96-90-96-90 27 160 39 68-123-117 42 440 42 108-134-108 44 440 46 98-181-84 42 440 61 175-175 28 570 55 180-194-166 28 640 53 210-210 28 670 73 193-208-204 38 880 61 121-181-95-112 32 960 54 92-145-145-145-92 42 1150 40 155-155-155-155 42 1150 63 160-200-200-160 28 1270 48 124-248-124 44 1270 66 122-115-122 40 1430 29 93-113-113-104 44 2000 30 Weights are from actual designs and may be subject to variations according to the design method used. Bridge Design Manual 7-116 TxDOT 12/2001 . but toughness is a function of strength. method of manufacture. Widely Spaced I-Girders Span Lengths Girder Spacing Deck Width Structural Steel Weight (ft) (ft) (ft) (lbs/ft2) 150-190-190-150 26 36 26 150-190-190-150 17 44 27 175-200-175 16 40 35 200-250-250-200 14 37 41 100-280-200 20 50 44 200-290-200 20 50 40 220-310-220 17 44 46 220-310-220 27 68 53 220-310-220 28 70 46 240-340-240 24 62 52 320-480-320 23 58 79 Weights are from actual designs. Structural Steel Weights for Tangent. Some weld details have been shown by numerous laboratory tests to be highly susceptible to fatigue crack initiation in bridge girders and I-beams. Steel structures have a history of fatigue cracking and brittle fracture. Crack initiation is independent of steel strength. Growth of the crack and possible subsequent brittle fracture are influenced by the toughness of the steel. fabrication is complicated by variable web depth.3125 in. Except for a few notable mistakes. Bridge Design Manual 7-117 TxDOT 12/2001 . Erection Issues Girder sections tend to be unstable during erection if the top flange plate is too narrow. including falsework and girder support while welding or bolting splices. span girders were 20 ft. In recent years a new high performance steel. There is a quenched and tempered version of this steel that has high toughness characteristics. thick were barely tolerable. Web plates 0. varying geographically with service temperature and amplified for structures considered “fracture critical. its use is now encouraged where applicable. deep over the supports. require close coordination with the contractor. has been made available. For widely spaced girders. the steel industry. Texas has managed to get its curved girders erected without significant mishap. For several years. The specification allowed very thin web plates. but all vertical field splices were welded. Quenched and tempered (Q&T) steel was notorious for brittle fracture and was later disallowed for flexural members. Although somewhat more costly than conventional grade 50W steel. The 480 ft. Improvement of weld details was a continuous process for several years.” Fabrication Issues Weld details are very critical to good performance and also to economical fabrication. thick were impossible to fabricate without significant cupping between stiffeners. The horizontal splice was bolted. The tendency is to rely on judgment until trouble occurs. Shipping problems are more significant because of excessive depth of the negative moment sections and length of the positive moment sections. Close coordination between field and design personnel is vital. braced floor beams.375 in. Composite design tends to make the top flange narrow. Accurate analysis of deflections and stresses during erection stages is very complicated and time consuming. Texas has only one bridge containing this material in a flexural member. but cutting flange plates to the curvature is preferred by some fabricators. and a horizontal field splice was required in the web plate so the pieces could be shipped. and overall steel weight was minimized by using the thinnest web possible. Erection procedures. and temperature. Distortion due to weld shrinkage is a persistent problem with thin web plates. Web plates 0. and FHWA argued about the level of toughness required for bridges and how to ensure that the steel used has the toughness required. The specifications allow heat curving. For curved girders. longitudinal stiffeners. the main problem in fabrication is maintaining the proper horizontal curvature. Erection is especially critical for curved girder units. There is now an acceptable program of Charpy impact testing. HPS70W. Thin web plates are a source of concern. and lateral bracing.Chapter 7 — Superstructure Design Section 26 — Steel Plate Girder Spans thickness of plate. AASHTO. wide. Except for paint. Texas experience does not appear to justify the severity of the current AASHTO fatigue requirements. Usually. In one instance after a design error forced the need for false bents and composite action for slab dead load. there have been few maintenance problems with curved girders. Although paint and painting techniques have been improved somewhat in recent years. This usually occurs on heavily skewed units with diaphragms perpendicular to the girders. Paint has not been a maintenance problem for widely spaced girders since most were fabricated of weathering steel and left bare. The lack of fatigue and fracture problems has been attributed to good weld quality control and mild climate. At least 45 percent of these were constructed after 1951 using all welded details. to prevent shrinkage cracking. Positive moment pieces that were too long to ship in one piece were usually spliced on the ground and lifted into place. In 1978 after serious weld flaws were discovered in a 1957 vintage girder. completely erected but without a deck slab. Falsework was usually provided at the end span splices at the inflection points. Deck cracking is sometimes more severe due to long continuous slab placement. A recent inspection discovered the possibility that loss of section is continuing under the bridge in the variable depth section. It started at a shop splice in the bottom flange of a girder constructed on a county road and inspected by county personnel in the early 1950s. it was 0. One instance of brittle fracture was reported in 1977. One of these. Once started. splice welding was continuous until at least half of the splice in each flange was complete. but the Construction Division says it is rusting. ultrasonic inspection was performed on girder units constructed during that time frame. Most of TxDOT’s large.000 steel girder bridges on the state highway network. One such unit. The theory is that the seclusion of this area discourages wind drying of the saltladen atmosphere. cracking in the negative moment area was particularly prominent. The Bridge Design Section considers it to be weathering. When the flange crack was discovered. the Bridge Division encourages the use of Bridge Design Manual 7-118 TxDOT 12/2001 . is subject to controversy.75 in. The structure was built in the late 1970s and is serving traffic well. but no weld cracking was observed. Maintenance Issues There have been a few cases of cracking due to out-of-plane bending. Numerous welds were found to be outside the current limits of acceptable ultrasonic performance. was observed to develop harmonic response to a 60 mph norther. End span splices over the falsework were welded. The weathering steel bridge near the coast has not been painted. located about 2 miles from the Gulf of Mexico. There are more than 3. Amplitude of the center span deflection ranges was estimated to be 3 ft. the center span section was hung from the two cantilevered ends of the haunched section by temporary shear plates and aligned with clamps. All of these bridges were redundant so no action was taken to repair the questionable welds. Subsequent inspection revealed no evidence of damage to the welded connections. widely spaced girders are in three-span continuous units. Deck cracking may be a little more prevalent on curved girder units but not to the extent that causes extra maintenance procedures. Painting has been unreliable. The structures with problems previously mentioned are still carrying traffic.Chapter 7 — Superstructure Design Section 26 — Steel Plate Girder Spans Erection problems are increased by the size and weight of the girders. Specification loads and load distribution apply without modification. If the girders are to be painted. will be discussed first. including weathering steel. therefore. ♦ ♦ ♦ Service load design is recommended. Closely spaced girders are usually not considered fracture critical. If HPS70W is used. several grades of steel may be used. 50. With little to no cost advantage over Grade 50 or 50W steel. Grade 50W was A 588 (Grade 345W) For girders that are to remain unpainted. the use of Grade 36 steel should be minimal. followed by specific considerations for curved girders and widely spaced variable depth girders. Grade 36 was A 36 (Grade 250) A709. including tangent constant depth girders. Closely spaced constant depth girders should be used for tangent spans up to about 300 ft.Constant Depth . the use of hybrid girders with Grade 36 webs is discouraged.Closely Spaced The 1989 AASHTO Specifications announced new American Society of Testing and Measurement (ASTM) steel specification as follows: ♦ ♦ ♦ A709. and for all horizontally curved units. Current Status Steel I-shaped plate girders should be used when the span lengths and/or horizontal curvatures exceed the capabilities of prestressed beams. Widely spaced variable depth girders should be considered for use on tangent spans greater than 300 ft.Chapter 7 — Superstructure Design Section 26 — Steel Plate Girder Spans unpainted weathering steel except in northern urban regions of the state where de-icing salts are used heavily.Tangent . weathering steel is required. Design Recommendations Girder design in general. where vertical clearance is not critical. These grades are A709 Grades 36. it should be in a hybrid girder with a Grade 50W web if the girder is to remain unpainted or a Grade 50 web for painted girders. General . Note. Preliminary analysis may be done with the help of influence lines such as published in “Ten-Division Influence Lines for Continuous Beams. Grade 50 was A 572 (Grade 345) A709. and HPS70W.39 7-119 TxDOT 12/2001 Bridge Design Manual . 50W. Grade 50W should not be required for painted girders. Unpainted weathering steel is preferred for all girders not subject to continuous moisture or in northern urban areas where de-icing salts are used.”38 Final analysis can be done with the B30 computer program. Available grades of weathering steel are A709 Grades 50W and HPS70W.” and “Moment Shears and Reactions for Continuous Highway Bridges. The minimum thickness should be 0. except that splices will not be allowed where a 40 ft. they should be thick enough to eliminate the need for transverse stiffeners except within two times the web depth adjacent to the interior supports. within . Web plates should be no less than 0. for this purpose. The following girder fabrication note should be placed on the details: Except at changes in section.000 cycles. A check for ultimate strength is not required if service load design is used. Shop flange splices shown on the details should be minimized. Additionally. details which cover stud shear connectors and transverse stiffeners welded to the flange. Shop web splices should preferably not be shown on the details. Transverse stiffeners to which diaphragms are connected should be welded to the top and bottom flanges. Thickness changes considered appropriate could be made at the field splices.” Field splices should be located near the dead load inflection points in each span.500 average daily trucks in one direction. Stress range should be limited to the allowable for Category C. Spacing of stud groups should not exceed 24 in. nor within the range between . Flange and web splices shall be made by full penetration groove welds in accordance with the Item “Steel Structures. The minimum flange plate width should be the length between field splices divided by 80 for erection stability. Planning and Programming Division traffic count near the proposed bridge indicates more than 2. Case II loading cycles should be used.” at 500. “Additional Connectors to Develop Slab Stresses” should be spaced at 6 in. In keeping with past practice. Unless a current Transportation. all field splices are to be shown in the plans as welded splices. The fabricator may lengthen thicker plates if the contractor approves any change in thickness at field splices. and the maximum width 24 times the thickness. The flange thickness should not exceed 4 in. thick because of fabrication problems. shop flange and web splices in plate girders may be located as desirable to optimize plate lengths and erection procedures. or less unspliced length would suffice.500 combination and three-axle trucks in each direction. Longitudinal stiffeners should not be used. There should be no less than five rows spaced at 6 in.30S and . Permissible field splices should be used to keep piece lengths less than 130 ft. neither will tension flange splices be allowed within . Shear connectors should be provided according to specification requirements for “Fatigue.10S either side of the centerlines of interior spans. 7-120 TxDOT 12/2001 ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ Bridge Design Manual . This will minimize out-of-plane bending of the web.50S from the end bearings (S = length c/c bearing of span in which the splice is made). A new requirement is to also design and detail bolted field splices as an option to welded splices. It takes a lot of traffic to generate more than 2.Chapter 7 — Superstructure Design Section 26 — Steel Plate Girder Spans ♦ Deck slab should be composite with girder in the dead load positive moment zones only.75 in.5 in. centers at the contraflexure points and also at the ends of the unit. Usually one in the bottom flange either side of the maximum positive moment location and one in both flanges either side of interior supports will suffice.05S either side of interior bearings. The guide specification is then subjugated to local practice. Load distribution and allowable stress from the AASHTO Specifications for tangent girders are used except as hereinafter noted. in all respects. Blast-cleaned faying surfaces are specified in construction specifications for System II paints. it is subject to a complicated procedure considering torsion and lateral flange bending.steelbridge. ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ Establish a preliminary design using B-30. like a unit on tangent alignment. For bolted connections or splices it is recommended that ASTM A325 bolts be used with a design allowable shear stress of 15 ksi (assuming 0. Calculate V-Loads for non-composite dead load and also for any composite dead load.33 slip coefficient). Weld details are given in TxDOT construction specification Item 441 “Steel Structures” and in the AASHTO/AWS Bridge Welding Code. Curved Girders . that will approximately reproduce each maximum live load moment.org.Constant Depth . Welding symbols are explained in the ANSI/AWS A2. Select the maximum percentage increase in live load moment caused by the V-Loads. the combination of high shear and bending should be checked adjacent to interior supports. 7-121 TxDOT 12/2001 Bridge Design Manual . which is considered sufficiently accurate for most situations. If not. If the central angle is less than allowed by the guide. Miscellaneous details are shown on standard drawings SPGD. Remove all dead load and live load from the B-30 input and make a separate B-30 run for each concentrated loading configuration. In the Bridge Design Section this guide is applied to the longest span in a continuous unit. design of the unit is treated. The allowable compressive stress shall conform to the AASHTO Specifications. Calculate V-Loads from the moments due to each loading. Recommendations of the Texas Steel Quality Council should be followed in the design and detailing of structural steel girders and can be found at the web site www. An approximate method of analysis has been developed using the V-Load concept41 and B30 computer program. Material selection criteria are the same as for tangent girders and loads. Service load design is recommended.Closely Spaced The AASHTO Guide Specification for Horizontally Curved Highway Bridges40 gives maximum central angles beyond which torsional effects must be considered in primary bending calculations. Apply the V-Loads as composite P-Loads in a separate B-30 run for each configuration. concentrated at each diaphragm. Find the maximum live load moment in each span and at each support. Compute separate loadings. For hybrid girders.Chapter 7 — Superstructure Design Section 26 — Steel Plate Girder Spans ♦ ♦ Bottom compression flanges are considered supported at the diaphragms and bearings. that is. the absolute maximum from the several live load/V-Load runs.4-93. 4. This method produces a final moment B-30 run for each girder that will usually suffice for stress ranges.Chapter 7 — Superstructure Design Section 26 — Steel Plate Girder Spans ♦ Make a final B-30 run for each girder with dead loads and live load using the following adjustments: 1. and flange plate cut-off. Engineering judgment may suggest modifications to this procedure as the design progresses. it will usually be sufficiently accurate to calculate the maximum lateral flange bending stress in each size of bottom flange. 2. The method is approximate but conservative. Although theoretical movement of the girder ends is along a chord to the curvature. Since the load case giving the largest percentage increase in live load moment is used to adjust the distribution factor. other areas of the outside girder will be overdesigned. Since the top flange is laterally restrained by the slab against live load effects. 3. If desired the design may be made using the DESCUS I computer analysis program. diaphragm spacing and flange plate width are significant design items. Apply composite dead load V-Loads as composite P-Loads to each girder. The amount of over design will usually be an insignificant portion of the total stresses in the girder. 5. Lateral flange bending stresses must be calculated by hand and added to the final stresses above. Careful attention to girder reactions at the ends of unit is appropriate because of uplift tendencies at the inside girder. for the outside girder. Formulae for lateral flange bending are as follows: Lateral Flange Bending Moment M(d )2 = = Mb (Bimoment) 12hR where: M = Maximum moment (including V-Loads) within the flange plate size under consideration H = Girder web depth R = Radius of horizontal curvature of the girder d = Diaphragm spacing Flange Tip Stress Due 6M b = (Conservatively applied at all locations along girder) to Lateral Bending tw 2 where: t = Flange thickness w = Flange width These formulas can be quickly applied to the appropriate moment from the final B-30 run to produce an additional stress due to lateral flange bending. as necessary. Bridge Design Manual 7-122 TxDOT 12/2001 . Do not increase or decrease live load to girders inside of the centerline. Because of lateral bending stresses in the flanges due to non-uniform torsion. Increase live load proportionally for other girders outside of the centerline of the superstructure. which can be added to the combined DL/LL stresses and stress ranges. expansion bearings are usually set square with each girder. shear connector spacing. Increase the live load distribution factor by the percentage selected above. Apply non-composite dead load V-Loads as non-composite P-Loads to each girder. south of Dallas). The additional cost of fabrication according to the “Fracture Control Plan” is currently about 15 percent. Flange plate width should be no less than one-fourth of web depth. Four in. Tangent Girders . Widely spaced variable depth girders are not recommended for curved structures. it has not been used for that purpose by the Bridge Design Section. one-third of the web depth is preferred. Standard drawing MEBR(S) contains some empirical controls on the erection of curved girders.55 Fy. Standard diaphragms must be checked against actual forces. Widely spaced girders with a twoway deck slab will usually be more economical than closely spaced girders but will require more depth. 50W. Elimination of this false bent requires temporary support of two or three girders until they can be made stable by diaphragm connection and/or partial splicing.Chapter 7 — Superstructure Design Section 26 — Steel Plate Girder Spans Lateral bracing is never required to resist wind forces in properly designed curved girders. or HPS70W steel should be used. Under no circumstances should X diaphragms without top horizontal members be used. Modifications to the standard details are often required.Variable Depth . 7-123 TxDOT 12/2001 ♦ Bridge Design Manual . Quenched and tempered Grade 100 should never be used for flexural members. Additional design considerations for curved girders are as follows: ♦ ♦ ♦ ♦ ♦ Diaphragm spacing should be less than 20 ft. (IH 45 over the Trinity River. TxDOT’s longest constructed span with this framing system was 480 ft. Vertical clearance is usually critical. Miscellaneous details are shown on standard drawing SPGD. Flange plates will be large on long span units. Diaphragm connection stiffeners should always be welded to both girder flanges. and variable depths are not considered aesthetic on a curve. Calculated bending stress plus lateral flange bending stresses should not exceed 0. thickness should be considered the absolute maximum. Material selection is similar to the other girder types. which will be unacceptable if traffic must be maintained beneath the unit. Grade 50. An example configuration of diaphragm floorbeams is shown on Figure 7-38. Live load deflection is calculated for the outside girder assuming only one lane loaded. Current policy requires the contractor to prove the adequacy of any erection procedure that does not conform to MEBR(S). Application of these controls will often require a false bent between field splices.Widely Spaced Fabricators discourage the use of variable depth girders. The recommended minimum span length for this type of construction is 300 ft. The allowable live load deflection is one eight-hundredth of the span along the outside girder from bearing to bearing. Although research42 suggests that lateral bracing increases the resistance of the girder to torsion. ♦ Two-girder units are considered fracture critical and should be avoided. However. 5 in. Web thickness should be no less than 0. constant depth girders are also applicable. especially in the deeper negative moment zone. Other suggestions for tangent. the recommended connection details from standard drawings SPGD are reproduced on Figure 7-40 for emphasis. This will be discussed in later sections. This will usually produce members with more than enough strength to withstand wind forces. but longitudinal and transverse stiffeners may be desirable to reduce weight. Since longitudinal stiffeners are potential Category E fatigue details. ♦ ♦ ♦ ♦ Bridge Design Manual 7-124 TxDOT 12/2001 . If lateral bracing is required it is recommended that l/r not exceed 140. Example bracing details are shown in Figure 7-39. Bearing and expansion joints will be more specialized for the longer units. Example details of the compression flange connection at interior bearings are given in Figure 7-41.Chapter 7 — Superstructure Design Section 26 — Steel Plate Girder Spans ♦ Current AASHTO design methods will seldom require lateral bracing. closely spaced. ) Bridge Design Manual 7-125 TxDOT 12/2001 . Widely Spaced Girder Details (Online users can click here to view this illustration in PDF.Chapter 7 — Superstructure Design Section 26 — Steel Plate Girder Spans Figure 7-38. ) Bridge Design Manual 7-126 TxDOT 12/2001 .Chapter 7 — Superstructure Design Section 26 — Steel Plate Girder Spans Figure 7-39. Example Lateral Bracing Details (Online users can click here to view this illustration in PDF. Chapter 7 — Superstructure Design Section 26 — Steel Plate Girder Spans Figure 7-40. Longitudinal Stiffener Details (Online users can click here to view this illustration in PDF.) Bridge Design Manual 7-127 TxDOT 12/2001 . Variable Depth Girder Details (Online users can click here to view this illustration in PDF.Chapter 7 — Superstructure Design Section 26 — Steel Plate Girder Spans Figure 7-41.) Bridge Design Manual 7-128 TxDOT 12/2001 . Shipping and erection may require special measures. and the girder sections are necessarily large and heavy. trapezoidal box girders became an acceptable structure type. Example details are shown in Figure 7-42. especially in the urban areas.Chapter 7 — Superstructure Design Section 27 — Trapezoidal Box Girders Section 27 Trapezoidal Box Girders Background Except for the one unorthodox through box girder in Austin. and the Bridge Design Section has even designed a few. compared to I-girders. especially on a horizontal curve. TxDOT avoided the use of steel box girders until the late 1980s.. However. Beginning with the toll roads constructed by the Metropolitan Transit Authority in Houston. Current Status Trapezoidal box girders are recommended in interchanges where aesthetics dictate the use of U-beam approaches. Fabrication is more complicated. Center spans of these continuous units are usually well over 200 ft. girders with sloping webs and weathering steel have existed since 1968 on the road system of the Dallas-Fort Worth airport. consultants have designed some. Bridge Design Manual 7-129 TxDOT 12/2001 . The Houston District has now designed several such structures. being in the core region of an interchange. They may be removed after the slab has cured. The B-30 computer program can be used for tangent girders and preliminary curved girder sections. Lockable doors must cover each access hole. In addition to the diaphragms within the box. DESCUS II is available and may be used for analysis of curved units. using 1 in. thick Class S concrete slab should be supported during placement with permanent metal deck forms. Access holes into each girder should be provided near the end bearings. diameter. Bridge Design Manual 7-130 TxDOT 12/2001 . Example Trapezoidal Steel Box Girder Details (Online users can click here to view this illustration in PDF. Crawl holes must be provided in the bearing stiffener/diaphragms at interior bents. Field splices should be bolted. Faying surfaces should be sandblasted. Other recommendations are as follows: ♦ ♦ ♦ ♦ ♦ Service load design should be used. A minimum 8 in.50 in. A325 bolts with a design allowable shear stress of 15 ksi. temporary crossframes should be provided between girders to help the girders deflect together during slab placement.) Design Recommendations Material selection criteria should be the same as for conventional girders.Chapter 7 — Superstructure Design Section 27 — Trapezoidal Box Girders Figure 7-42. The minimum tension flange thickness should be 0. 11 “The Analysis of Continuous Beams for Highway Bridges IV. 15 “Bridge Design Practice. 2. Automation Division. 1955. CTR. 1989. 1982.” Leyendecker. and H.” Panak.J. Prestressed Sub-Deck Panels and Cast-in-Place Decks. H... and L. which adds a considerable amount to fabrication costs. 1973. 1977." S. E. Recommendations of the Texas Steel Quality Council should be followed in the design and detailing of trapezoidal box girders and can be found at the web site www.” Matlock. TTI. 1972..T. 20 “Economical Precast Concrete Bridges. and others. CTR. R.. 9 “Application of Transverse Prestressing to Bridge Decks. John Wiley and Sons. Report 56-25. 8 “Experimental Use of High Strength Reinforcing Steel” Newton. Furr. 1966. and 316-3F. CTR.Chapter 7 — Superstructure Design Section 27 — Trapezoidal Box Girders A long depth transition is desirable to meet the approach span depth. Final Report 25-1F. 1975.O.. 1990.” Bieschke. high load-multirotational bearings (usually pot bearings) may be required because of the large reactions..” Poston. and others. I-Beams and Box Beams for Bridges.steelbridge. 1983.W. Reports 316-1.” Poston. Departmental Research. Y.” Guyon. J. “Behavior of Prestressed Panel Cast-in-Place Concrete Bridge Decks. and 316-3F.” Computer Program PSTRS 14. H. 16 “Guide Specifications for Design and Construction of Segmental Concrete Bridges. 316-2. 1982.G. CTR. 1989. 3 “A Discrete-Element Method of Analysis for Orthogonal Slab and Grid Bridge Floor Systems. Departmental Research. & 4F.P.J.” Panak. Adolf and Springer-Veriag Wein.” American Association of State Highway and Transportation Officials (AASHTO). CFHR. Final Report 94-3F. Report #158-1F. Every attempt should be made to use steel reinforced neoprene bearings.” AASHTO. Matlock.” AASHTO. J. CFHR. and others. CFHR. Breen.” Poston.. 1968.” Jones.V. L. and others. 19 “Tentative Standards for Prestressed Concrete Piles.” Georgia D. Report 56-5.L.” Joint AASHTO and PCI Committee Report. 1972. SDHPT. 13 “A Computer Program to Analyze Beam Columns Under Moveable Loads. Final Research Report 254F.” Panak. and R. R. 14 “Design of Slender Non-Prismatic and Hollow Concrete Bridge Piers. E.E. 1 “Standard Specifications for Highway Bridges. One or more bearing manufacturers should be contacted before bearing seat dimensions and connection details are finalized. J. Report 56-25.L. 1955. 1962. 1971. 24 “Differential Camber in Prestressed Concrete Beams. Bridge Design Manual 7-131 TxDOT 12/2001 .. Breen. Enhanced by SDHPT.W. Sixteenth Edition (1996). and J. Final Report 226-1F. 23 "The Effect of Diaphragms in Prestressed Concrete Girder and Slab Bridges. Reports 316-1.J. and H. 1985. 17 “Prestressed Concrete.A. Taylor.” Guyon. 5 “Influence Surfaces of Elastic Plates. 1970 & 1972.W. 6 “Standard Specifications for Highway Bridges. CTR. Spans with two box girders in the cross section are considered fracture critical.. Lateral bracing within the box should be bolted to the top of the flange to mitigate fatigue problems and to avoid connection difficulties to the sloping web beneath the top flange. J.L. 1969. Walker.” American Association of State Highway and Transportation Officials (AASHTO).” Pucher.” Furr. 316-2.G.E.org. Department of Transportation. However. 3. Center for Highway Research. Report 303-1F. 22 “Precast Prestressed Concrete Short Span Bridges – Spans to 100 Feet. H. John Wiley and Sons. Latest Revisions. 1985. Reports 145-1. Sixteenth Edition (1996). Matlock. TTI. Report 193-1F. 7 “Behavior of Concrete Slab and Girder Bridges. New York. Klingner. Y. R.. and H. 18 “Guide Specifications for Design and Construction of Segmental Concrete Bridges. 2 “A Study of Prestressed Panels and Composite Action in Concrete Bridges Made of Prestressed Beams. 21 “Prestressed Concrete Girder Design.” Prestressed Concrete Institute. 12 “Prestressed Concrete. 1977. 10 “Application of Transverse Prestressing to Bridge Decks. Sengupta and J. and T. 4 “A Discrete-Element Method of Analysis for Orthogonal Slab and Grid Bridge Floor Systems. Slabs.” State of California. P.” AASHTO. 1962.” Kelly. 1989. 1984. AISC Project 308. 1987.” Jones.J.” Overman. 38 “Ten-Division Influence Lines for Continuous Beams. 1970 & 1972. 1971. 1987. TTI.” Menon. 1968.” Joint AASHTO and PCI Committee Report. and others. CTR. CFHR. 31 “A Computer Program to Analyze Beam Columns Under Moveable Loads. Slabs.” Schelling. “Tentative Standards for Prestressed Concrete Piles. CTR. 1977. New York.D. 1977.” C.. Est. H./Feb. 35 “Automated Design of Continuous Bridges with Precast Prestressed Concrete Beams. TTI. 41 “Time Dependent Deflections of Pretensioned Beams. Reports 145-1. and others. L.T. and T. PCI Journal.R.J. Report 381-1. H.” 1883-1952. 37 “Iron and Steel Beams. 2. PCI Journal. T.” NCHRP Project 12-29. 1977. 29 “Optimum Design or Reinforcement for Notched Ends of Prestressed Concrete Girders.” Kelly. Jan. Report 56-5.O. 28 “A Study of Prestressed Panels and Composite Action in Concrete Bridges Made of Prestressed Beams.” Martin. New York. 30 “Optimum Design or Reinforcement for Notched Ends of Prestressed Concrete Girders. Prestressed Sub-Deck Panels and Cast-in-Place Decks.” Menon. G. Freyermuth. April 1969. Report 381-1. CTR.J. G. 40 “Guide Specifications for Horizontally Curved Highway Bridges..” Furr. Ing. D. 33 “Time Dependent Deflections of Pretensioned Beams. Furlong. CFHR.. American Institute of Steel Construction. D.” Georgia D. 1987. 42 “Effects of Bracing on I-Girder Bridges.” American Institute of Steel Construction. “Moment Shears and Reactions for Continuous Highway Bridges. 3..” Matlock. D. G. and others. Fifth Printing 1968. 27 “Time Dependent Deflections of Pretensioned Beams. Prestressed Concrete Girders. 36 “Design of Simple-Span Precast Prestressed Bridge Girders Made Continuous. Final Report 196-1F. Report 22-1F. 34 “Design of Continuous Highway Bridges with Precast. 1966. H. CTR. 1974.. and R. I-Beams and Box Beams for Bridges. 1980 with 1986 revisions. Program Documentation. CTR. 1956.” Kelly. Taylor. and others. Dr. 32 “A Rational Method for Estimating Camber and Deflection of Precast Prestresseed Members. Final Report.W. Furlong. Report 381-1.L. 39 “The Analysis of Continuous Beams for Highway Bridges IV. Volume II. Frederick Zinger Publishing Co.Chapter 7 — Superstructure Design 25 26 Section 27 — Trapezoidal Box Girders “Fatigue Behavior of Pretensioned Concrete Girders. and others. Final Report 196-1F.L.W. and others. and R.A. Enhanced by SDHPT.” Eighth Edition.L. Bridge Design Manual 7-132 TxDOT 12/2001 . & 4F. Report 300-2F. .......8-3 Section 2 — Interior Bents..............................8-35 Section 4 — Pier Protection................................................................................................................................................8-8 Section 3 — Piers...........................................................................................8-41 Bridge Design Manual 8-1 TxDOT 12/2001 ..........................................................................................Chapter 8 Substructure Design Contents: Section 1 — Abutments ................................................... Chapter 8 — Substructure Design Section 1 — Abutments Bridge Design Manual 8-2 TxDOT 12/2001 . U type abutments had two side walls and a front wall resting on spread footings below natural ground. Abutments must be compatible with the bridge approach roadway. the longer the sidewalls. however. on a reasonable slope. The extra length was justified on the basis of cost and aesthetics. constructed by driving piling or drilling shafts through the finished fill and placing a cap backwall and wing walls on top. They usually have wing walls to keep the sideslopes away from the structure and to transition between the guard rail and the bridge rail. The bridge must be considerably longer than with U type abutments but slightly shorter than with cantilever types. Cantilever type abutments had variable width rectangular columns supported on spread footings below natural ground. The fill was built around the columns and allowed to spill through. most of the abutments in Texas were of the “stub” or “perched” type. They must have backwalls to keep the embankment from covering up the beam ends and to support possible approach slabs. However. and they complicate detailing and construction. most bridges require them. Figure 8-1 shows this relationship. A cap was placed on top of the columns to support the superstructure. By 1940. The side walls were long enough to keep the embankment from encroaching on the bridge opening. Bridge Design Manual 8-3 TxDOT 12/2001 .Chapter 8 — Substructure Design Section 1 — Abutments Section 1 Abutments Overview Abutments present special problems in bridge design. and earwalls were added to the ends of the cap to keep the fill away from the bearing area. A great number of these types of abutments were constructed in Texas. Many have low structural ratings because of bending in the abutment piling. into the bridge openings. Background Timber piling with timber lagging to hold back the embankment were once the cheapest solution. A number of these still exist on county roads across the state. were severe and so was the cost. The header bank was sloped from the top of the wing wall through the intersection of the cap and backwall into the bridge opening. The taller the abutment. and they performed very well. Early concrete abutments were called U type or cantilever type. They make design methods problematic. Detailing and construction problems. Abutments (Online users can click here to view this illustration in PDF.Chapter 8 — Substructure Design Section 1 — Abutments Figure 8-1.) Bridge Design Manual 8-4 TxDOT 12/2001 . behind the abutment backwall. Commonly. usually with unsatisfactory results. retaining walls are often required at the ends of bridges. tied-back walls. the Bridge Design Manual 8-5 TxDOT 12/2001 . Abutments are generally the most complicated detailing problem in a bridge. or even spread footing type walls. With nailed type walls. The well known “bump at the beginning of the bridge. Some important features that may be included in the design and construction of abutments include the following: ♦ ♦ ♦ Wing walls Retaining walls Approach slabs Wing Walls. Design of this type of retaining wall abutment can become complex.” caused by fill settlement. Current Status Most bridges designed in Texas have “stub” abutments. breaking it off at the cap. Vertical moments and shears in the cap are insignificant because of the participation of the relatively deep backwall and because support spacing is kept smaller than for interior bents to increase horizontal resistance. Because of increased urban rehabilitation work and restricted right-ofway. Wing walls greater than 12 ft. is hard on abutments. In cut situations. the wall may cross in front of the abutment cap. walls pass alongside and in front of the abutment creating a U-shaped wall. The available Texas Department of Transportation (TxDOT) “Standard Details” for abutments include wing wall details. Reinforcing bars that tie the backwall and the wing wall together must extend on each side of the joint enough to develop the strength of the bar. The wall and bridge abutment will often become a single structure in these cases. the settling fill can drag the wing wall down. be sure to include a live load surcharge in the analysis. Horizontal movement of the fill has caused torsion cracking in the caps. Wing walls can be cantilevered or founded. Additional information can be found in the TxDOT Bridge Detailing Manual. These details should be adhered to if the case is appropriate. In other situations. A wing wall confines the abutment backfill material and roadway soil at the sides. In some cases these walls may pass by the sides of the abutment cap. If these supports are stopped too close to natural ground. in length must be founded by drilled shaft(s) or pile(s). eliminating the wing walls.Chapter 8 — Substructure Design Section 1 — Abutments Although more economical. stub abutments are associated with certain maintenance problems. If designing a wing wall. The design of abutments with backwalls has been standardized through trial and error. Longer wing walls require a pile or drilled shaft to support their weight. Retaining Walls. and should be a cooperative effort between bridge and geotechnical engineers. There have been some attempts to secure the abutment to the superstructure. Several types of walls may be used in conjunction with bridge abutments. the walls will often be cantilevered drilled shaft type walls. Soil or rock nailed walls may also be used to support abutments in cut situations. The length of the cantilevered wing wall is limited to 12 ft. therefore drilled shaft or piling foundations must be provided. Therefore. at the two wing walls and the abutment backwall. this has not been a common practice in Texas. Its intended purpose is to provide a smooth transition from roadway pavement to bridge slab. The standard approach slab is not reinforced for this situation. Bridge Design Manual 8-6 TxDOT 12/2001 . If used. nor are the wing walls designed to carry the load. An approach slab is a 13 in. the approach slab becomes a slab supported on three sides. Experience has shown that compaction of the backfill is difficult. Approach Slabs. Contact the Bridge Design Section or Geotechnical Branch for suggested limits for the CSS. and is resistant to the moisture gain and loss of material that is common under approach slabs. and that the loss of backfill material can take place. The use of approach slabs is optional. TxDOT is currently supporting the placement of a cement stabilized sand (CSS) “wedge” in the zone behind the abutment. and follows the abutment at the end of the bridge. In the most common. while others have had success without their use. the Bridge Design Section strongly discourages supporting the approach slab on wing walls. the walls will usually be mechanically stabilized earth (MSE) walls. and some districts have had success with their use. thick lightly reinforced concrete slab that precedes the abutment at the beginning of the bridge. CSS solves the problem of difficult compaction behind the abutment. Although the abutment cap can be placed directly on the MSE fill without deep foundations. an appropriate backfill material is essential.Chapter 8 — Substructure Design Section 1 — Abutments abutments are usually standard type “stub” abutments that are completely separate from the walls. The foundations are required to be installed prior to construction of the MSE wall. in order to avoid damage to the wall reinforcements during foundation installation. fill type situation. The use of CSS has become standard practice in several districts and has shown good results. It is suggested that the approach slab should be supported by the abutment backwall and the approach backfill only. Without the bearing on the backfill. Cantilever wing walls should be used where possible. Retaining type abutments in questionable soils may justify a more accurate analysis. backwall. Structural analysis will not be required. above the roadway surface. wing wall support. The back pile should not be allowed to go into tension due to the lateral load. ♦ ♦ ♦ ♦ ♦ ♦ Bridge Design Manual 8-7 TxDOT 12/2001 . will suffice for calculating horizontal forces. wing wall lengths. of surcharge. and various other standardized items should be as shown in the TxDOT Bridge Detailing Manual or the applicable bridge standard drawings. The maximum spacing of drilled shafts or pile groups should not exceed 16 ft. the use of 40 pounds per cubic foot equivalent fluid pressure with 2 ft. Drilled shaft loads may be calculated as the total vertical load on the cap divided equally among the cap shafts. with beams 40 in. For pile foundations. A construction joint should be provided in abutment caps longer than 90 ft. The joint should clear the bearing seat areas. with beams of greater depth. Wing wall shaft or pile load is usually taken as 10 tons per shaft or pile. Recommended design practice for standard type “stub” abutments with backwalls is as follows: ♦ The position of the backwall. if no approach slab is used. battered pairs of piling should be used for all abutments that are not otherwise restrained from horizontal movement.5 ft. Pile loads may be calculated as the total vertical load on the cap divided equally among the cap piling plus the load caused by 40 pounds per cubic foot fluid pressure from the bottom of the cap to 2 ft. Examples of sufficient restraint are slab spans and pan form spans that are doweled into the abutment. and wing wall reinforcing should also conform to the TxDOT Bridge Detailing Manual. Battered piling should never be used adjacent to MSE walls because of the difficulty of installing the backfill. Cap. and less in depth nor 12.Chapter 8 — Substructure Design Design Recommendations Section 1 — Abutments As a general rule. and abutments within a mechanically stabilized fill. Bridge Design Manual 8-8 TxDOT 12/2001 .and single-column bents. most columns were square. pitch because of concrete placement difficulties and later to #4 spirals at 9 in. No failure has been experienced. Single-column bent caps usually have sloping soffits. basically rectangular reinforced concrete beams. timber trestle bents were the earliest type. Trestle pile bents. most columns are round. Square columns are still used occasionally in situations where aesthetics are considered. In the early days they tended more to chamfers. Post-tensioned concrete caps in various forms have also been used in these situations. form liners. and still are. Concrete piling were reinforced with mild steel until the 1950s when prestressed piling took over. Now they are mostly prismatic except that the cantilever soffits are sloped. followed by concrete walls and then concrete columns with cap. These caps are considered fracture critical. Prestressed piling are easier to handle and more durable. Many have been fitted with concrete collars to correct the problem.Chapter 8 — Substructure Design Section 2 — Interior Bents Section 2 Interior Bents Background The design of interior bents is more direct than it is for abutments. pitch in columns with longitudinal bars that are #11 and larger. They have been used in multiple. although they are designed as tied columns. Engineers have chosen round. rounded end rectangular. Structural steel box girders have been used for caps on bents that require a long span between columns to clear a roadway below. Originally. two or more concrete columns. other shapes in attempts to enhance the appearance of the bridge. Now. In Texas. Inverted tee caps were introduced in the 1970s to reduce the clutter of deep caps in congested environments. Some have a sloping soffit but many are constant depth. spirals were #3 at 3 in. Moments and shears are calculated at various locations for comparison to strength of the member. The loads are carried on down to the foundation and delivered to the earth. Spiral reinforcement is used around the main steel. Multiple-column bents are composed of a concrete cap. Until the late 1940s. Typical trestle pile and multiple column bents are shown in Figure 8-2. Wheel loads can be followed through the beams into the cap and columns. and. They often had large chamfers or radii at the cap and occasionally had decorative collars or were tapered in width. occasionally. Special surface finishes such as exposed aggregate. This was soon increased to 6 in. and lateral earth pressure is not usually a factor. Column sizes are limited to 6 in. Bent caps have been. rectangular. after the 1930s. pitch. or texture paint have also been used. and other decorative features. and hidden foundations. Steel piling have been used extensively but have been susceptible to severe corrosion at the ground line. had steel or concrete piling with concrete caps. incremental diameters for standardization and reuse of forms. Load paths are cleaner. which requires careful attention to details. square. Metal forms are required to ensure a smooth and even finish. radii. Early engineers put their ideas of aesthetics into the columns and caps. Bridge Design Manual 8-9 TxDOT 12/2001 . Column members are subject to simultaneous axial compression and bending. These uncertainties have apparently resulted in conservative designs since no column malfunctions in Texas can be attributed to the effects of service loads. which may be cracked to varying degrees at different loading stages and different locations along the column length. Pile footings are designed as beams subject to considerations in the “Foundations” section of the American Association of State Highway and Transportation Officials (AASHTO) Specification. Hollow sections have been used for extremely large columns. They help alleviate the congested appearance by minimizing the number of columns. neither of which is subject to accurate determination in reinforced concrete members. Manageable analysis procedures have been developed based on magnification of the moments due to external loads using simplified assumptions for elastic behavior of the column. Variations in shape have been more extensive for single columns. usually as a reinforced concrete beam. Too many tie bars to allow efficient vibration of the concrete was a common complaint at one time. An iteration process is required to reach a condition of stability.Chapter 8 — Substructure Design Section 2 — Interior Bents Single-column bents came into use on ramps and connectors in urban interchanges. Internal stiffening is often required for stability of the reinforcing cage during erection and concrete placement. which in turn generates a little more deflection. a secondary moment is generated in the amount of the axial load times the deflection. Lap splices in main reinforcing can impede concrete flow. Design of reinforced concrete columns is very complex. which can aggravate already crowded conditions. although some of the effect is lost because single columns are larger than multiples. Design of caps is more straightforward. Deflections are inversely proportional to moment of inertia and modulus of elasticity. When a column deflects due to any primary cause. They may be prismatic or tapered in one direction. Construction Issues Construction problems have been caused by congestion of reinforcing. Accurate determination of the amount of bending is especially difficult. Rectangular is the basic shape with circular or chamfered end faces as variations. Columns have also been weakened by scour and moved by drift or migrating soils. internal heat damage to the concrete. lose their concrete cover layer due to corrosion of the reinforcing steel. Bridge Design Manual 8-10 TxDOT 12/2001 . Caps designed by load factor methods can have very high dead load steel stresses. occasionally.Chapter 8 — Substructure Design Maintenance Issues Section 2 — Interior Bents Maintenance problems have primarily been caused by catastrophic or corrosive conditions. This has caused alarming cracks in a few caps that required epoxy injection for appearance and corrosion protection. On one occasion a 54 in. the bridge will fall. in time. If there are only two columns per bent under simple spans with open joints. in diameter can be demolished by an errant truck. and the column continued to support the bridge. Fire has also caused loss of concrete cover due to expansion of the steel and. Columns up to 30 in. column was sheared by an airborne tank truck from a ramp above. The reinforcing steel was offset but unbroken. Columns under leaking joints in bridges on which salt is used for ice removal will. Typical Interior Bents (Online users can click here to view this illustration in PDF.) Bridge Design Manual 8-11 TxDOT 12/2001 .Chapter 8 — Substructure Design Section 2 — Interior Bents Figure 8-2. acceptable aesthetics can be achieved by eliminating as many columns as practical and maintaining smooth lines with unobtrusive caps. If clearance to the cap is critical. Bridge Design Manual 8-12 TxDOT 12/2001 . producing a multi-tiered bent. Multiple-column bents are used in the majority for both stream crossing and grade separations. Piers. Column size may change within the bent height. Inverted tee caps are often used for prestressed beam spans. Columns are usually round but may occasionally be square or rectangular. Multi-tiered bents with web walls are considered to be piers. See Section 3. if clearance permits. in this chapter for information on piers. Caps may be rectangular or inverted tee with or without sloping cantilever soffit. It is conceded that column shape will be a controversial item on most interchange projects. The preferred column shape is rectangular and prismatic. Rectangular caps are recommended with continuous steel girder units. a steel cap framed into the girders can be used. A tie beam may be provided at the size change if necessary to control sidesway moments and unbraced column length.Chapter 8 — Substructure Design Current Status Section 2 — Interior Bents Trestle pile bents are acceptable for stream crossing structures. Post-tensioned concrete or steel box beam caps are recommended for straddle bent caps beyond the span limit of reinforced concrete. A concrete cap cast around and prestressed to the steel girders has been used. Single-column bents are recommended for interchanges and elevated highways where aesthetics dictate a minimum of columns. Except for historical and decorative bridges. Design heights are usually limited by lateral soil properties and the bending strength of piling regularly used. The live load is distributed to the stringers assuming the slab hinged at each stringer. abutment foundation load calculations). box. cantilever regions of the cap actual bearing pad locations for box and U-beams may need to be considered. Assumptions recommended for use with this program or with longhand analysis are as follows: ♦ Dead load reactions due to slab and beam weight are applied as point loads at centerline of beam. increments and not less than the cap width. Live load plus impact reactions per lane are based on the governing truck or lane loading..) ♦ ♦ Class C concrete (f ′c = 3. medians.1 The program is used regularly for multiple-column bents and can be used for single-column bents. Model of Total Live Load Reaction per Lane for Bent Cap Design (Online users can click here to view this illustration in PDF. and I. sidewalks. Bridge Design Manual 8-13 TxDOT 12/2001 . In the middle 1960s a bent cap analysis program (CAP 18) was developed at the University of Texas on a research project. Cap depth should be in 3 in. ♦ Figure 8-3. except the outside stringer. Note that the 26 kip force is used when the uniform load extends in one direction only (e.Chapter 8 — Substructure Design Design Recommendations – Rectangular Caps Section 2 — Interior Bents Trestle pile and multiple-column caps may be analyzed as beams on knife edge supports at the center of piling or columns. and overlay can usually be distributed evenly to all the beams. The use of the lighter 18 kip force for interior bent reactions has been the philosophy of the department since the 1950s. Dead loads due to railing. wider then the columns. For lane loading the 18 kip concentrated load should be used. Typically the total reaction is then modeled as shown in Figure 8-3.g. This applies to prestressed and steel. Lane boundaries should be carefully considered so as to produce maximum stress at various critical locations along the cap.and U-shaped girders. Caps are typically 3 in. Higher concrete strengths are sometimes used in large caps supporting very long spans.600 psi) and Grade 60 reinforcing steel are typically used. In short. clear spacing requirement to facilitate concrete placement and vibration. Consideration should be given to staggering or alternating laps in adjacent bars to minimize congestion.Chapter 8 — Substructure Design ♦ Section 2 — Interior Bents A construction joint should be used in multicolumn bents when the distance between outside columns exceeds 80 ft. Design negative moments are taken at the effective face of the column. Mixing of bar sizes is usually not justified. To simplify design. Attempts should be made to locate these laps in compression or very low tension zones. In addition. For large caps with heavy reinforcement. A second layer may be placed 4 in. and the maximum number in a layer is limited by a 2 1/2 in. While #9 or #10 bars are sometimes used. bars should usually be conservatively extended Ld past an inflection point rather than adhering to the complex requirements in AASHTO. or stage construction. for round or irregular shaped columns. For bottom reinforcement. Location of effective face requires engineering judgment but is generally the face of a square or rectangle. These top and bottom bar cut-off criteria apply to conventional caps with moderate amounts of reinforcement. Flexure design: use the strength design method (load factor) with the check for distribution of flexural steel using z = 170 for moderate exposure conditions. follow the provisions in AASHTO. bundling bars in two-bar bundles is sometimes used to maintain necessary clear spacing. Lap lengths should be based on tension lap requirements (see TxDOT Bridge Detailing Manual). The joint should be located close to a dead load inflection point but not under a bearing seat buildup. The use of 130 for z is typically only justified for bridges over coastal waterways. For additional information concerning reinforcing steel. Typically the minimum number of bars is four top and bottom. limit the number of bars across a column and into a cantilever to three or four to avoid congestion with vertical column steel. Section 5: Stage Construction ♦ ♦ ♦ ♦ ♦ ♦ ♦ Bridge Design Manual 8-14 TxDOT 12/2001 . it is our policy to limit the stress in the steel due to unfactored dead loads to 22 ksi to further minimize cracking that has been observed in numerous bent caps in Texas. Layered and/or bundling bars should comply with AASHTO. For most caps anything in excess of four top bars can be cut off in compression zones between columns. This 22 ksi limit will usually control the final design on longer span structures. based on equivalent area. in the majority of cases #11 bars are used to simplify design and construction. Bars longer than 60 ft. A horizontal tie bar tied to the vertical stirrup legs should support second and/or third layers. Additional bars should end at the column face. Maximum and minimum reinforcement ratios should be limited by the requirements in AASHTO. on center from the outside layer. refer to the following: • • Chapter 5. In heavily reinforced caps. A third layer should only be used in very deep caps. will require laps. Mechanical couplers or welded splices may be specified for stage construction. Section 1: Materials Chapter 4. For shear. This is slightly more conservative than AASHTO Specifications but simplifies design. 8-15 TxDOT 12/2001 ♦ ♦ Bridge Design Manual . Caps 3 ft. b is the bottom width for negative moment and top width for positive. Between columns. Try to minimize the number of stirrup spacing changes. The standard distance from centerline beam to end of cap is 1 ft. Longitudinal skin reinforcement should be used in accordance with AASHTO in caps deeper than 3 ft. shear need not be considered unless the distance from center of load to effective face of column exceeds 1.2d. For moment. research conducted at the University of Texas in the mid 1960s indicated that the reaction from the outside beam provides a clamping effect and that a bar extension of 15 in. minimum and 12 in. For most conventional caps. Distribution of reinforcement and dead load stress limits are the same as for rectangular caps.-9 in. #6 stirrups are typically used. design for shear at the face of the column and extend required stirrup spacing to a convenient distance beyond the centerline of the beam but not less than 1 ft. d may be taken at the inside edge of bearing. use #5 stirrups with a 3 in. For longer cantilevers with sloping soffits.. ♦ ♦ ♦ ♦ ♦ A detailing item that merits attention from the design engineer is the bearing seat build-up for prestressed beam spans. which should be shown on the detail. beyond the center of the beam will develop the bar. Double stirrups may be required close to column faces. The design recommendations for rectangular caps shown above also apply to inverted tee caps. Note that bearing seat build-ups taller than 3 in. but it will rarely control. maximum spacing. In cantilever regions. However. The TxDOT Bridge Detailing Manual shows typical bearing seat configurations. For large heavily reinforced caps. torsion should be considered in single-column bents. require reinforcement. Provide stirrups at 6 in. Since the bearings are relatively far from the center of the cap. and less should have two #5 bars equally spaced in each side face. b is the stem width. Other salient features of inverted tee cap design are as follows: ♦ ♦ Since the caps are usually deeper than 3 ft.Chapter 8 — Substructure Design ♦ Section 2 — Interior Bents Many cantilevers are too short to allow full development length for the #11 Grade 60 top reinforcement. beam side reinforcing should be provided according to Figure 8-4. Extreme grades and skews can produce conflicts between the bearing seat or bent cap and the beams or bearings if the seats are not shown properly on the bent details. spacing. Design Recommendations – Inverted Tee Caps Inverted tee caps have been the subject of considerable research. which should be considered as a minimum for new designs. Shear design: use the strength design method (load factor) with Vc = 120bd.2 Appendix B illustrates the recommended design method.. Primary moment and shear design is similar to that for rectangular caps. ♦ Figure 8-4. Beam Side Reinforcement. Web reinforcing must be sized for hanger loads and for vertical shear. Extra vertical reinforcing should be provided across the end surfaces of the stem to resist cracking. Hanger load stresses. shear friction. Inverted Tee Caps (Online users can click 8-4 to view this illustration in PDF. which has been observed in existing bridges. are not added to shear and torsion stresses. possibly in combination with torsion. which usually control.) Bridge Design Manual 8-16 TxDOT 12/2001 . The former practice of welding bars together for development of ledge reinforcing has been discontinued.Chapter 8 — Substructure Design ♦ ♦ Section 2 — Interior Bents Ledge depth and reinforcing is determined by punching shear.4 provides sufficient development. or moment. It has been determined that widening the ledge and reinforcing similar to Figure 8. Since single. In extreme cases it may be desirable to add shears due to St. for access by fracture critical inspectors. Current practice is to provide a removable hatch cover at both ends. For several years. Bolted connections must be used for this condition. and to paint the inside regardless of whether the outside is painted or bare.or two-column steel bent caps are considered fracture critical. Venant torsion according to Prestressed Concrete Structures3 or Theory of Simple Structures. but the usual case has simple span beams supported by brackets on the sides of the box girders. Box girders may be below the span beams if clearance permits. Figure 8-5 shows a proven detail for attachment of prestressed beam support brackets. Deck slabs continuous across steel box girder caps are discouraged. Anchor bolts can be located inside the box to improve aesthetics. Painting inside the box girder is controversial. The usual occurrence is on two-column bents straddling a lower roadway. Box girder caps have been pierced by continuous steel I-girders. so the exposed depth can be minimized.Chapter 8 — Substructure Design Design Recommendations – Steel Box Girders Section 2 — Interior Bents Steel box girders may be used if the cap design span is beyond the practical limits of reinforced concrete.4 Overturning should be considered. The contractor should be advised if certain erection or slabbing sequences will not be permitted. Box girders may be painted or be of weathering steel. Web plates should preferably be designed to resist shear and bending without transverse stiffeners. ♦ ♦ ♦ Bridge Design Manual 8-17 TxDOT 12/2001 . recommendations of the Bridge Design Section were to weld the girders air tight and not paint inside for either regular or weathering steel. Drip beads and pans may be required to protect concrete from weathering steel run-off. Three options for web-to-flange welding have evolved to accommodate different fabricators’ procedures (see Figure 8-5 and Figure 8-6). Items to consider are as follows: ♦ ♦ Service load design is recommended. bad fatigue details must be avoided. but this practice is discouraged because it can create a terribly fatigue-prone detail. ) Bridge Design Manual 8-18 TxDOT 12/2001 . Cap Supporting Prestressed Beams (Online users can click here to view this illustration in PDF.Chapter 8 — Substructure Design Section 2 — Interior Bents Figure 8-5. Steel Box Girder Bent Cap Example. ) Bridge Design Manual 8-19 TxDOT 12/2001 . Access Hole (One End Only) (Online users can click here to view this illustration in PDF.Chapter 8 — Substructure Design Section 2 — Interior Bents Figure 8-6. Steel Box Girder Bent Cap Example. single-column bents are also usually required. regardless of the anchor bolt strength. If weathering steel is used for steel girder units. Any fluctuation of tension in non-prestressed anchor bolts must be within the allowable stress for a Category E detail. Because of horizontal clearances. At this time. but the bearing should be fixed against translation. The integral or framed steel bent cap has been used many times in many ways in the past. Items to consider in design are as follows: ♦ ♦ ♦ ♦ ♦ Service load design is recommended. Figure 8-7 shows an acceptable detail for bolted integral steel bent caps. Web plates preferably should be designed to resist shear and bending without transverse stiffeners. Design Recommendations – Integral Prestressed Concrete Projects have been built in Texas with concrete caps surrounding and post-tensioned to continuous steel I-girders. Bearing stiffeners should be used over floorbeam reactions. I-girders are the overwhelming choice of shape because of difficult framing details and questionable fatigue performance of pierced box girders. It remains to be seen if there are sufficient advantages to make this solution desirable. Provision for rotation due to girder deflection should be provided. numerous cracks have been found and it is currently considered more appropriate to use bolted connections. steel girders were framed into steel bent caps using field welded connections. the Bridge Design Section does not recommend this method. the same is recommended for integral bent caps. This is virtually impossible to do without creating a Category E detail in the girder. Transverse overturning should be carefully considered. Prior to the early 1980s. Bridge Design Manual 8-20 TxDOT 12/2001 .Chapter 8 — Substructure Design Design Recommendations – Integral Steel I-Girders Section 2 — Interior Bents Interchange structures often contain continuous steel girder units requiring a minimum of cap projection below the bent cap for vertical clearance reasons. Although there have been no catastrophic failures in 25 years of usage. There is a disadvantage in erection of the steel girders because the interior columns cannot be used for support until the cap is stressed. This design is supposed to be more consistent in appearance with approaching inverted tee concrete caps and avoid the complicated framing details of a steel cap. Chapter 8 — Substructure Design Section 2 — Interior Bents Figure 8-7.) Bridge Design Manual 8-21 TxDOT 12/2001 . Girder Connections. Integral Steel Girder Bent Caps (Online users can click here to view this illustration in PDF. Secondary effects are calculated and biaxial bending is treated accurately. Horizontal loads are wind.Chapter 8 — Substructure Design General Design Recommendations for Columns Section 2 — Interior Bents Accurate column analysis is virtually impossible. horizontal loads are estimates. Specifications have adopted conservative approximations of column action for use by the designer. Section 2). longhand calculations will usually suffice for moments due to horizontal forces. and centrifugal forces as described earlier (see Chapter 6.) below the top of the shaft. Column response to these forces is influenced by incipient buckling and secondary bending. and the manner in which the forces due to these loads distribute through the structure is indeterminate. Some of the computer programs that attempt to predict scour are overly conservative. scour has to be considered. Vertical loads are statistical. Analytical studies have shown that fixity is usually achieved with considerably less embedment. longitudinal. and splice lengths can Bridge Design Manual 8-22 TxDOT 12/2001 . using input nonlinear properties of the concrete. some consideration should be given to the possibility of some future minor excavation around the shafts. In any case. For bents in erodeable stream beds. Vertical loads and moments are factored and the section strength compared by an ultimate strength computer program8 PCACOL or charts shown in Appendix C. Strong consideration should be given to designing columns with Grade 40 reinforcing steel even though 60 ksi steel is now required for all reinforcement on TxDOT projects. Biaxial bending is considered in terms of a load contour method (not AASHTO) unless the program PIER is used. solid or hollow. lateral load-deflection properties of the soil can be input into a nonlinear laterally loaded foundation program such as COM624. computer programs6 such as BMCOL 51 can calculate secondary effects directly based on input section properties. It uses a fiber model to analyze any shape. Design of Slender Non-Prismatic and Hollow Concrete Bridge Piers5 contains a good discourse on column design. The computer program PIER7 is the most advanced reinforced concrete column analysis tool available to local designers. if the designer wishes to make a more accurate prediction of the depth of fixity. Columns on single drilled shafts may be assumed fixed at three shaft diameters (but no more than 10 ft. Columns on footings with multiple drilled shafts or piling in both transverse and longitudinal directions are considered fixed at the top of footing. even in soft soils. Impact is included.) below the bottom of the footing. and consultation with a qualified geotechnical and/or hydraulic engineer is recommended to give a better estimate as to the top of the ground for column design purposes. However. Moments may be magnified for design according to the specifications or computed with programs that account for P∆ effects. Experience indicates that the design of the majority of bridge columns is controlled by the AASHTO minimum reinforcement ratio of one percent. For single-column bents on footings with two drilled shafts. Vertical loads are reactions from the bent cap design loads. Additionally. Much has been written and researched on columns. Stress requirements can usually be satisfied with a Grade 40 design and significantly shorter embedment. For single-column bents and single-tier. which depend on flexural section properties usually varying with the forces. transverse fixity is assumed at the top of footing and longitudinal fixity at three shaft diameters (but no more than 10 ft. multiple-column bents. 600 psi concrete reinforced with Grade 40 reinforcing steel. B.600 psi is required. Recognition of conditions that deserve extensive analysis is characteristic of experienced designers. except for very unusual conditions. They are greatly preferred for multi-column bents with rectangular caps. VI(Mod) beams 24 inch 24 inch 30 inch 36 inch When analysis for axial load and bending is necessary. the critical section will usually be in the drilled shaft. Class A concrete may be allowed in the drilled shaft. this method is highly conservative and should only be used when Bridge Design Manual 8-23 TxDOT 12/2001 . Round columns are currently used for a majority of structures. the following column sizes may be used without analysis for axial load and bending. For columns on a single drilled shaft. This is primarily a constructibility issue and is the philosophy used in the current state standards. In the transverse direction the top of the column should be assumed to be free to translate but not rotate. Earthquake effects are not considered at all in the design of columns by the Bridge Design Section. However. Design of single-tier bent columns can be completed by modeling individual columns as fixed against rotation and deflection at some assumed fixity point (see discussion on fixity under General Design Recommendations for Columns) into the soil while free to rotate and deflect in the longitudinal direction. which make it difficult for bridge columns to bend far enough to break. Available sizes.Chapter 8 — Substructure Design Section 2 — Interior Bents then be justified. Occasionally. C beams Types IV. higher strength concrete is required. since there is no reliable computer program available to the Bridge Design Section.000 psi concrete. and recommended height limits are shown on Figure 8-9. Class C concrete should be specified for shafts by plan note. minimum and usual reinforcing. It is evident that considerable redundancies exist. horizontal forces must be resolved into components parallel and perpendicular to the bent by longhand. and with reasonable column spacing as used on the state standards. Column Design – Multi-Column Bents Most of the columns under Texas bridges have 3. Moments can be magnified to account for slenderness (P∆) effects by using the manual method described in the AASHTO Specifications. If the section is adequate for 3. If 3. Slab spans Pan form spans Prestressed beam spans Types A. Within these height limits. Designers are encouraged to take comfort in past performance and avoid complicated studies in column design whenever possible. even for single-column bents. k. may be taken as 1. Effective length factors. secondary effects and biaxial bending may be investigated using the PIER program. Experience has been insufficient to justify size selection without calculation. Transverse and longitudinal moments should be magnified separately. Multiple-column bent tiers with web walls could be considered braced in the transverse direction. Round and square columns in multi-tier bents should be analyzed with the FRAME 11 program (sidesway allowed). Desirable column to tie beam connection details are shown in Figure 8-10. such as BMCOL 51. Effective length factor may be taken as 1.Chapter 8 — Substructure Design Section 2 — Interior Bents a computer program. Square columns are occasionally used for aesthetic enhancement of a structure. Alternatively. Bridge Design Manual 8-24 TxDOT 12/2001 . Factored loads and moments may be compared to column strengths obtained from the interaction curves of Appendix C. Longitudinally.0 transversely and 1. Column capacity in the longitudinal direction is not considered affected by the web wall.0 transversely.5 longitudinally. use of the BMCOL 51 program is encouraged because of the interaction of different lengths and column sizes in different tiers. Recommended tie reinforcing is shown in Figure 8-9. that considers secondary effects is not available. Factored loads and moments should be compared to column strength using the computer program PCACOL. but this is immaterial since there will be no transverse moment to magnify in the braced tier. Design methods may be the same as for round columns except that transverse and longitudinal moments must be magnified separately and combined in an interaction equation. ) Bridge Design Manual 8-25 TxDOT 12/2001 .Chapter 8 — Substructure Design Section 2 — Interior Bents Figure 8-8. Minimum Column Steel Requirements for Round Columns (Online users can click here to view this illustration in PDF. Examples of Typical Square Columns (Online users can click here to view this illustration in PDF.Chapter 8 — Substructure Design Section 2 — Interior Bents Figure 8-9.) Bridge Design Manual 8-26 TxDOT 12/2001 . Chapter 8 — Substructure Design Section 2 — Interior Bents Figure 8-10.) Bridge Design Manual 8-27 TxDOT 12/2001 . Column to Tie Beam Connections. Multi-Tier Bents (Online users can click here to view this illustration in PDF. loads and moments may be calculated in a manner similar to that described above for multiple-column concrete bents.003 concrete strain and linear variation of strain with steel stresses limited to 60 ksi for mild reinforcing and the average stress in prestressing steel at ultimate load. Bridge Design Manual 8-28 TxDOT 12/2001 . For column heights over 100 ft. for prismatic solid columns and preferably for non-prismatic or hollow columns.Chapter 8 — Substructure Design Column Design – Single-Column Bents Section 2 — Interior Bents Single columns are usually rectangular with square. Beyond this size and when mild steel reinforcing is used in addition to prestressed reinforcing.. The height capabilities of these are fairly well established by past practice using standard details. Unfactored and unmagnified loads and moments should produce no more tensile stress than allowed by the AASHTO Specification. Extremely large columns are often hollow. Steel Columns Virtually the only steel columns likely to be used on Texas bridges are H-shaped trestle piling. For these situations. or chamfered ends. Column strength may be estimated using PCACOL. Longhand methods are appropriate for application of horizontal loads and calculation of column loads and moments in single-column bents. Longitudinal and transverse moments must be magnified separately using the AASHTO Specification methods or BMCOL 51. Wind Forces on Structures9 may be of assistance in this regard. Alternatively. Stresses due to the unfactored loads and moments should be compared to the service load allowables given in the AASHTO Specification. Factored and magnified loads and moments should not exceed the ultimate capacity of the cracked section. Effective length factor may be taken as 2. consideration should be given to wind loads more appropriate for the location and height. Should a more accurate analysis be required. it is recommended that loads and moments be calculated in a manner similar to reinforced concrete columns above. longhand methods can be used assuming 0. there have been other columns prestressed for crack control or to allow precast segmental construction.0 in both directions unless it can be shown. A once-used hollow column section is shown in Figure 8-12. analytically. the PIER program can be used to consider secondary effects and biaxial bending. circular. The PCI Design Handbook10 contains ultimate strength interaction curves for square members up to 24 in. Typical single columns are shown in Figure 8-11. It has occasionally appeared desirable to extend trestle piling to greater than standard heights. Prestressed Concrete Columns Besides prestressed concrete trestle piling. that restraints provided by the superstructure sufficiently limit secondary moments. with eight seven-wire strands. They may be tapered in the transverse or longitudinal width but not in both. Chapter 8 — Substructure Design Section 2 — Interior Bents Figure 8-11. Single Column Examples (Online users can click here to view this illustration in PDF.) Bridge Design Manual 8-29 TxDOT 12/2001 . ) Bridge Design Manual 8-30 TxDOT 12/2001 . Hollow Column Example (Online users can click here to view this illustration in PDF.Chapter 8 — Substructure Design Section 2 — Interior Bents Figure 8-12. there is a probability that foundation tip elevations will be set differently for each column. but usually it will suffice to calculate the loads in kips as follows: P MxCx MyCy Foundation Load = N + Ix + Iy P = Vertical load at bottom of footing (k) N = Number of shafts or piling M = Moment at top of footing (k ft. This practice is strongly discouraged because of differential settlement possibilities. of the maximum number of lanes that can occupy the roadway width times the lane reduction factor. Also. For single drilled shaft foundations. Bridge Design Manual 8-31 TxDOT 12/2001 . Single-column bents usually have multiple shaft or pile footings. Extremely large or complicated footings may justify more accurate procedures. is the “calculated drilled shaft load” or “calculated pile load” shown for the bent. This is given in tons as the “calculated pile load” for the bent. and the foundation loads should be the maximum. The rationale for this is that slight settlement of the foundation will cause redistribution of actual column loads. for typical multiple-column bents the vertical load at the bottom of each column is the weight of the bent plus dead load reaction of the superstructure plus the live load reaction.) C = Centroid to extreme shaft or pile (ft. Thus. tending toward equalization. converted to tons.Chapter 8 — Substructure Design Foundation Loads Foundation loads must be based on service load design. For individual pile footings of typical multiple-column bents. the average load at the bottom of a column is the “calculated drilled shaft load. all divided by the number of columns.) I = Moment of inertia of pile group about centroid (ft. the footing weight is added to the average bottom of column load and is divided by the number of piling in the footing. Use of this method will result in the same number of piling in each footing of a bent. Theoretical pile load variation due to moment at the top of the footing is ignored except for unusual situations. considering all appropriate loading groups and including moment effects at the top of the footing. if different loads are given for each column in a bent. without impact. Section 2 — Interior Bents It is a policy of the Bridge Design Section to calculate foundation loads based on the average vertical load on each column.2) The maximum of the calculated loads divided by the allowable overstress for the causative group loading.” which will be shown on the plans and used in the foundation design. dia. dia. it may be corrected by thickening the footing. dia. It will usually be acceptable to calculate moments and shears assuming all shafts or piling to experience the maximum load simultaneously. and to some extent the allowable load.Chapter 8 — Substructure Design Section 2 — Interior Bents The type of foundation. If the shearing stress should exceed the specification maximum. 20 in. dia. adding shear reinforcement. depends on the soil profile. 48 in. or both. sq. Maximum calculated loads recommended by the Bridge Design Section are given in the " Maximum Allowable Loads on Foundation Elements" table below. 72 in. dia. and embedment are illustrated in Figure 8-13 and Figure 8-14. Note: Trestle piles should be checked for structural adequacy.1: Maximum Allowable Loads on Foundation Elements ♦ Maximum Allowable Load (tons) Size Trestle Pile Pile Footing 12 x 53 40 70 14 x 73 60 100 16 in. 18 in. Type Steel H Piling Table 8. dia. Footings Service load design without impact is recommended for drilled shaft or pile footings. Certain design concepts and load recommendations for top and side reinforcing. spacing. clearances. sq 24 in. 30 in. 60 in. 36 in. Shear seldom controls except on extremely large footings. sq. dia. sq. Moments and shears are calculated according to the rules given in the AASHTO Specification. Refer to the TxDOT Geotechnical Manual for a more complete discussion. 42 in. 66 in. 54 in. A geotechnical engineer should be consulted before finalizing the foundation design. Bridge Design Manual 8-32 TxDOT 12/2001 . 75 90 110 140 125 175 225 300 275 400 525 700 900 1100 1300 1500 Concrete Piling Drilled Shafts ♦ Based on service load design Ability of the pile or drilled shaft to transfer load to soil may limit these loads. dia. Typical Pile Footing (Online users can click here to view this illustration in PDF.) Bridge Design Manual 8-33 TxDOT 12/2001 .Chapter 8 — Substructure Design Section 2 — Interior Bents Figure 8-13. Figure 8.14: Typical Drilled Shaft Footing (Online users can click here to view this illustration in PDF.Chapter 8 — Substructure Design Section 2 — Interior Bents Figure 8-14.) Bridge Design Manual 8-34 TxDOT 12/2001 . tower supports and all other substructure under the unit may be referred to as piers. the U. as shown in Figure 8-16. For piers adjacent to the Intracoastal Canal. After the Sunshine Skyway disaster in Florida. Most piers on major river crossings are constructed with round columns and web walls extending full height. On cable-stayed bridges. Coast Guard required a flush face on the traffic side. if the pier was outside of the normal water line. usually supported by two largediameter caissons excavated to the founding elevation. Most of the caissons were excavated by clamshell working inside. Usually. considerable attention was directed to protection of piers from the consequence of impact from an ocean-going vessel. Large hollow columns under major waterway bridges are also called piers. the caisson could be dewatered and the footing and shaft constructed in the dry. The tops of drilled shafts were located about a foot above water or. and columns were constructed on top of the shafts or tie beam. One has large dolphins to ward off ship impact. Piers adjacent to the Intracoastal Canal may be required to have a smooth face. Many river piers were constructed with a single shaft under each column. Piers adjacent to ocean-going traffic require special design against the catastrophic effects of ship impact. These piers had square columns with a wall between. as shown in Figure 8-15. Large-diameter drilled shafts in water became practical in the 1960s.11 Texas has three bridges that were subject to these considerations. and the others have large sand and rock islands built around the critical piers. Web walls were often constructed between columns to strengthen the pier against drift carried by a flood. Current Status Piers are considered to be any substructure composed of reinforced concrete walls or columns with full or partial height walls adjacent to a waterway. Piling replaced caissons. A few were deep enough and wet enough to require excavation within a pneumatic chamber. These are called dumbbell piers. could be stopped a few feet below natural ground. The piers were sturdily designed to resist the destructive forces associated with rivers and bays. excavation was performed inside the cofferdam and piling were driven below the water using a follow block.S. Seal concrete was then placed around the piling. A tie beam between shafts was sometimes used. The shaft could be constructed from a temporary soil island or by using double casing and a floating drilling rig. The weight and the cutting edge on the bottom allowed the caisson to sink as the material inside was removed. and construction was performed in a cofferdam usually formed with steel sheet piling. After curing of the concrete.Chapter 8 — Substructure Design Section 3 — Piers Section 3 Piers Background Early piers were solid walls or webbed square columns. Bridge Design Manual 8-35 TxDOT 12/2001 . Figure 8.15: Typical Dumbbell Pier (Online users can click here to view this illustration in PDF.Chapter 8 — Substructure Design Section 3 — Piers Figure 8-15.) Bridge Design Manual 8-36 TxDOT 12/2001 . Intracoastal Type Pier (Online users can click here to view this illustration in PDF.Chapter 8 — Substructure Design Section 3 — Piers Figure 8-16.) Bridge Design Manual 8-37 TxDOT 12/2001 . intracoastal piers. Bridge Design Manual 8-38 TxDOT 12/2001 . Spans are long and high and usually constructed by unusual methods. Stability of the riverbanks and possible future channel migration should be considered. Piers in rivers or other significant flowing waterways deserve careful attention by engineering personnel in the district. Pier caps usually cantilever past the outside columns for appearance and consistency with interior bents in the bridge. Web walls are assumed to fully support connected caps and tie beams and to prevent transverse moment in the connected columns. geotechnical group. The Bridge Design Section recommends a maximum ratio of 7. hydraulics group.or multi-cell concrete boxes. piers should have a minimum factor of safety of 1.0 under Group II loading. If dumbbell piers are used. The span-to-thickness ratio for hollow columns is subject to controversy. Typical details are shown in Figure 8-15. including scour due to the presence of the pier. Because these piers are more sensitive to longitudinal moments and more likely to be used with longer spans. Acceptable details are shown in Figure 8-16. Crash wall design is nominal. Web walls should be no less than 1. Pier sections will probably be single. Intracoastal Piers. Orientation parallel to the stream flow is critical for minimizing scour and overturning force of floodwater on the pier. and bridge design group. Columns are usually square so the crash wall can be connected smoothly. web walls should extend above the elevation of any possibility of drift in the area. Future scour depth. but designing for unimpeded impact from ocean-going vessels is considered impractical. Piers adjacent to ocean-going channels are usually subject to modified design procedures. but the portion between columns can be nominally reinforced if supported by the web wall. They are not considered to affect the longitudinal section properties of the columns. such as cantilever segmental concrete box girder or cable-stayed segmental. Web wall design is nominal and according to past practice. thick to facilitate concrete placement. Ship impact should preferably be resisted by protective dolphins or islands. Ocean-Going Channel Piers. Wind Forces on Structures. Piers should be sturdy near the water line. Wind loads may be increased because of the height and further modified for the shape of the superstructure based on wind tunnel tests.Chapter 8 — Substructure Design Design Recommendations Section 3 — Piers A few design considerations will be given for dumbbell piers.5 based on the research reported in Design of Slender Non-Prismatic and Hollow Concrete Bridge Piers. Foundation elements must extend sufficiently below the scour line to resist design loads. Intracoastal Canal piers should be subject to the same design considerations given above for dumbbell piers. Dumbbell Piers. Magnitude and application of wind loads during cantilever erection conditions will be negotiated. it appears appropriate to use 19 pounds per square foot for longitudinal superstructure wind instead of the usual approximation. creating additional loads to the piers. cast-in-place or precast and post-tensioned. and oceangoing channel piers followed by a discussion of seal design.12 can be used as a guide Design controls for a recent cable-stayed bridge are shown in Figure 8-17. The cantilever is designed as for any multi-column bent. which is common to all three.13 This standard has been relaxed when it was proven that the column capacity was more than twice the factored load and moment. should be estimated by the best available methods and taken into account in the structural design. At the anticipated scour depth.25 ft. For extreme spacing of piling or drilled shafts. Piling or drilled shafts may be assumed to resist uplift in the amount of 10 pounds per square inch of contract area. Arguments have occurred in the past when pier footings were constructed during high water and the contractor claimed TxDOT was liable for not having provided an adequate seal.Chapter 8 — Substructure Design Seal Design Section 3 — Piers Seal design should be based on hydrostatic pressure created by normal water elevation. Reinforcing steel is not used in seals. For river crossings this level may be difficult to establish. Recommended allowable stresses are 300 pounds per square inch in tension and 80 pounds per square inch in shear. The construction specification is now very specific in this regard. Seal concrete is assumed to weigh 150 pounds per cubic foot. and buoyancy is taken as 63 pounds per cubic foot times the depth of water to the bottom of the seal. Friction between the seal and cofferdam should not be considered.000 pounds per square inch. Seals are always placed with Class E concrete. which has six sacks of cement per cubic yard and minimum compression strength of 3. shear and bending should be investigated in the plain concrete section. The assumed elevation for design must be shown on the plans. Bridge Design Manual 8-39 TxDOT 12/2001 . ) Bridge Design Manual 8-40 TxDOT 12/2001 .Chapter 8 — Substructure Design Section 3 — Piers Figure 8-17. Figure 8.17: Example Design Wind Loads for a Recent Long-Span Bridge (Online users can click here to view this illustration in PDF. Special coatings have been developed to protect steel in marine environments. lights. It has been determined that fender systems will sustain less damage if they consist of vertical piling only. Besides bracing the piers with crash walls or mass concrete as appropriate. Criteria for the Design of Bridge Piers with Respect to Vessel Collision in Louisiana Waterways14 is a good general discourse on problems and probabilities in the Gulf Coast area. Corps of Engineers can furnish valuable information about this. Much has been researched and written. Federal Highway Administration (FHWA)-coordinated research. the Intracoastal Canal. the U.S. This requires an elaborate coating repair procedure that may still be the weak spot in the corrosion protection system. Because of fit-up between the wales and driven piling. The U. Design of any pier protection system should be referred to the Bridge Design Section.S. preferably. attempted to develop an acceptable guide specification for pier protection.18 ( using pooled funds from several coastal states. Good information about the size. usually based on maintenance experience. Coast Guard requires that non-sparking material be used for the horizontal wales that will contact the vessel to minimize the possibility of ignition of flammable material. A probabilistic risk analysis is usually performed to determine the most desirable level of protection. or barricades actuated manually or by span failure detectors. Large elastomeric energy absorbers are available to ease the force on the support members. sand and rock islands for ocean-going traffic. local practice suggests fender systems for barge traffic and large dolphins or. and barge canals. Design of pier protection against ocean-going vessels is even less defined. Protection for the pier may be large dolphins or islands. Casual impact will be a regular occurrence. Some districts prefer timber and others prefer steel. navigable rivers. Barge tows can generate large forces on impact. but usage has been minimal in Texas. as evidence suggests that barge operators like to use fenders as navigational aids under certain conditions. Bridge Design Manual 8-41 TxDOT 12/2001 . accurate analysis of a fender system is very complicated. Fender systems have been used for many years. Large timber pile clusters or small concrete dolphins are often used at the ends and angle breaks of the fenders. instead of bracing with battered piling according to past practice.Chapter 8 — Substructure Design Section 4 — Pier Protection Section 4 Pier Protection In coastal areas along ship channels. and operational characteristics of marine traffic is essential. speed. Currently.16 but application to a particular situation is tedious. Given the forces to be resisted. General requirements for recent Intracoastal Canal fender systems are shown in Figure 8-18. Each situation will require careful consideration of the size and speed of marine traffic and operating characteristics of the channel. horns. connections will usually be welded. Steel rusts badly. but the energy released by a large ocean-going vessel in collision can be astronomical. but some believe the average damage due to impact is less severe and easier to repair. piers need protection from marine traffic. Computerization of Fendering Systems 15 gives an insight into this . Steel members must have a timber or plastic facing.17 Protection for motorists may be required in the form of warning signs. ) Bridge Design Manual 8-42 TxDOT 12/2001 .Chapter 8 — Substructure Design Section 4 — Pier Protection Figure 8-18. Typical Fender Configuration (Online users can click here to view this illustration in PDF. Final Report. John Wiley and Sons.” Modjeski and Masters. Report to LADOTD and FHWA. CFHR. Chmn. New York 1979.” Derucher and Heins. 1983. Transactions. 3269.” Knott and Flanagan. and others.” Poston. “Computing in Civil Engineering. Marcel Derucher.. 6 “A Computer Program to Analyze Bending of Bent Caps.M. 3 “Prestressed Concrete Structures. Second Edition. Volume 126. R. “Bridge and Pier Protection Systems. for FHWA.” Third Edition.” Derucher and Heins.” J. 1966. Part 2.” Matlock. 1983.” Line. Bridge Design Manual 8-43 TxDOT 12/2001 . Final Report. Final Research Report 254F.. 10 “PCI Design Handbook. 1961. Report to LADOTD and FHWA.” J. June 1978..A.M. 1961. Draft Report. New York 1979. Marine Board.B. 18 “Vessel Passage at Navigable Waterway Bridge Crossings. and W.W. 1985. July – August 1985. T.” Furlong and Mirza.. Transactions. Inc. 13 “Design of Slender Non-Prismatic and Hollow Concrete Bridge Piers. Briggs. Marcel Derucher. R. 12 “Wind Forces on Structures. 1988.” Portland Cement Association. H. Third Edition. 3269. Research Report 56-2. CTR.W. Heins and Schelling. 8 “Strength Design of Reinforced Concrete Sections.” Conference Paper.B. and Ned H. Burns. CFHR. M.” Shedd. PCI Journal. 1983. 1985. Preliminary Report. and others. and others.” Poston. and J. National Research Council. Task A. CTR. IABSE Colloquium. 5 “Design of Slender Non-Prismatic and Hollow Concrete Bridge Piers. Prestressed Concrete Institute. Volume 126. 17 “Bridge and Pier Protection Systems. John Wiley and Sons. Briggs. CTR. 2 “Design of Reinforced and Prestressed Concrete Inverted T-Beams for Bridge Structures. American Society of Civil Engineers. and others. 1983. Chmn. Vawter.” Knott. Part 2. American Society of Civil Engineers. 9 “Wind Forces on Structures. Ingram. 1966.” Poston. 1985. Final Research Report 254F. 1966. and W. “Ship Collisions with Bridges.” Modjeski and Masters. Computer Program.C. 14 “Criteria for the Design of Bridge Piers with Respect to Vessel Collision in Louisiana Waterways. Final Research Report 254F. Ingram.” Committee on Ship-Bridge Collisions. T.Y.” Derucher. 15 “Computerization of Fendering Systems.W. 11 “Pier Protection for the Sunshine Skyway Bridge.. 1983. Research Report 56-2. 4 “Theory of Simple Structures. R. 7 “Design of Slender Non-Prismatic and Hollow Concrete Bridge Piers.” Matlock. 16 “Criteria for the Design of Bridge Piers with Respect to Vessel Collision in Louisiana Waterways.Chapter 8 — Substructure Design Section 4 — Pier Protection 1 “A Computer Program to Analyze Bending of Bent Caps. H. American Society of Civil Engineers. Paper No. Paper No. ..................9-113 Section 20 — High Mast Illumination Poles ............................................................................................................................9-13 Section 3 — Deck Replacements...........................9-101 Section 15 — Reinforced Concrete Pipe ...........................9-61 Section 10 — Expansion Joints .....................9-17 Section 4 — Raisings....................9-89 Section 13 — Deck Drainage.......................................................................................................................................................................................................................................................................................................................................................................................................................................9-97 Section 14 — Reinforced Concrete Box Culverts .....9-117 Section 21 — Traffic Signal Poles.......................9-122 Bridge Design Manual 9-1 TxDOT 12/2001 ..............................................................................................................................................................................9-62 Section 11 — Bearings ........................................................................................................................9-105 Section 16 — Corrugated Metal Pipe ...............................................................................................................................................................................9-2 Section 2 — Strengthenings...............9-53 Section 8 — Long Span Bridges...............................................................................................9-49 Section 7 — Historic Bridges ...............................................................................................................................................................................................................9-121 Section 23 — Wildlife Issues........................9-112 Section 19 — Sign Support Structures ......................9-29 Section 6 — Pedestrian Underpasses...................................................................................................................................................9-109 Section 17 — Structural Plate Structures...Chapter 9 Special Designs Contents: Section 1 — Widenings .....9-19 Section 5 — Railroad Underpasses...........................................................................................................................................................................9-54 Section 9 — Bridge Railing ..........................................................................9-119 Section 22 — Sound Walls .................9-111 Section 18 — Long Span Structural Plate Structures .......9-72 Section 12 — Anchor Bolts ..................................... and replaced with concrete posts or steel H piling supported by a footing cast around the timber piling below ground. Prestressed beams have since been used extensively for widening steel beam spans and even cantilever/drop-in and continuous steel units. and double tee beams have been used in isolated instances.) was widened with cantilever/dropin prestressed beams with the deck continuously reinforced across the notched ends. The deck expansion joint is over the interior bents in the widened portion with longitudinal open joints connecting to the existing deck joint at the cantilever end. Section 3 of this manual. prestressed box beams. as discussed in Simple Slab Spans in Chapter 7. Old concrete girder spans were widened occasionally with slab spans but usually with smaller concrete girders or with prestressed concrete beams. Steel I-beam spans with timber piling have been widened and the exposed portions of the timber piling removed. Continuous steel I-beam units have been widened in kind but more often with prestressed beams continuous for live load or with a continuous deck on simple-span prestressed beams. There is a way to widen the H10 high curb design to make it theoretically adequate for H20 loading. bridge widening accounted for about 5 percent of the total bridge construction cost. In 1988. Farm-to-market road slab spans provided a widening challenge. under traffic. Bridge Design Manual 9-2 TxDOT 12/2001 . A continuous steel plate girder unit (100-140-100 ft. many widenings currently include replacement of the existing slab. except for one continuous deck truss on which the required width was attained with cantilever frames from each panel supporting longitudinal stringers. Steel I-beam spans were widened with steel until it was decided that prestressed concrete beams could be used without the difference in stiffness causing deck distress. Pan form girders. ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ Because of bridge deck deterioration. but the method is so complicated as to make it impractical. widenings accounted for about 3 percent of the total deck area of bridges let to contract. In 1963. Trusses have not been widened. Cantilever/drop-in steel I-beams have been widened with simple-span prestressed concrete beams.Chapter 9 — Special Designs Section 1 — Widenings Section 1 Widenings Background Widening of bridges began in the 1950s and has been a steady source of work for designers and contractors. Pan form girders and prestressed beam spans are usually widened in kind. The various types of bridges have been widened as follows: ♦ ♦ Old slab spans were widened with slab spans. In 1997. this number was 10 percent. pile footings. Upward movement of the spread footings. Until recently. Breakback of existing concrete can cause damage to the remaining structure if done improperly. Maintenance Issues Maintenance problems are rare. Round columns with web wall have been used to widen solid wall piers and tapered square column/web wall piers. There have been a few cases where the new foundation. Round columns have been used to widen square columns or even trestle pile bents. Now. Spread footings have been used for widening foundations when the existing bridge is on spread footings. The extreme case involved existing spread footings in a swelling clay with well anchored drilled shafts in the widening. Thick overlay is susceptible to rutting. caused differential vertical movement. or otherwise unsymmetrical with the existing centerline. Current Status Bridge widening can be an economical improvement to roadway traffic and safety conditions. raised the new beams clear of the bearing seat. Trestle piling. Limber columns with no tie to the existing cap have allowed uncomfortable lateral deflection before the deck connection is made. most plans are prepared in the Bridge Design Section and the metropolitan districts. If diaphragms are in place between the new and old beams. acting through the new diaphragms. Caps have been extended with different size caps. creates variable and thick overlay to accommodate the new crown. Caps have not been extended and the new beam supported on a column only. Most widened deck slabs have an asphaltic concrete overlay that tends to hide any surface cracking. There seems to be no appreciable effect on the slab even when placed very close to heavy traffic. they may be broken loose. Span length variation in old bridges has caused the designed structure to misfit. The possibilities appear to be endless. Widening one side only. and drilled shafts are also used. loading up the pile foundation in the widening and causing extensive cracking. being drastically different from the old. A load rating and bridge condition survey must be completed before plans are Bridge Design Manual 9-3 TxDOT 12/2001 . Drilled shafts predominate. In another bridge the spread footing settled due to drying of the founding material. some of which create a strange appearance. most bridge widening plans were made by district personnel from design sketches prepared by the Bridge Design Section. Deck problems have not been reported. Design can be interesting because of the many variations in geometry and structure.Chapter 9 — Special Designs Section 1 — Widenings Substructure design has taken many forms. Construction Issues Construction problems have occurred because of differential dead load deflection between the new beam and the existing. Deck slabs not broken back over a beam are particularly susceptible to damage underneath. or double tees: Prestressed beams are recommended. Continuous steel I-beam units can be widened with prestressed beams or steel I-beams: Simple-span prestressed beams with the slab continuous are recommended. Additional observations are given below. Section properties of old I-beams are given in Iron and Steel Beams. Alternates using double tees may be appropriate for certain situations. It is generally conceded that existing structures with an Inventory Rating of H20 and above are suitable for widening. Curbs may be removed after the new slab has cured. Design Recommendations Design guidelines for the various elements of new bridges may also be applied to widenings. framing. Ratings should be based on the current American Association of State Highway and Transportation Officials (AASHTO) Specification except that allowable stresses should be based on the minimum material strengths used on the original construction. ♦ Slab spans can be widened with slab spans: Skewed slabs with main reinforcing perpendicular to the bents will be weak if the edge beam is removed under traffic. Steel I-beams may be used if depth. An Inventory Rating of H17 is sometimes acceptable for widening structures with low average daily traffic (ADT).1 Allowable stresses for reinforcing steel may be taken from the "Chronology of Reinforcing Steel Specifications (1918-1953)" table and the Chronology of Reinforcing Steel Specifications (1957-1988 Interim) table. Farm-to-market slab (FS) spans of any loading should not be widened. especially the bridge deck. The survey must evaluate the condition of all structural elements. or aesthetics is a problem. Chloride concentration must be measured if there is evidence that the deck has been salted. Box beams may be used if depth is a problem. Allowable concrete stresses may be as shown for slabs on the "Chronology of AASHTO Specification Requirements for Concrete Slabs Reinforced Perpendicular to Traffic" table. The edge should be shored under this condition. Allowable stresses for structural steel may be taken from the "Chronology of Simple Steel I-Beam Standards" table or for standard designs from the "Chronology of AASHTO Specification Requirements for Structural Steel Plate Girders Era" table . Alternatively. Concrete girder spans can be widened with concrete girders. dowels can be grouted into the existing slab edge and the widening placed with reinforcing parallel to the centerline of roadway. Pan form girders can be widened with pan forms.Chapter 9 — Special Designs Section 1 — Widenings begun. prestressed beams. or double tees: Pan forms are recommended. Steel I-beam spans can be widened with prestressed beams or steel I-beams: Prestressed beams are recommended. prestressed box beams. prestressed box beams. Cantilever/drop-in steel I-beam units can be widened with prestressed beams or continuous steel I-beams: Simple-span prestressed beams are recommended with 9-4 TxDOT 12/2001 ♦ ♦ ♦ ♦ ♦ Bridge Design Manual . Widened portions should be designed for HS20 loading. The slab should have standard reinforcing and be tied to the existing slab. Chapter 9 — Special Designs Section 1 — Widenings expansion joints over the bents connected by longitudinal open joints to the existing expansion joint at the notches. Some examples of typical widening details are shown in: ♦ ♦ ♦ ♦ ♦ ♦ ♦ Figure 9-1 Figure 9-2 Figure 9-3 Figure 9-4 Figure 9-5 Figure 9-6 Figure 9-7 Bridge Design Manual 9-5 TxDOT 12/2001 . ♦ ♦ Continuous steel plate girder units can be widened with continuous steel plate girders or with prestressed beams if the span is 140 ft. or less. Prestressed concrete beam spans and units should be widened in kind. Chapter 9 — Special Designs Section 1 — Widenings Figure 9-1.) Bridge Design Manual 9-6 TxDOT 12/2001 . : No Longer Recommended Practice of Widening of FS Concrete Slab Spans (Online users can click here to view this illustration in PDF. ) Bridge Design Manual 9-7 TxDOT 12/2001 .Chapter 9 — Special Designs Section 1 — Widenings Figure 9-2: Examples of Widening Concrete Girder Spans (Online users can click here to view this illustration in PDF. Figure 9.3: Examples of Widening Pan Form Girder Spans (Online users can click here to view this illustration in PDF.) Bridge Design Manual 9-8 TxDOT 12/2001 .Chapter 9 — Special Designs Section 1 — Widenings Figure 9-3. ) Bridge Design Manual 9-9 TxDOT 12/2001 .Chapter 9 — Special Designs Section 1 — Widenings Figure 9-4: Examples of Widening Using Double Tee Beams (Online users can click here to view this illustration in PDF. : Examples of Widening Steel I-Beam Spans (Online users can click here to view this illustration in PDF.Chapter 9 — Special Designs Section 1 — Widenings Figure 9-5.) Bridge Design Manual 9-10 TxDOT 12/2001 . Chapter 9 — Special Designs Section 1 — Widenings Figure 9-6.) Bridge Design Manual 9-11 TxDOT 12/2001 .: Example of Widening Cantilever/Drop-In Steel I-Beam Unit with Prestressed Concrete Beams (Online users can click here to view this illustration in PDF. Chapter 9 — Special Designs Section 1 — Widenings Figure 9-7. End diaphragms are required at ends of units only. Bridge Design Manual 9-12 TxDOT 12/2001 . Online users can click here to view this illustration in PDF.) Explanatory Notes for Figure 9-7 Longitudinal deck reinforcing is continuous across interior supports. Interior diaphragms are not required.: Example of Widening Continuous Steel I-Beam Unit with Prestressed Concrete Beams (See following explanatory notes. There have been other maintenance procedures to restore strength lost to design error. Dire need to retain portions of existing structures for traffic handling during construction has resulted in retrofitting some cover plate ends by bolting as shown in Figure 9-8. Structural steel girder flange plate found to contain extensive laminations and nonmetallic inclusions: Long cover plate was bolted to the flange after erection of the girder. Many steel girder bridges with noncomposite decks have been redecked with new composite slabs with the addition of shear connectors. it became apparent that these strengthening details could create other problems that are worse than theoretical overstress.Chapter 9 — Special Designs Section 2 — Strengthenings Section 2 Strengthenings Background Strengthening has not been a prevalent practice for Texas highway bridges. Under the growing emphasis of fatigue design. Bolting was used to attach plates or shapes to improve moment capacity of chords carrying direct deck load or to decrease slenderness of compression members. or vehicular impact. Concrete columns have been reinforced by encasement. fire. there have been other bad fatigue details in the structure that make it desirable to measure actual stress ranges in the bridge before strengthening. Early widenings of H15 I-beam bridges often required cover plates or angles welded to the bottom flanges. Rusted rivets were replaced with highstrength bolts. This is usually a maintenance procedure required by damage due to chloride attack. Concrete superstructures are hardly ever strengthened. and are expensive. Such procedures are always done under pressure. A few examples are given below: ♦ Design failure to evaluate erection stresses due to curvature of continuous steel girders: Required additional shoring during erection to remain in place until the concrete slab had cured. Usually. construction mistakes. Design error in sizing bent cap shear reinforcing: Required external hanger bolt installation. material deficiency. and hostile environment. Steel trusses have been strengthened to accommodate heavier decks. 9-13 TxDOT 12/2001 ♦ ♦ ♦ Bridge Design Manual . Lower concrete strength in prestressed beams than required by the design: Beams were shored near mid-span until the concrete slab had cured. challenge ingenuity. Current fatigue specifications make it virtually impossible to weld cover plates to old or new steel beams. install new ties before encasement. Columns damaged by fire or chloride intrusion: Remove soft concrete. and strengthening of footings. Pin and hanger damage due to wear and skew on redundant structures: Replace pin and hangers under traffic. Suspected pin fracture on non-redundant structure: Install temporary hangers and replace pins while closed to traffic. Prestressed concrete beams hit by overheight load: Dry pack. or replace sections of girder depending on severity of damage. Steel piling corroded at ground line. Brittle fracture in a redundant steel girder unit: Install temporary hangers and reweld the flange under partial traffic. and weld stiffeners to the flange in the vicinity of bents. General softening of concrete piling due to sulphate attack: Construct new outboard foundation to pick up the load on prestressed footing beams. Concrete piling broken or exploded: Encase with concrete. replace sections of web and flange. Out-of-plane bending cracks in a skewed steel girder unit: Gouge and weld cracks. revised cap design. pneumatically grout. Insulate new columns from moving river bank with compressible materials. or replace beam depending on severity of damage. Steel beams hit by overheight load: Heat straighten. epoxy inject.Chapter 9 — Special Designs Section 2 — Strengthenings ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ Bridge deck slabs constructed with insufficient effective depth: Concrete overlay was bonded to deck. Columns broken by vehicular impact: Shore and encase or replace column and part of cap depending on severity of damage. Foundation elements mislocated during construction: Various solutions included straddle footings. Epoxy grout small dowels and encase. ♦ ♦ Bridge Design Manual 9-14 TxDOT 12/2001 . Bents undermined by scour. If spiral is corroded. Piers moving with the river bank: Construct outboard foundation and transfer superstructure reaction to new cap. or bridge engineers in the metropolitan districts. Bridge Construction Branch.) Current Status Strengthening to increase load-carrying capacity should be carefully evaluated in terms of fatigue susceptibility and cost. : Example of Retrofit Cover Plate Ends (Online users can click here to view this illustration in PDF. Bridge Design Manual 9-15 TxDOT 12/2001 .Chapter 9 — Special Designs Section 2 — Strengthenings Figure 9-8. Repairs should be coordinated with the Bridge Design Section. Rating should be based on AASHTO Specification service load methods. Steel beams and girders with Category D.Chapter 9 — Special Designs Section 2 — Strengthenings Design Recommendations Generally. E or E' fatigue-prone details should also be evaluated for “remaining useful life” before significant modifications are made. This is considered justified by the conviction that a more complicated and accurate structural analysis. structural members with an inventory rating of H17 or better may remain in place without strengthening. such as with a computerized grid program. Equipment exists in the Bridge Design Section for field measurement of stress range histograms and prediction of remaining life. Bridge Design Manual 9-16 TxDOT 12/2001 . New composite decks are often added to steel girder bridges. would reveal an excess of strength. Construction of 2 in. replaced with Class C concrete. Most deck replacements are accompanied by bridge widening. and overlaid with 2 in. Advancements in field stud welding methods have eased the addition of shear connectors. all of which created a unique and complicated structure to build. as a means of strengthening the girders. Section 25 for design recommendations for steel I-beam span shear connectors. Maintenance procedures have not yet been required. of concrete. it was considered sufficient to scrabble off an inch and overlay the existing deck with 2 in. overlays have some cracking and possible delamination. although some of the earlier 2 in. especially with the dense concrete type. In isolated cases. in one continuous slab bridge. Section 26 for design recommendations for steel plate girders. of dense concrete. The continuous slab that required shoring also was widened on the outside and in the median and was on a horizontal curve. The new. When decks are replaced on non-composite steel I-beam or plate girder spans. shear connectors are usually installed on all existing beams. if the chloride content in the second inch below the top of the old slab was 2 pounds or more per cubic yard of concrete. Chloride-laden concrete was removed to a maximum depth of 6 in. regardless of their condition. Removal of decks appears to be no great problem. overlays is tedious. there have been an increasing number of bridge decks replaced. thicker decks are made composite with the girders through the addition of stud connectors. when deck deterioration was recognized as a serious problem. Generally. the existing deck was replaced. Decks on older noncomposite steel girder bridges are often replaced. Deck replacement on concrete girder spans has not been considered practical. If the chloride content in the second inch was less than 2 but the top inch was 2 or more. and see Chapter 7. which requires special mixing and screeding equipment. Shoring was required because of exposed negative moment reinforcing. prestressed beam structures with sufficient structural capacity have been overlaid with a full depth reinforced concrete slab in lieu of replacement.Chapter 9 — Special Designs Section 3 — Deck Replacements Section 3 Deck Replacements Background Since the 1960s. See Chapter 7. Bridge Design Manual 9-17 TxDOT 12/2001 . Design Recommendations New deck slabs shall be designed as provided in Chapter 7 of this manual. Current Status Decisions to replace or rehabilitate a deteriorated bridge deck should be based on the results of a load rating and a thorough condition survey. A complete discussion of bridge load ratings and condition surveys is presented in Chapter 3 of the Bridge Project Development Manual. considering a portion of the old slab composite for new slab load plus the usual width of new slab composite for live load. general warrants for overlaying an existing deck with a full-depth reinforced slab are: ♦ ♦ The existing structure should be composite. The width and thickness to be used in the analysis will be a matter of judgment based on the deck condition. Concrete over the beam should be sufficiently sound to provide some composite action for dead load.Chapter 9 — Special Designs Section 3 — Deck Replacements Although seldom used. ♦ ♦ Bridge Design Manual 9-18 TxDOT 12/2001 . The beam should not be overstressed according to current allowables. Dowels between old and new are not considered necessary to develop composite action. The old deck should be scrabbled before placement of the new slab. For various reasons the low clearance structures on certain routes have demonstrated a propensity for high load impact damage. Lifting points must be realistically analyzed for highway bridges. Current Status Clearance-deficient prestressed concrete beam and steel beam or girder bridges are raised as deemed appropriate by the responsible highway engineers. was allowed. for interstate highways in 1957 to provide for national defense movements. Vulnerability to damage from overheight loads is often the motivation. Sometimes the perpetrator is apprehended and required to make restitution. vertical clearance of 14 ft. permitting of overheight loads is routine and desirable to facilitate the movement of equipment within the state. Pedestal design has been somewhat variable. Even so. None of the structures raised has been hit again. to date none have collapsed. Many highway bridges have been raised. (14 ft. This alleviated the damage problem but did not eliminate it. Jacking from falsework adjacent to traffic is dangerous. Although the maximum legal height of highway vehicles in Texas is 13 ft. It was soon adopted for all major highways. Continuous units raised a different amount at each support require more complicated analyses and usually acceptance of some theoretical overstress. Bridge Design Manual 9-19 TxDOT 12/2001 . Maintenance requirements have not appeared. Simultaneous raising of continuous beams requires coordination of many jacks. Many high loads are moved without a permit.6 in.Chapter 9 — Special Designs Section 4 — Raisings Section 4 Raisings Background In the early days of freeway and interstate highway construction. for automobile carriers).-6 in. Although some of the pedestal supports lack the sturdy appearance of the former bearings. mostly over interstate routes. Construction problems usually involve raising the bridge.-6 in. there is usually significant disruption of highway personnel and highway users until the damage is repaired.-6 in. Design Recommendations Raising of bridge types other than prestressed beam or steel is likely to be as expensive as replacement and is generally not recommended. General rehabilitation of a section of highway may also require raising the crossovers to full 16 ft. and others are higher than that stated on the permit. clearance above a freeway that may have been raised by pavement strengthening or gradeline improvement. Required vertical clearance was established at 16 ft. This has been proven for most diaphragm details by field experience. Load factor design could be invoked to establish the allowable variation. Pedestals for prestressed concrete beams are shown in Figure 9-14. Jacking from falsework is discouraged for safety reasons. was raised. See Figure 9-16. Experience has shown that these can be separated from the beam during lifting. The most common type is fabricated from rolled W or HP shapes to minimize welding. Steel bridges are easily fitted with welded jacking supports. The resultant of dead load reaction and horizontal force applied at the top of the pedestal should fall within the middle third of the base plate width to prevent overturning. Figure 9-15. These probably do not conform to the above design recommendations for overturning. An exception is the thickened slab end allowed by current prestressed concrete beam span standards. Bracing between adjacent pedestals has often been used for lateral stability for taller pedestals.Chapter 9 — Special Designs Section 4 — Raisings Steel and prestressed beam bridge raising can be cost effective. Currently. Liberal analyses of prestressed beam diaphragms will usually reveal them capable of carrying jack reactions to the beams. because a smoother appearance can be achieved. Figure 9-12 and Figure 9-13 show pedestals for continuous steel I-beam units on another project. Steel pedestals are recommended between the cap and bearing for both steel and prestressed concrete beams. Continuous steel units can accommodate some variation in the amount adjacent supports are raised. The prestressed beam bearings were assumed to transfer most of the horizontal load. Skew and horizontal curvature can be accommodated. square or rectangular steel tubing is recommended for pedestal design. Two jacks per beam space are recommended. One such design used on a recent continuous steel I-beam raising is shown in Figure 9-9. Strengthening of the main members is not recommended because of the possibility of doing more harm than good. Figure 9-10. The steel I-beam unit. particularly those exceeding 2 ft. Various types of steel pedestals have been used. eight beams wide. Bridge Design Manual 9-20 TxDOT 12/2001 . Compressive stress in the pedestal should not exceed the specification allowable for concentrically loaded columns.-6 in. and Figure 9-16. if the diaphragms are not suitable. then widened with four prestressed concrete beams on each side and a new deck slab placed over all 16 beams. Jacking from bent caps is usually practical. These figures indicate types that can be adapted to different design conditions. and Figure 9-11. The raised bridge was five beams wide and not widened. ) Bridge Design Manual 9-21 TxDOT 12/2001 .Chapter 9 — Special Designs Section 4 — Raisings Figure 9-9. : Example Pedestal Details for Raised Steel Beams – End Expansion (Online users can click here to view this illustration in PDF. Chapter 9 — Special Designs Section 4 — Raisings Figure 9-10. : Example Pedestal Details for Raised Steel Beams – Interior Expansion (Online users can click here to view this illustration in PDF.) Bridge Design Manual 9-22 TxDOT 12/2001 . ) Bridge Design Manual 9-23 TxDOT 12/2001 . : Example Pedestal Details for Raised Steel Beams – Interior Fixed (Online users can click here to view this illustration in PDF.Chapter 9 — Special Designs Section 4 — Raisings Figure 9-11. : Pedestal Details for Raised Steel Beams – Example 2.) Bridge Design Manual 9-24 TxDOT 12/2001 .Chapter 9 — Special Designs Section 4 — Raisings Figure 9-12. View 1 (Online users can click here to view this illustration in PDF. ) Bridge Design Manual 9-25 TxDOT 12/2001 . : Pedestal Details for Raised Steel Beams – Example 2. View 2 (Online users can click here to view this illustration in PDF.Chapter 9 — Special Designs Section 4 — Raisings Figure 9-13. ) Bridge Design Manual 9-26 TxDOT 12/2001 . : Example Pedestal Details for Raised Prestressed Concrete Beams – View 1 (Online users can click here to view this illustration in PDF.Chapter 9 — Special Designs Section 4 — Raisings Figure 9-14. : Example Pedestal Details for Raised Prestressed Concrete Beams – View 2 (Online users can click here to view this illustration in PDF.Chapter 9 — Special Designs Section 4 — Raisings Figure 9-15.) Bridge Design Manual 9-27 TxDOT 12/2001 . Chapter 9 — Special Designs Section 4 — Raisings Figure 9-16.) Bridge Design Manual 9-28 TxDOT 12/2001 . : Example Pedestal Details for Raised Prestressed Concrete Beams – View 3 (Online users can click here to view this illustration in PDF. Some examples of the material and structure types that have been used are as follows: Deck Types.Chapter 9 — Special Designs Section 5 — Railroad Underpasses Section 5 Railroad Underpasses Background A railroad underpass allows vehicular traffic to pass under railroad traffic. Types of beams have included the following: Bridge Design Manual 9-29 TxDOT 12/2001 . The AREMA Manual for Railway Engineering governs railroad bridge design. railroad personnel or consultants performed most railroad underpass designs over state highways. During the early development of the highway system. The association has since changed its name back to American Railway Engineering and Maintenance of Way Association (AREMA) and currently publishes the Manual for Railway Engineering (1999). All of these designs were performed by TxDOT or a consultant retained by TxDOT. During the 1950s the task of designing railroad underpasses over state highways began to transition from the railroad company to the Texas Department of Transportation (TxDOT). Superstructure and substructure elements of railroad underpasses have been composed of various materials and have included many different structure types. These early designs usually consisted of a meager opening framed by steel beam spans and vertical abutment walls. In 1911 the association changed its name to the American Railway Engineering Association (AREA) and published several subsequent manuals. A basic illustration of a railroad underpass and an example layout sheet can be found in Chapter 2 of the Bridge Detailing Manual. the American Railway Engineering and Maintenance of Way Association published the Manual of Recommended Practice for Railway Engineering and Maintenance of Way as guidance to aid individual railroad companies develop their own policies and practices for railroad design. Types of deck have included the following: ♦ ♦ ♦ ♦ Open timber Closed timber Steel plate Cast-in-place reinforced concrete Beam Types. in a similar fashion to the AASHTO Specifications for highway bridges. During the past 35 years. Most railroads were in existence before the highway system was created. there have been approximately 150 railroad underpasses let to contract over state highways. which is as far back as the records can be documented. In 1905. with the similar goal of providing a guide from which railroad companies can develop their own policies and practices for railroad design. 60. Prestressed concrete I-beams were used only on about 30 percent of the above underpasses because of: ♦ ♦ The relatively short span for a given depth that prestressed concrete beam bridges can carry or The railroad companies’ concerns over difficulties in repairing damaged prestressed concrete beam bridges quickly with minimal disruptions in rail service Through steel plate girders were disallowed for a while because of fear of failure caused by a derailed train. reinforced concrete box beams (deck) Cast-in-place reinforced concrete slab Cast-in-place post-tensioned concrete gull wing girder Prestressed concrete gull wing girder Precast prestressed box beams (through) Interior Support Types. 44 Spl. Recently. IV. and 52 in. C(Mod).Chapter 9 — Special Designs Section 5 — Railroad Underpasses ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ Steel I-beam (deck) Steel I-girders (deck and through) Precast prestressed concrete beams (deck) types A(Mod). either cantilevered drilled shaft or cantilevered spread footing Note: Square-up approach slabs are used for both abutment types on skewed structures. Types of interior supports have included the following: ♦ ♦ Solid wall piers Closely spaced round columns with cap Abutment Types. C. AREMA 42 in. 46. however. through steel plate girder bridges have been used for a large percentage of RR bridges because they have been the best way to carry long spans within the vertical clearance constraints. Types of abutments have included the following: ♦ ♦ Stub type with rip-rap protected slopes Retaining wall type. 34 Spl. The most economical railroad (RR) bridges are constructed with prestressed concrete I-beams. and 72 TxDOT precast prestressed concrete box beams (deck) in depths of 34. 54. Foundation Types. Types of foundation have included the following: ♦ ♦ ♦ ♦ Spread footing Trestle pile Foundation pile Drilled shaft Recent Use. Bridge Design Manual 9-30 TxDOT 12/2001 . 54(Mod). Chapter 9 — Special Designs Section 5 — Railroad Underpasses Examples of typical sections for some of the railroad underpass types that have been constructed in Texas are illustrated in the following figures: Prestressed concrete beams Through steel I-girders Gull wing girders TxDOT box beams TxDOT and AREMA box beams A unique system A system with common details Figure 9-17 Figure 9-18 Figure 9-19 Figure 9-20 and Figure 9-21 Figure 9-22 Figure 9-23 Figure 9-24 The letting year and the low bid unit cost for the entire underpass are included in the above figures. However. the figures should produce a sense of the range of structural possibilities. likely variables. Bridge Design Manual 9-31 TxDOT 12/2001 . and generally relative economy. The costs may not be completely reliable due to economic conditions. bidding abnormalities. and locations of the structures. Chapter 9 — Special Designs Section 5 — Railroad Underpasses Figure 9-17. : Prestressed Concrete Beam Railroad Underpass (Online users can click here to view this illustration in PDF.) Bridge Design Manual 9-32 TxDOT 12/2001 Chapter 9 — Special Designs Section 5 — Railroad Underpasses Figure 9-18. : Through Steel I-Girder Railroad Underpass (Online users can click here to view this illustration in PDF.) Bridge Design Manual 9-33 TxDOT 12/2001 Chapter 9 — Special Designs Section 5 — Railroad Underpasses Figure 9-19. : Gull Wing Girder Railroad Underpass (Online users can click here to view this illustration in PDF.) Bridge Design Manual 9-34 TxDOT 12/2001 Chapter 9 — Special Designs Section 5 — Railroad Underpasses Figure 9-20. : TxDOT Box Beam Railroad Underpass – Example 1 (Online users can click here to view this illustration in PDF.) Bridge Design Manual 9-35 TxDOT 12/2001 Chapter 9 — Special Designs Section 5 — Railroad Underpasses Figure 9-21. : TxDOT Box Beam Railroad Underpass – Example 2 (Online users can click here to view this illustration in PDF.) Bridge Design Manual 9-36 TxDOT 12/2001 Chapter 9 — Special Designs Section 5 — Railroad Underpasses Figure 9-22. : TxDOT Box Beam/AREMA Box Beam Railroad Underpass (Online users can click here to view this illustration in PDF.) Bridge Design Manual 9-37 TxDOT 12/2001 Chapter 9 — Special Designs Section 5 — Railroad Underpasses Figure 9-23. : Railroad Underpass – Unique System (Online users can click here to view this illustration in PDF.) Bridge Design Manual 9-38 TxDOT 12/2001 Chapter 9 — Special Designs Section 5 — Railroad Underpasses Figure 9-24. : Railroad Underpass – System with Common Details (Online users can click here to view this illustration in PDF.) Current Status TxDOT or a consultant retained by TxDOT now designs virtually all railroad underpasses over state highways. Railroad companies are reimbursed for the cost of checking these designs. The TxDOT Bridge Division-Bridge Design Section, and the TxDOT bridge project manager, should be closely involved with any discussions with the various railroad companies concerning railroad bridges designed and built with federal and state funding for or by TxDOT. Bridge Design Manual 9-39 TxDOT 12/2001 which was pioneered by Southern Pacific Railroad. is no longer allowed by Union Pacific Railroad because of the concern that it would be difficult to repair and the repair would disrupt train movements. Prestressed concrete I-beams with concrete deck (30 degree maximum skew). Railroad traffic handling is an important part of the overall design that is usually carefully negotiated during preliminary planning for the underpass. Concrete box culverts. Work is done during short interruptions of rail traffic. 3.Chapter 9 — Special Designs Section 5 — Railroad Underpasses Railroad right-of-way. This means that anything done on or across railroad right-of-way is subject to their review and approval. usually precast boxes that are jacked and bored under the track so as not to disrupt rail traffic. approximate 2000 cost = $150/sf Steel I-beams or steel plate girders with concrete deck on top (30 degree maximum skew). Review by the railroads of the design and plan details for railroad work should be coordinated through the TxDOT bridge project manager. The TxDOT Bridge Division has the in-house expertise to turnkey design railroad bridges and all appurtenances. are the property of the individual railroad companies. approximate 2000 cost = $225 to $275/sf Rolled-in superstructures (complete superstructures placed on the substructure as a unit) have generally not been allowed because the railroads were concerned that TxDOT’s contractors could not complete them in a short enough period of time (usually 3 to 4 days). It should be noted that the cast-in-place post-tensioned trough. Other structure types not mentioned above may be acceptable to the railroad company but must be negotiated on a case-by-case basis with the TxDOT Bridge Division and the railroad company involved. The Bridge Project Development Manual contains information concerning project lead times and agreement requirements. A formal railroad agreement must be executed prior to any such work. in order of preference: 1. Information concerning the handling of rail traffic can be found in Chapter 4 of the Bridge Project Development Manual. approximate 2000 cost = $125/sf Box beams with concrete deck (15 degree maximum skew). 4. 2. and all structures within the railroad right-of-way. Complete agreement between the railroad company and the TxDOT Bridge Division should be obtained regarding the design features before beginning any design work. and any delays in opening a structure to rail traffic can be very costly to a railroad. approximate 2000 cost = $175 to $200/sf Steel through girders with steel floor beams and concrete deck (30 degree maximum skew). Superstructure Type TxDOT prefers the following superstructure types. or assist in the negotiations with a railroad company for the design Bridge Design Manual 9-40 TxDOT 12/2001 . are another commonly acceptable railroad supporting structure. On structures with low vertical clearance. Fatigue-prone details should still be avoided. however. both requirements unique to Union Pacific Railroad. 2. Since they are temporary they may be designed for Cooper E72. walkways. A steel through girder bridge designed in 1996 is shown in Figure 9-26. Bridge Design Manual 9-41 TxDOT 12/2001 . A steel deck girder bridge designed in 1999 for the Union Pacific Railroad is shown in Figure 9-25. and the beams bear on the substructure with only a masonry plate (no bearing pad or shoe).-6 in. clear distance between bottom beam flanges and the slab extending to the bottom of the top flange. These structures are temporary. or shoo-fly structure on an alternate alignment. These are usually open deck structures. but it brings the live load closer to the wing walls. This simplifies superstructure details over the cantilevered walkways shown in Figure 9-24. Substructures are Class A concrete with no surface finish. 3.Chapter 9 — Special Designs Section 5 — Railroad Underpasses features of any railroad bridge or associated items. Union Pacific Railroad has required in some cases that there be no welded connections to the bottom or tension flange because of repairability and fatigue considerations. The ballast curb is a modified T-501 traffic rail shape and the roadbed is wide enough to accommodate the required walkways for trainmen. so they are designed to minimize the expense to construct and facilitate dismantling of the structure upon project completion. thus possibly resulting in a heavier wing wall design. This wealth of knowledge and experience can help assure the successful outcome of any railroad project. and AREMA fatigue requirements are usually waived. Sometimes this necessitates the construction of a temporary. Fatigue concerns are for notches in the tension flange from over-height load impacts. Beams are usually short span wide flanges with no paint and the ties. The clearance width was increased in accordance with AREMA requirements for a structure on a curve. Recent Examples Features and typical sections of three recent railroad underpasses designed by TxDOT are included below: 1. Note the 1 ft. with railroad approval. The structure depicted in this figure meets all Union Pacific Railroad requirements except those for a bolted built-up bottom flange. The general notes must be written to waive these requirements in the TxDOT Standard Specifications. and handrails are untreated lumber. A shoo-fly bridge superstructure is shown in Figure 9-27. This type of structure is used where the apparent superstructure depth is required to be held to a minimum for spans over approximately 40 ft. with the ties connected directly to the beams with hook bolts. Rail traffic must run throughout the duration of a bridge replacement project. Chapter 9 — Special Designs Section 5 — Railroad Underpasses Figure 9-25. : Example Steel Deck Girder Bridge (Online users can click here to view this illustration in PDF.) Bridge Design Manual 9-42 TxDOT 12/2001 . Chapter 9 — Special Designs Section 5 — Railroad Underpasses Figure 9-26.) Bridge Design Manual 9-43 TxDOT 12/2001 . : Example Steel Through Girder Bridge (Online users can click here to view this illustration in PDF. Chapter 9 — Special Designs Section 5 — Railroad Underpasses Figure 9-27. unless a railroad company has design guidelines other than.) Design Recommendations The American Railway Engineering and Maintenance of Way Association’s Manual for Railway Engineering governs railroad bridge design and shall govern all railroad structure designs. : Example Typical Section of a Shoo-Fly Railroad Underpass (Online users can click here to view this illustration in PDF. Bridge Design Manual 9-44 TxDOT 12/2001 . the AREMA guidelines. or in addition to. all design requirements must be determined by the bridge designer during the preliminary planning stages of the project since these additional requirements may greatly affect the design. special exceptions may be entertained on a case-by-case basis. Chapter 15. The Union Pacific Railroad design requirements must be maintained on all Union Pacific Railroad work unless variance can be secured before design begins. This is the usual maximum amount of vertical clearance allowed by the Federal Highway Administration (FHWA) for federally funded projects. Outside beams will not be placed at a lower low-chord elevation in an effort to protect inside beams from an over-height load hit. in which case the specifications refer to the AREMA provisions. of vertical clearance between the low chord of a replacement or new construction railroad structure and the lower roadway. The Texas Standard Specifications (1993) govern the construction of railroad bridges and are generally approved as satisfactory to meet the requirements of most railroad companies. structural steel fabricators must be made aware of the AREMA provisions that govern fabrication. No other railroad company currently has a written bridge design policy. but they all have their own requirements that must be followed. such as additional railroad track approach work. Therefore. TxDOT will not provide sacrificial beams ahead of the bridge that are not connected to the bridge slab as a remedy for insufficient vertical clearance. Section 3. ♦ TxDOT will furnish no more than 16 ft. always refer to the above-listed guidelines and specifications for complete design and construction requirements of railroad underpasses. However. Many times it is costly to provide additional vertical clearance when replacing an existing railroad bridge because the line and grade of the existing track can be quite low and allowable grade on the railroad track can typically be less than 1 percent. The following includes some general design information that may be helpful to the designer. Since a railroad company’s design policy and practice may differ from those presented in the AREMA Manual for Railway Engineering. The one exception is in the area of structural steel fabrication. However. provided they are negotiated before any design work has begun and provisions for the railroad company to bear any additional cost involved.Chapter 9 — Special Designs Section 5 — Railroad Underpasses Union Pacific Railroad has a written set of design requirements that must be adhered in addition to the AREMA Manual for Railway Engineering.-6 in. a limited study by TxDOT has indicated that sufficient vertical clearance is the most important factor in guarding against over-height load hits. A copy of Texas Standard Specifications (1993) should be included along with the design notes and plan details for railroad review. Union Pacific Railroad design requirements will generally suffice for most railroads. have been agreed to in the railroad agreement and associated contract documents. This is based on the uncertainty of the applied load during an over-height load strike event and the ensuing public safety concerns. Vertical clearance at a site where an existing railroad bridge is to be replaced will only be increased above existing conditions as is economical and practicable for TxDOT. All beams in a railroad bridge will be placed at a similar lowchord elevation. ♦ Bridge Design Manual 9-45 TxDOT 12/2001 . Additional information can be found in Chapter 4 of the Bridge Project Development Manual. Additionally. Clearances for railroad underpasses are listed in Chapter 4 of this manual. Lateral live load 9-46 TxDOT 12/2001 Bridge Design Manual . A part of the Union Pacific Railroad Design Guide that deals with beam spacing significantly affects the choice of beam type for a railroad structure. in many cases Burlington is not as stringent if asked. and dead load factor of 1. While a through girder structure has the thinnest overall superstructure depth for a given span length it is also the most costly to build. and length of span. Because of a narrow composite flange and the magnitude of Cooper E80 loading. the superstructure must be carried on steel plate girders. Design is unusual for highway bridge designers because railroad live loads are so much heavier. a span length greater than 55 ft. Includes loading groups. A steel plate deck instead of a concrete deck may be used to decrease overall superstructure depth when vertical clearance is tight. Distribution of live load is very conservative and can be open to interpretation by the various railroad companies. Impact factors for diesel locomotives are used exclusively. higher strength concrete than TxDOT Class S must often be specified for slabs on prestressed concrete or steel beams. but they still vary with structure element. The maximum span length now attainable with AASHTO Type IV beams spaced 18 in. where TxDOT is replacing the railroad underpass and widening the lower roadway. Burlington Northern Railroad requires only 12 in. clear distance between bottom beam flanges in most cases and prestressed beams are still feasible for spans up to about 69 ft. Spans up to about 95 ft. clear under E80 loading before Union Pacific Railroad instituted this requirement. type. TxDOT Class F concrete with a minimum 28-day compressive strength of 5. ♦ ♦ ♦ ♦ AREMA Specifications Salient features of the AREMA specifications affecting underpass design are: Chapter 8 – Concrete Structures and Foundations ♦ Section 2 – Reinforced Concrete Design. it generally endorses the Union Pacific Railroad Design Guide. Similar to AASHTO. At most grade separations. However. clear under E80 loading is about 55 ft.000 psi should be used. Impact factors are usually larger than AASHTO and vary from member to member in the load path. however. Although Burlington Northern Railroad does not have a published guide. is required and. All underpasses are designed for Cooper E80 live load except for shoo-fly bridges.Chapter 9 — Special Designs Section 5 — Railroad Underpasses ♦ All skewed railroad bridges must have an approach slab that squares up the edge of the concrete deck with the railroad track to evenly support the first track tie on the bridge to prevent derailments caused by uneven track support. In such cases. Note the 25 percent longitudinal force provision. were achievable with AASHTO Type IV prestressed concrete beams spaced 1 in. Beam spacing is always close and therefore deck forms often cannot be removed. under E80 loading for a Burlington Northern Railroad structure. Higher strength concrete than TxDOT Class C must be used in interior bent and abutment caps because of the high beam bearing pressures under railroad live load. which can be designed for Cooper E72 live load with railroad approval. therefore.4 is larger than AASHTO. Union Pacific Railroad now requires a minimum of 18 in. clear distance between bottom beam flanges to facilitate beam and slab inspection. Live load is applied to the culvert regardless of depth of fill. Contains clearance. requires that all bearing pads be doweled for a railroad bridge.000 and over 2.000 psi are anticipated. Four percent minimum longitudinal reinforcement is required. Load factor design is not allowed. one of which is the requirement for a 200 psi minimum dead load stress. A useful table of simple span moments and shears due to Cooper E80 loading is contained at the end of this section. Section 17 – Prestressed Concrete Design. The mean impact fraction used for fatigue design is different than that used for strength design.-6 in. See text and commentary for complete provisions. and load distribution information. railroad tie. ♦ ♦ ♦ Chapter 15 – Steel Structures ♦ Section 1 – Design. Section 16 – Reinforced Concrete Box Culverts. Anchorage provisions. Similar to AASHTO Section 9. No final tension in the concrete is allowed in prestressed concrete members. Minimum cover to bottom slab steel is larger. Impact factor calculation is different from reinforced or prestressed concrete design. loading.000. Bridge Design Manual 9-47 TxDOT 12/2001 . railroad tie. If pressures greater than 1. Distribution of live load is very conservative and can be open to interpretation by the various railroad companies. TxDOT’s experience and research tends toward a more conservative design. Yield strength of reinforcement limited to 60 ksi. especially the distribution of live load through earth fill. Very similar to AASHTO provisions adopted in 1985. Union Pacific requires an additional Cooper E65 loading check on the superstructure assuming a non-composite deck after designing the girders for Cooper E80 loading with a composite deck. Similar to AASHTO. Contains design method for culverts under railroads. ♦ Section 5 – Retaining Walls and Abutments. Calculation of impact factor for prestressed concrete is different than that for reinforced concrete members.-6 in.000.Chapter 9 — Special Designs Section 5 — Railroad Underpasses distribution should be based on an 8 ft. Note the 10 in. minimum slab and wall thickness requirement with 2 in. Minimum shear reinforcement is greater and proper detailing of lap lengths for shear reinforcement in box beams requires careful attention. AREMA “fracture critical” is the same as AASHTO “non-redundant. which are the only loading frequencies allowed. Methods vary for different railroad companies. Lateral live load distribution should be based on an 8 ft.” Some railroad companies use a conservative interpretation of what constitutes a fracture critical member. Also includes Pier Protection (Railroad Crash Wall) Provisions. Retaining wall abutments must be designed for Cooper E80 surcharge with limited lateral distribution through the fill and without any support from the span. approval should be obtained from the railroad company. Refers to Section 2 for loading and lateral live load distribution. Allowable fatigue stress ranges are the same as AASHTO for 2.000 cycles of loading. except allowable bearing pressure. of cover over the reinforcement. The railroads can be very conservative in their design approach for these elements. Mechanically stabilized earth retaining walls are only acceptable for temporary installations. Section 18 – Elastomeric Bridge Bearings. unless specifically agreed upon by the railroad.2 contains a few instructions regarding continuous structures. Section 5 – Special Types of Construction.1 covers composite design of simple structures.-6 in. most railroads are reluctant to build them. but some have been used in the past. There is no provision for composite continuous structures. Minimum reinforcing steel areas usually govern. Chapter 28 – Clearances ♦ This chapter contains clearance diagrams for railroad bridge and track design.” and the TxDOT plan details for Waterproofing and Deck Drainage. Note the requirement for full shop assembly of a railroad bridge before shipment to the job site. Section 5. Crash Walls for Railroad Overpasses Although the preceding section is about railroad underpasses. usually in the case of an existing condition at a site. Horizontal clearance can not usually be less than 9 ft. however. they are not recommended for use on TxDOT railroad projects due concerns most railroad companies have regarding their performance. Section 2 for complete provisions and the Commentary for AREMA Chapter 8 for a useful graphical representation of the provisions. Use of the specification item and plan details will adequately cover these requirements. but greater than or equal to 12 ft. thick. Section 5. ♦ Chapter 19 – Bridge Bearings ♦ All bridge bearing provisions will be collected from the various chapters and included in this section in the near future. Crash walls must extend 4 ft. Item 458 “Waterproofing for Structures. See AREMA Chapter 8. above the top of the rail is required. a crash wall extending 12 ft. When the clear distance from the centerline of the track to the face of the columns is less than 12 ft. Sufficient horizontal clearance between an interior support for a highway bridge and an under-crossing railroad track must be provided or a crash wall is required to reduce the chance of the highway bridge collapsing after a hit from a shifted load on a train or a train derailment. These provisions have been included in the Texas Standard Specifications (1993).Chapter 9 — Special Designs Section 5 — Railroad Underpasses ♦ Section 3 – Fabrication.. the need for crash walls is an important aspect of railroad overpass design and deserves mention here. However. above the top of the rail is required. The TxDOT Standard Specification refers to this section for the control of railroad structural steel fabrication. Chapter 29 – Waterproofing This chapter contains all specifications for waterproofing of railroad structures. When the clear distance from the centerline of the track to the face of the columns is less than 25 ft. Refer to the Bridge Detailing Manual for detailing practice for crash walls.. below grade at the base of the wall and be at least 2 ft. Bridge Design Manual 9-48 TxDOT 12/2001 . a crash wall extending 6 ft. It was necessary to drill holes in the deck to drain the water over traffic. Substructures have been predominantly single-column bents on drilled shaft foundations. One maintenance problem that should be remembered concerned a through prestressed beam underpass with the deck slab resting on the lower flanges. Since 1980 there have been 19 on-system and 7 off-system pedestrian underpasses constructed by TxDOT. Bridge Design Manual 9-49 TxDOT 12/2001 . The following superstructure types have been used: ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ Steel truss unit Steel arch unit Steel I-girder unit Steel I-beam unit Combination steel I-beam and plate girder unit Cast-in-place reinforced concrete tee girder unit Precast pretensioned concrete single tee beam span Precast pretensioned double tee beam span Cast-in-place reinforced concrete slab unit using both regular and lightweight concrete Precast pretensioned concrete beam spans – concrete deck on top Precast pretensioned concrete beam spans – concrete deck on the bottom flange of the beam Precast pretensioned concrete beams – deck on top – continuous for live load Contractor-designed alternates (commercial designs) More than half of these used precast pretensioned beams. with time. except for one pedestrian underpass designed with circular ramps composed of precast slabs resting on precast cantilever arms prestressed to a central cast-in-place column. Construction problems have been minor. creating a pond of water on the walkway. Access to the underpass has occasionally been by stairway. Twist and tolerance problems resulted in very difficult construction conditions. but mostly by tangent or circular ramps constructed with concrete slab units. The deck was cast to a level gradeline and the beams sagged. approximately four pedestrian underpasses per year were constructed by TxDOT.Chapter 9 — Special Designs Section 6 — Pedestrian Underpasses Section 6 Pedestrian Underpasses Background In the 1970s. A minimum grade of 1 percent is recommended as a straight grade or ending tangent to a vertical curve. Decks should not be constructed to a level gradeline. but the superstructure is bid as a lump sum item. Prestressed concrete beam camber should not be considered to improve the drainage. Vertical and horizontal clearances should be greater than for highway bridges in recognition of the probability of severe damage from vehicle impact. Prestressed concrete beams.-6 in. the cast-in-place deck can be placed on the lower flanges of the beams. issued in 1979. Required minimum vertical clearance over traffic is 17 ft. Enclosures should have rounded tops for aesthetics except that if any part is horizontally curved. Grades no steeper that 1:12 with level landings at 30 ft. If vertical clearance is critical. require ramps to be designed to accommodate people who are disabled.Chapter 9 — Special Designs Section 6 — Pedestrian Underpasses Current Status Pedestrian underpasses are not popular because of long ramps required and a record of infrequent pedestrian traffic. Reference the 1997 AASHTO Guide Specification for Design of Pedestrian Bridges for additional considerations. The contract plans provide substructure details. but there is an increased interest due to TxDOT participating more heavily in city projects. or one H5 truck. which are paid for as traditional bid items. The loading is the same and the minimum thickness of metal is 1/4 in. but calculated sag should be compensated by extra vertical curve or slope. Design live load is 85 pounds per square foot of deck. with cast-in-place concrete deck on top. but will still be more economical than other minimum depth alternatives. The usual width of walkway is 8 ft. ♦ ♦ ♦ ♦ ♦ ♦ ♦ Bridge Design Manual 9-50 TxDOT 12/2001 . are the recommended design. This adds to the expense because of detail complication and the loss of composite action. Examples are shown in Figure 9-28 and Figure 9-29. the enclosure should be flat-topped to avoid severe construction difficulties. A number of projects have been built recently using a one-time use special specification for a contractor-designed prefabricated pedestrian steel truss bridge. spacing produce very long ramps. Design Recommendations ♦ Federal requirements. Chain-link enclosure should be provided to discourage foreign objects being dropped onto the highway. Chapter 9 — Special Designs Section 6 — Pedestrian Underpasses Figure 9-28.) Bridge Design Manual 9-51 TxDOT 12/2001 . : Typical Section of a Pedestrian Underpass – Example 1 (Online users can click here to view this illustration in PDF. ) Bridge Design Manual 9-52 TxDOT 12/2001 .Chapter 9 — Special Designs Section 6 — Pedestrian Underpasses Figure 9-29. : Typical Section of a Pedestrian Underpass – Example 2 (Online users can click here to view this illustration in PDF. Chapter 9 — Special Designs Section 7 — Historic Bridges Section 7 Historic Bridges Background An increasing number of TxDOT projects involve “historic” bridges. Current Status The Historic Bridge Manual has been developed as a guide to planning projects involving historic bridges. The Environmental Affairs Division is responsible for making this determination of eligibility and for maintaining an inventory of historically significant on-system and off-system bridges in Texas. Bridge Design Manual 9-53 TxDOT 12/2001 . Design Recommendations For design guidance refer to the Historic Bridge Manual. These procedures are intended to minimize project delays that can occur as a result of the complex coordination that historic bridge projects require. It provides a detailed explanation of the federal laws relevant to historic bridge preservation and sets forth procedures for project development that comply with the federal preservation laws. Historic bridges are defined as bridges older than 50 years that have been determined to be eligible for listing on the National Register of Historic Places. Proper closing of continuous trusses was sometimes tedious. All steel trusses have been riveted. Wooden decks. or greater spans. For shorter spans. The "On-System Truss Bridges" table and the "OffSystem Truss Bridges" table show the distribution of truss spans remaining on the state highway system for on-system and off-system bridges. and ships. span. Many old trusses are listed in the National Register of Historical Places. overwidth loads. respectively. Most had riveted steel members. which makes replacement very difficult. used in the beginning. They have been hit by overheight loads. Through trusses are not compatible with high speed or high volume traffic. Through trusses were used for 120 to 300 ft. but one early standard had wooden chords and diagonals with steel verticals in the typical Howe configuration (see Figure 9-30). Long span bridges of the past are simple trusses in Texas and the span lengths are modest. Pony trusses were used to approximately 80 ft. Fabrication was intensive because of the many relatively small and complicated members. so many of the old ones have been replaced. These were predominantly of the Parker configuration with reinforced concrete decks (see Figure 9-31). Construction is complicated by the need for shoring during erection. Most pony trusses were of the Warren configuration with verticals at even panel points. This would govern the design of trusses with 500 ft. errant vehicles. Design Recommendations. Design has been for service loads at working stresses according to classical textbook methods. it has been necessary to replace some rusty rivets with high-strength bolts. but some of the old ones may be subject to Historical Bridge Restoration. gave way to reinforced concrete in the middle 1920s. Standard details can be found dating from 1918 for a variety of truss spans. rivet crews were not easy to find. Many had eyebars for diagonals and lower chords. By far the most frequent maintenance required is the repair of portal bracing and truss members damaged by impact. Trusses carrying highway loading are no longer constructed for the Texas highway system. spans. Shop assembly before reaming field connection holes was required to assure the proper camber after erection. Current Status. Bridge Design Manual 9-54 TxDOT 12/2001 .Chapter 9 — Special Designs Section 8 — Long Span Bridges Section 8 Long Span Bridges Truss Spans Background. service load design would be required in accordance with the current AASHTO Specification. The last simple through truss span bridge was completed in 1951. In recent years. except for eyebars. In the latter days. The AASHTO Guide Specification for Strength Design of Truss Bridges2 was published in 1985. Chapter 9 — Special Designs Section 8 — Long Span Bridges Figure 9-30.) Bridge Design Manual 9-55 TxDOT 12/2001 . : Configurations of Standard Half Through Trusses (Pony Trusses) (Online users can click here to view this illustration in PDF. Half Hip. Parallel Chord Parker Truss. Parallel Chord Warren Truss. Parallel Chord Pratt Truss.Chapter 9 — Special Designs Section 8 — Long Span Bridges Figure 9-31. Parallel Chord Warren Truss. : Configurations of Standard Through Trusses (Online users can click here to view this illustration in PDF.) Configuration Pratt Truss. Parallel Chord Continuous Truss Other Truss. Polygonal Top Chord On-System Truss Bridges Combination Deck Deck/Through 2 3 4 1 Partial Through Through 2 3 21 Bridge Design Manual 9-56 TxDOT 12/2001 . Polygonal Top Chord Lenticular Truss. Parallel Chord Whipple Truss.Chapter 9 — Special Designs Camelback Truss. Parallel Chord Warren Truss. Polygonal Top Chord 1 Other Truss. Parallel Chord 3 Other Truss. Parallel Chord Parker Truss. Parallel Chord Pratt Truss. Half Hip. Parallel Chord Pratt Truss. Polygonal Top Chord Wichert Continuous Truss Total Section 8 — Long Span Bridges 6 1 1 4 1 3 1 1 1 10 4 39 Off-System Truss Bridges Configuration Combination Deck Deck/Through Other than Metal Truss or Other Metal 1 Pratt Truss. Half Hip. Parallel Chord 1 Camelback Truss. Polygonal Top Chord Lenticular Truss. Parallel Chord Pratt Truss. Parallel Chord Camelback Truss. Polygonal Top Chord 2 Pratt Truss. Parallel Chord Parker Truss. Polygonal Top Chord Camelback Truss. Polygonal Top Chord Pennsylvania Truss. Parallel Chord 8 Whipple Truss. Parallel Chord Bedstead Truss. Polygonal Top Chord Other Truss. Parallel Chord 1 Warren Truss. Polygonal Top Chord Pennsylvania Truss. Parallel Chord Warren Truss. Parallel Chord Warren Quadrangular Truss. Polygonal Top Chord Camelback Truss. Parallel Chord 1 Continuous Truss 2 Other Truss. Parallel Chord 1 Partial Through Through 54 2 9 1 3 6 4 1 1 21 10 380 4 3 2 38 2 3 1 2 Bridge Design Manual 9-57 TxDOT 12/2001 . Parallel Chord Continuous Truss Other Truss. Polygonal Top Chord Bowstring Truss. Parallel Chord Warren Truss. Polygonal Top Chord Vierendell Truss Other Truss. Polygonal Top Chord Camelback Truss. Polygonal Top Chord Pratt Truss. Polygonal Top Chord 2 Warren Truss. two hinge. before the state highway system was created. deck arch span was constructed in the late 1950s as part of an interchange with the Dallas-Ft. Early arch bridges in Texas were cast-in-place concrete. arch span was designed by the Bridge Design Section and completed across Lake Austin in 1982. The city engaged a private engineering firm to design a wider superstructure while retaining the unloaded arches and spandrel walls for aesthetic and historical reasons. Worth Turnpike. Other concrete arch bridges were constructed on the system until the late 1930s. Analysis of the ribs was performed with the linear computer program FRAME 11 followed with nonlinear FRAME 51. Early Concrete Arches. The bridge was widened in the 1950s without changing the basic structural elements. completed in 1941 in Austin. The person who wrote this booklet was retained as an overview consultant.Chapter 9 — Special Designs Total 1 22 Section 8 — Long Span Bridges 466 81 Arch Spans Background. and was instrumental in changing the original tied arch concept to the two-hinge design. which have become a vital part of the city’s image. Terms like “intrados. fixed end. Those constructed under the National Recovery Act were especially ornate and of excellent workmanship. Steel Arches. Design was interesting for the one steel arch span designed by the Bridge Design Section. and taken off system again in 1986. A steel box rib. 600 ft. A steel Igirder. The FHWA booklet on arch bridges3 was helpful in preliminary stages. Rising labor costs had made the method uneconomical. marked the end of cast-in-place arch construction in Texas. Several tied arch bridges were constructed in the 1970s in other states. The result is considered quite pleasing.” “extrados.” and “springing” reflect the antiquity of this art form. Fabrication of the span was performed in South Korea with plant inspection by TxDOT personnel. changed to a loop when the interstate highway was constructed. It was incorporated into the primary system in 1930. The new bridge was constructed with TxDOT supervision using federal and state funds. In the 1970s distress was noted in the cantilevers from its spandrel walls. This eliminated erection stresses that would have been locked in due to placing the segments on falsework. A temporary hinge was provided at the crown so forces would be determinate for the effects of rib weight. Steel arches remained as aesthetic alternatives for longer spans. An example of such workmanship can be seen in the Guadalupe River Bridge in Comal County. part through. A similar bridge. but the finished product was of good Bridge Design Manual 9-58 TxDOT 12/2001 . There were many problems and much delay before the methods and workmanship conformed to state specifications. One of the earliest was completed in Austin with county funds around 1910. The arch is one of the oldest means of framing an opening. It is now a roosting place for thousands of bats. Current Status. Bridge Design Manual 9-59 TxDOT 12/2001 . Weathering steel conforming to A588 was used. Some of the tied arches in other states developed serious cracks in the tie. Field splices were bolted with A325. An innovative design was performed in the Houston District for four tied arch bridges to carry city streets over U. The Houston bridges were more complicated due to the construction sequence and posttensioned tie. Cable-Stayed Spans Background. or fixed. whether two-hinge. A true arch. Service load analysis. which allowed for additional vertical clearance under the structures. using FRAME 11. For the Austin bridge. was used for deflections and stress ranges. it could find other applications. since space for bridges is getting more critical. Texas’ first was completed in 1990 and its second in 1995. Anticipate expensive abutment design to withstand this thrust. load factor design methods were used for the arch ribs. but no significant oscillation has been observed. This should eliminate cracking in the tie. Type 3 bolts. long will have a depth of 1 ft.Chapter 9 — Special Designs Section 8 — Long Span Bridges quality. Neoprene cable grippers were installed on the Austin bridge because of this. Single-span structures about 229 ft. Construction was accomplished without serious difficulty. with allowable stresses and plate thicknesses according to the AASHTO Specification. The first modern cable-stayed bridge was constructed in Sweden in 1955. Deck slab concrete was pumped a considerable distance and deposited segmentally so as to load the arch ribs symmetrically. three-hinge. 59. There are no arch bridges under consideration for Texas at the present time. Two false bents on steel piling were used for cantilever erection of each half span. Cable-stayed bridge concepts are said to have begun in the seventeenth century. Avoid tied arches that require extra-high stresses in the tie. from roadway surface to the bottom of the tie. so careful attention should be given to fatigue-resistant details and fracture-critical fabrication. delivers a large horizontal component of reaction to the abutments. Stress ranges due to live load can be severe. No maintenance problems have been experienced on the Lake Austin bridge except for grass growing in the finger joint drainage troughs and graffiti on the arch rib.S.4 Significant development for long span bridges did not occur until after World War II in Europe. Design Recommendations. Analysis by FRAME 51 included secondary effects of deflection and rib shortening so that specified amplification factors were not required. The tie consists of a steel tube encased in concrete and post-tensioned to create no stress in the steel due to full dead load. Two barge-mounted cranes were used to erect most of the components. If this design fares well under traffic. Computer programs STAAD 3 and StruCAD were used for analysis. and all shop connections were welded. The first in the United States was completed in the early 1970s. which was widened to add high occupancy vehicle lanes and shoulders. Wind-induced oscillation of floorbeam hangers was a problem in another state. One was a strutted steel I-girder unit and the other a balanced cantilever castin-place prestressed concrete box girder unit. It is located across the Houston Ship Channel near Baytown. The second bridge has severe oscillation of the stays in a mild wind. Alternatives designed by the Bridge Design Section were provided for the channel unit. The contractor was allowed to change the cast-in-place concrete deck slab to precast concrete. Stays are groups of prestressing strands in polyethylene sheaths anchored with strand chucks and grout. The PTI Recommendations for Stay Cable Design and Testing5 provided some relief. Private engineering firms designed alternates. Center spans were 1. and systems to change the aerodynamic response of the cables have been investigated. The American Segmental Bridge Institute (ASBI) has been formed to provide a forum where owners.250 ft. effective center span. all with varying degrees of success. Current Status. Design Recommendations. Texas’ second cable-stayed unit went to contract in 1986. designers. The stays are groups of prestressing strands in polyethylene sheaths anchored with strand chucks and grout. Both had approximately 630 ft. and suppliers can meet to further refine the procedure. No design recommendations will be given. Design is very complicated. The first bridge has mild oscillation. Texas. The Bridge Design Section does not recommend further use of cable-stayed bridges on the Texas highway system until proven systems are available to eliminate stay oscillations. contractors could also submit an optional design for any or all of the bridge. The lowest bidding contractor chose the structural steel superstructure. The concept had been approved before bidding and the final design and details were approved later. Bridge Design Manual 9-60 TxDOT 12/2001 .Chapter 9 — Special Designs Section 8 — Long Span Bridges In 1984 bids were taken for a bridge across the Neches River between Orange and Port Arthur. Various methods such as cable dampening. According to the specifications. Both of these bridges have sustained damage in the anchorage zone of the stays. one with prestressed concrete box girder superstructure and the other with composite concrete structural steel I-girder superstructure. constructors. A considerable amount of research by universities and forensic consultants has been performed on the bridge stay systems since the bridges have been completed. Texas. cable restraining. Negotiation of parameters was hampered by a lack of authoritative specifications. The lowest bidding contractor submitted a cable-stayed segmental concrete box girder design for the channel crossing. A program was developed. concrete walls alone. to make these old rails. Current Status. Current Status For a complete discussion on the current status of bridge railing. concrete posts with steel or concrete rails. and is still in progress. steel post railing of many configurations. With the advent of the new railing designs. Bridge Design Manual 9-61 TxDOT 12/2001 . safer for the traveling public. the Bridge Design Section has offered at least a hundred designs for use on Texas highway bridges over the last 50 years. of many and varied types. concrete walls with aluminum railing on top. and concrete safety shapes. For a complete discussion on the current status of retrofit railing. Individual districts added many other designs or modifications to Bridge Design Section standards to satisfy their own conception of appearance and functionality. Beginning with weak timber handrails or no railing at all and progressing through massive concrete monstrosities. Retrofit Railing Background. refer to the Bridge Railing Manual. attention was directed to the many miles of presumably unsafe railing already constructed on the Texas highway system. refer to the Bridge Railing Manual. aluminum post railing.Chapter 9 — Special Designs Section 9 — Bridge Railing Section 9 Bridge Railing Background No component of modern bridges has undergone as many changes in design and configuration as bridge railing. stainless steel sheet metal. Design of expansion joints has not been satisfactorily accomplished with analytical methods. then vertical plates with anchors embedded in the deck. Continuous steel girder units. The following are descriptions of some of the expansions joints that have been used: ♦ ♦ Early timber bridges could be designed jointless because of their resilience and loose fit. Details were changed several times to enhance performance. Open steel plate armor joints with rows of studs soon replaced them. Early concrete slab spans. this too causes problems when improper provisions are made to accommodate deck expansion. and require detailed attention while being maintained throughout the life of the bridge. However. concrete girder spans. Expansion joints are a product of ingenuity. A joint with a neoprene membrane clamped between plates and embedded angles was designed in 1967 for use where leakage was particularly undesirable. Its performance was highly sensitive to the installation method. Due to these unattractive traits. This joint was used extensively with various degrees of success. Steel I-beam details changed to open expansion joints. They are problematic during design and construction. During the past 20 years there have been many different expansion joint designs. Prestressed concrete beams had small steel angles anchored with a single row of studs welded to the inside corner of the angles. total movement. and some steel I-beam spans had premolded bituminous expansion joint material topped with poured asphalt in the expansion joints. Truss spans with concrete deck usually had a plate attached to an embedded angle on one side of the joint and sliding over the top of an embedded angle on the other side. The poured asphalt was later replaced with a polysulfide compound.Chapter 9 — Special Designs Section 10 — Expansion Joints Section 10 Expansion Joints Background Bridge deck expansion joints have always been considered a necessary evil for bridges. expanded through finger joints. Neoprene compression seals known as preformed joint seals (PJS) were installed in the joint opening in de-icing salt areas and in grade separation structures beginning in the 1960s. using embedded angles first. which must be proved by tests and actual service conditions. Finger joints have been used for as much as 9 in. Refer to Figure 9-33 for an example of an armor joint with PJS. Numerous configurations of finger joints have been used. A polyurethane compound joint seal (PUJS). and later steel truss units. 9-62 TxDOT 12/2001 ♦ ♦ ♦ ♦ ♦ ♦ ♦ Bridge Design Manual . Refer to Figure 9-32 for an example of a finger joint. attempts are being made to minimize their usage. which could be extruded or poured into the armor joint opening. was used to a limited extent instead of neoprene compression seals. or heavy steel plate to direct the joint water away from the bent. including those with drainage troughs made with neoprene. The epoxy usually cracks and delaminates from the concrete due to a significant difference in coefficients of thermal expansion. Maintenance Issues Maintenance problems involving expansion joints have been numerous. where required expansion exceeds the capacity of a 5 in. Anchors drilled into the slab are of questionable integrity.Chapter 9 — Special Designs Section 10 — Expansion Joints However. 9-63 TxDOT 12/2001 Bridge Design Manual . The common name for this system is sealed expansion joint (SEJ). Some examples of the maintenance problems encountered include the following: ♦ ♦ ♦ ♦ Unarmored concrete corners have spalled. Embedded anchor bolts for joints installed after the slab has been set may not fit. Considerable research and testing has been done by TxDOT in developing a generic specification for elastomeric concrete expansion dams. Small modular joints have been used. Refer to Figure 9-35 for an example of a SEJ. Large expansion joints may require anchor bolts and a slab block-out for proper installation. Strip seals are used in the joint openings. Expansion joints must be carefully graded to avoid excessive roughness of the riding surface. Some problems during construction to be aware of include the following: ♦ ♦ ♦ ♦ ♦ The embedded parts of a joint must usually be supported securely while the concrete deck is placed and finished. Embedded anchors have broken. The open joint with small angles and one line of studs consistently malfunctioned. Rotation of the small angles under traffic was the probable cause. Sliding plate joints always slap under traffic and are difficult to repair. instead of finger joints. ♦ Neoprene strip seals threaded or snapped into recesses in extruded or fabricated steel rails have been used for several years. ♦ ♦ ♦ Construction Issues Placement of expansion joints during construction is tedious. The shape and anchorage of the rails has changed several times over the years in an effort to enhance performance. Refer to Figure 9-34 for an example of this joint. the joint was removed as a standard in 1998. This system usually failed miserably and should not be used. Several experimental installations are being monitored. joint. Epoxy dams placed on top of concrete decks with asphaltic concrete overlay have been used. The openings between dams were often filled with polyethylene rope and a silicone sealer to withstand small movements. Elastomeric concrete or polymer nosing: A specially designed and constructed material to rebuild spalled corners. The most promising methods to date are the following: ♦ Asphaltic plug: A slab of rubberized asphaltic concrete placed over a 1/8 in. or geometric problems. studs and another line of studs in the vertical leg. Should be limited to 3 in.Chapter 9 — Special Designs Section 10 — Expansion Joints ♦ ♦ ♦ ♦ The top plates of membrane joints will become loose and dangerous if not carefully installed. ♦ ♦ ♦ ♦ There is a serious effort underway. Postmortem examinations revealed voids between the horizontal flange and the concrete due to poor concrete consolidation. to repair existing expansion joints that are leaking or otherwise malfunctioning. Epoxy expansion dams often crack and separate from the deck. The studs broke under traffic. Membrane joints may leak at changes in geometry such as a skew break or turn up into a concrete rail. Can be used with various sealing systems to waterproof the joint. monitored by the TxDOT Bridge Division-Bridge Construction and Maintenance Branch. Aluminum joints with drilled anchors have consistently come loose under heavy traffic. Neoprene expansion dams have leaked between the elastomer and the top of the concrete deck. installation methods. have been accommodated. Neoprene expansion dams always lose their bolt hole caps and also tend to slap under traffic. ♦ ♦ Bridge Design Manual 9-64 TxDOT 12/2001 . Compression seals leak and often fall out completely because of irregularities in the opening. Polyurethane compounds proved unsatisfactory for continuous units because of cracking due to first night movements. Total movements of 1 1/2 in. The latest problem involved the anchorage of strip seal rails. Class 7 silicone: A rapid curing sealing material that is placed on a foam filler in sandblasted armor joints. plate placed above the open joint. The rails had a horizontal top flange with a line of 1/2 in. opening and a total movement of 50 percent (+/-). This method is not recommended for new construction. Drainage troughs for finger joints become clogged with roadway debris. ) Bridge Design Manual 9-65 TxDOT 12/2001 .Chapter 9 — Special Designs Section 10 — Expansion Joints Figure 9-32. : Example of Finger Joint (Online users can click here to view this illustration in PDF. ) Bridge Design Manual 9-66 TxDOT 12/2001 . : Example of Preformed Joint Sealer (Online users can click here to view this illustration in PDF.Chapter 9 — Special Designs Section 10 — Expansion Joints Figure 9-33. : Example of Joint Type No Longer Used (Online users can click here to view this illustration in PDF.) Bridge Design Manual 9-67 TxDOT 12/2001 .Chapter 9 — Special Designs Section 10 — Expansion Joints Figure 9-34. ) Bridge Design Manual 9-68 TxDOT 12/2001 . : Example of Sealed Expansion Joint (Online users can click here to view this illustration in PDF.Chapter 9 — Special Designs Section 10 — Expansion Joints Figure 9-35. One inch thickness is used for continuous units. Bridges without ACP: Blast clean and seal with silicone ♦ ♦ Recommended joint types for stream crossings not in a de-icing zone are as follows: Slab Spans and Units: Type A joint with preformed expansion joint material and silicone sealed top.0 in. or less Finger joints with drainage troughs for larger movement Existing unsealed bridges: Bridges with ACP: Sealed elastomeric concrete or asphalt plug. movement. Asphalt plugs have service limitations and should only be used after consulting with the Bridge Construction and Maintenance Branch. Joints for all grade separation structures should be sealed. seal. for 4 in. armor joint sealed with silicone. Refer to the TxDOT standard detail sheet SEJ-A for the most current design details. Recommended joint types for all grade separations and for stream crossings in the de-icing zones are as follows: ♦ ♦ ♦ ♦ Pan form girder units: SEJ-A. or sealed elastomeric concrete Prestressed box beam units: SEJ-A. SEJ-P. Stream crossing structures may have open armor joints in the salt-free zones.Chapter 9 — Special Designs Section 10 — Expansion Joints Current Status Bridge deck continuity. The only membrane type joints approved for use are the following (see the TxDOT standard detail sheets): ♦ ♦ SEJ-A. Refer to the TxDOT standard detail sheet SEJ-P for the most current design details. ♦ Pan form girder units: Open steel plate armor joints 9-69 TxDOT 12/2001 Bridge Design Manual . The SEJ-A can be modified to allow a 5 in. which minimizes the number of expansion joints. All expansion joints in the de-icing zones should be sealed or drained. armor joint sealed with silicone. The SEJ-P has a significantly heavier steel rail and should only be used on bridges where heavy truck traffic is anticipated. or sealed elastomeric concrete Prestressed concrete I-beam units: SEJ-A Steel girder units: SEJ-P for required movement capacity of 5. Finger joints should be used for joints requiring movement greater than 5 in. is recommended.0 in. 1/2 in. or less Inverted tee bent with two sealed expansion joints Finger joints with drainage troughs for larger movements Concrete box girder units: SEJ-P for required movement of 5. for up to 5 in. movement. for simple spans. and Class S concrete is placed with the last adjacent slab. Tooth thickness to 2 in. if SEJ-P is used) below finish grade. but often there is insufficient space to accommodate this. an inverted tee bent cap with its stem extending through to finish grade has been used on a number of projects.42 times the total movement capacity unless the recommended installation width of proprietary seals requires otherwise. Contact the TxDOT Bridge Division-Bridge Construction and Maintenance Branch for details of the asphaltic plug. low modulus silicone is recommended. between steel armor plates. Finger joint design is subject to a variety of different support conditions. With either system 9-70 TxDOT 12/2001 ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ Bridge Design Manual .Chapter 9 — Special Designs Section 10 — Expansion Joints ♦ ♦ ♦ Prestressed box beam units: Open steel plate armor joints Prestressed concrete I-beam units: Open steel plate armor joints Steel girder units: Open steel plate armor joints or finger joints The latest TxDOT standard detail sheets should be used for armor joints and sealed expansion joints. The corbel can usually be detailed to make the bent aesthetically compatible with the other caps in the bridge. For joints with larger movements. should be 5 in. The cap is constructed about 8 in. Anchors installed in drilled holes should not be used. see Figure 9-36. The joint opening at 70 degrees should be set near 0. Design Recommendations ♦ The total movement required through a bridge deck expansion joint may be based on 120 degree temperature change (50 degree rise and 70 degree fall) for steel units and 70 degree temperature change (30 degree rise and 40 degree fall) for concrete units. Compression seals (PJS) and poured or extruded sealers (such as PUJS) are no longer recommended for use. (10 in. at the coldest design temperature. Example details can be furnished for finger joints and troughs. strip seal rails or membrane joint plates. Reinforcing steel strain in the upper pour due to live load should be checked to control cracking. To reduce the need for large capacity joints such as finger or modular joints. Teeth are usually sized for adequate flexural strength at working stress when loaded with their proportional share of the load footprint. such as pan girders or slab spans. A new armor joint standard is being developed that will utilize the silicone instead of PJS. A sealed expansion joint is placed at each face. Metal scuppers attached to one side of the joint with separate deflection baffles attached to the other side are preferable. Drainage troughs for finger joints are even more sensitive to configuration of structural members. a self-leveling. The maximum opening of the roadway surface. The TxDOT Bridge Division recommends a self-leveling low modulus silicone for joints with moderate anticipated movement. Stiffeners below the teeth are used when necessary to limit the thickness. is a nominal maximum. Nylon reinforced neoprene troughs accommodate joint movements well but are difficult to connect to structural members and downspouts. rapid curing. Aluminum joint components are discouraged. ) Bridge Design Manual 9-71 TxDOT 12/2001 . Figure 9-36.Chapter 9 — Special Designs Section 10 — Expansion Joints it is desirable to provide the maximum trough slope possible to minimize the probability of clogging by roadway debris. : Example of Double Expansion Joints at Inverted Tee Bent Cap (Online users can click here to view this illustration in PDF. TxDOT has used many types of bearing mechanisms over the years. National Cooperative Highway Research Program (NCHRP) Report 41 (1977). The following includes discussions on TxDOT’s experience with bridge bearings. TxDOT’s experience with bridge bearings is as follows: Timber spans began and ended with nothing between the stringer and cap but a vertical drift pin. fabrication. graphite. The tarpaper is now 30 pound roofing felt. fixed bearings and expansion bearings. Fixed bearings allow rotation of the superstructure but resist translation of the superstructure.Chapter 9 — Special Designs Section 11 — Bearings Section 11 Bearings Background Bearings transfer loads from the superstructure to the substructure. Expansion bearings allow both rotation and translation of the superstructure. the bottom plate slid on a flat base plate) Welded steel plate bolster shoes with a pin connection for rotation and a rocker plate for expansion Pan form girders started with roofing felt and still have it with oil. There are two basic types of bearings. and maintenance history. lead sheet between (for expansion. with oil and powdered graphite to make it slick. A strip of expansion joint material in the top corner of the cap allows a slight rotation of the spans without spalling the cap. fixed rigidly to the interior supports and slid across a convex steel plate at the ends A very few continuous concrete girders with lubricated bronze bearings For steel I-beam spans the evolution was: ♦ ♦ ♦ Tar paper under a concrete end diaphragm Bolted to a flat base plate with slots in the beam for expansion Concave/convex cast steel plates with a lead sheet between 9-72 TxDOT 12/2001 Bridge Design Manual . and bridge bearing design. Concrete girder spans underwent the following evolution: ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ Tarpaper Two layers of roofing felt separated by a copper sheet coated with graphite grease Cast steel curved plate sliding on a flat base plate Concave upper cast steel plate bearing on a convex lower plate with a 1/8 in. Concrete slab spans began with nothing at fixed bearings and tarpaper at expansion bearings. A good state-of-the-art report on bridge bearings nationwide is given in Bridge Bearings. construction. and cap corner protection The latest standard continuous concrete girders. Examples of bolster shoes are illustrated in Figure 9-37. Refer to the TxDOT standard detail sheet Elastomeric Bearing Details (For Steel Girders & Beams) for current standard design details. There is one roller nest installation on a long cantilever truss unit.) A low profile end expansion rocker plate. Pony trusses tended to have flat plate bearings while deck and through trusses had mostly bolster bearings pinned to the lower chord connection. Lately. A few were used on nonredundant plate girder units. a few disc and pot bearings have been used for steel plate girders as well. The pin and hanger connection is a type of bearing that was used for a brief period in the early 1940s. pinned through the beam web with doubler plates Convex plate bearing on a base plate with anchor bolts clear of the beam flange Fabricated steel or cast ductile iron bolster shoes. Most of them were on cantilever I-beam units.Chapter 9 — Special Designs Section 11 — Bearings ♦ ♦ ♦ ♦ Cast steel bolster shoes with continuous support on a pin and bolted to the beam Inverted convex fabricated steel plate with pintles to the flat sole plate Convex plate bearing on a base plate with anchor bolts through holes or expansion slots in the beam flanges Bolster shoes fabricated by welding steel plates together (Rotation was allowed by a pin transferring the load through the vertical bolster plates. welded to the beam A few of the later I-beam spans rest on plain or laminated elastomeric bearings End expansion bearings for a few continuous units used preformed fabric/Teflon/stainless steel sliding elastomeric bearings ♦ ♦ ♦ ♦ ♦ Steel plate girders had mostly bolster shoes. Expansion was accommodated by a rocker. Since the late 1960s many continuous units have been supported by laminated elastomeric bearings at the interior bents and sliding elastomeric bearings at the ends. Bridge Design Manual 9-73 TxDOT 12/2001 . : Examples of Bolster Shoes (Online users can click here to view this illustration in PDF.Chapter 9 — Special Designs Section 11 — Bearings Figure 9-37. Heavy concrete box girders. being constructed more frequently in the last few years.000 bearings for precast prestressed concrete beams used in Texas since the late 1950s.) There have been about over 500. place difficult demands on bearings. Usually. Except for a few steel bolster shoes. two bearings spaced close together on a capless Bridge Design Manual 9-74 TxDOT 12/2001 . they have all been plain and laminated elastomeric bearings. Some historical design experiences to be aware of are as follows: ♦ Design problems for steel shoes primarily have involved their ability to function under the conditions experienced in the bridge. One side will be heavily loaded while the other side may try to uplift. Expansion bearings must allow for horizontal movement of long continuous units and creep of the unit due to prestressing. Allowance for horizontal movement under these conditions is impractical. Proper embedment may also severely limit anchor bolt strength. Design Issues The design of bearings involves prediction of the loads and movements to be sustained. Bearings under steel box girder bent caps have been plain or laminated elastomeric and preformed fabric. Bearings under deck joints are subject to severe conditions of dirt and moisture plus chloride concentration in the de-icing areas. but columns are usually sufficiently limber to accommodate the deflection. but for one-column integral steel bent caps careful consideration is necessary. Analysis methods and allowable stresses are incidental. Standard bolster shoe details issued in 1965 are slightly 9-75 TxDOT 12/2001 Bridge Design Manual .Chapter 9 — Special Designs Section 11 — Bearings single-column bent carry the reaction of a wide roadway. Bearings under integral steel girder bent caps have been: ♦ ♦ ♦ ♦ ♦ ♦ Separated steel bolster shoes Single long steel bolster shoes Single long steel convex plate bearing on a base plate with prestressed anchor bolts Preformed fabric with high-strength anchor bolts Pot or disc bearings Separated shoes consisting of upright steel plates with convex top bearing in a concave depression in a sole plate This situation is usually severe because of the relatively close spacing of the bearings and large transverse overturning tendency due to off-center live load. Most of the steel trapezoidal box girders constructed to date have had pot bearings. disc bearings. Anchor bolt strength is usually not critical for bridge bearings. the possibility of a significant tensile stress range has caused concern in the past and encouraged prestressing of the anchor bolts. The 1973 AASHTO Specification settled it with a footnote stating that shoe pins were “subject to rotation” and thus had a lower allowable bearing stress than pinned truss members. high load sliding elastomeric bearings. A good bearing mechanism must survive this environment. Texas has used pot bearings. followed by invention of a mechanism to accommodate them for a reasonable service of life. and large laminated elastomeric bearings under these conditions. Because threaded bolts are weak in fatigue. There was a long period of controversy over the allowable bearing stress for shoe pins. NCHRP research7 in 1970 reminded bridge engineers of the variability of elastomeric performance. ♦ ♦ ♦ ♦ Fabrication Issues The quality of the manufactured bearing mechanism can affect a bearing’s performance as much as the design of the bearing. Now. and natural rubber was allowed as an alternative to neoprene. 9-76 TxDOT 12/2001 Bridge Design Manual . Texas was already using neoprene bearings under prestressed concrete beam spans. Performance of the Teflon interface would be questionable under higher loads. More extensive NCHRP research in 19828 led to a specification change in 1985 that the majority of Texas existing bearings could not meet because of shape factor limitations. ♦ Sliding elastomeric bearing design was justified by a few tests conducted by the Materials and Test Division during a period when various manufacturers were trying to develop Teflon/elastomeric combinations for bridge bearings. Allowable design pressure was arbitrarily set at 1. especially at cold temperatures. under the section on expansion bearings for steel structures. could not. In the late 1950s. Design of pot and disc bearings is largely based on manufacturers’ recommendations from the results of their own research and testing. it was found that preformed fabric could restrain the flowing tendency of the Teflon. Then. but no problems have been experienced. serious questions regarding the need for such a conservative design specification continue to confuse the picture. controversy over the ability of natural rubber to perform acceptably and continuous arguments over cold temperature requirements delayed the 1968 specifications. Round robin load tests performed in early 198010 revealed the ability of steel laminated elastomeric bearings to withstand large compression loads. Most elastomer fabricators find it awkward to get another company to fabricate and bevel the steel sole plate for sliding elastomeric bearings.000 psi. Further research has been conducted by the Center for Transportation Research (CTR) with a focus on TxDOT practice.Chapter 9 — Special Designs Section 11 — Bearings overstressed in this respect. without a metal interface. Dupont design data6 found its way into the AASHTO Specification in 1958. elastomeric bearings were given a section of their own in the specification. The method of analysis for base plates was usually debatable. Further NCHRP research9 proposed a more complicated empirical analysis method that allows higher bearing stresses. while other elastomers. Under vertical load and horizontal movement cycling. Ten years later. By then. Elastomeric bearings have undergone a controversial and largely empirical design development. neoprene bridge bearings had been used in Europe for several years and Dupont began to talk about this possibility in the United States. Some fabrication issues to be aware of include the following: ♦ ♦ Fabrication problems with steel bearings concern tolerances and welding procedures for steel plate bolster shoes. although the preformed fabric is capable of much higher loads. It was difficult getting neoprene accepted for bridge bearings initially. Texas had perhaps 20 or 30 bridges with pin and hanger connections of which only 3 or 4 could be considered non-redundant. For pot and disc bearings the condition of the steel backing behind the stainless steel sliding surface is critical because high loading can make the stainless steel assume the shape of the surface beneath. Replacement of these shoes has been extensive. Two of the redundant structures were found to have malfunctions of enough severity to repair. Since the failure of the Mainus River Bridge in Connecticut. which has resulted in some bearings projecting outside the limits of the beam. Bolster shoes have performed well.Chapter 9 — Special Designs Section 11 — Bearings ♦ The formulation and fabrication of laminated bearings is very critical and complicated. Flat steel sliding plates freeze due to rust. All components must be kept clean during the manufacturing process and the mold must be designed correctly to prevent separation of the elastomer or misplacement of the laminate plates during vulcanization. Elastomeric bearings for prestressed beams are easy to construct due to a lack of connection between the cap and beam. ♦ Construction Issues During bridge construction bearing mechanisms must be properly installed in order to function properly. Sliding elastomeric bearings must be floated into wet grout to achieve lateral restraint for the preformed fabric pad. ♦ ♦ Bridge Design Manual 9-77 TxDOT 12/2001 . Some maintenance issues to be aware of include the following: ♦ The older types of steel bearings have been the source of a few maintenance problems. Some construction issues to be aware of include the following: ♦ Construction problems with steel bearings concern getting an even bearing surface under the base plate and welding the top bolster to the girder without distorting the flange. This tends to cause splitting but not failure of the bearing. No problems have been reported with sliding elastomeric bearings. The raw elastomer must be compounded with other ingredients to insure ozone and oxygen resistance and adhesion to laminated plates. ♦ ♦ Maintenance Issues Properly maintaining bearing mechanisms also lends to their successful performance. considerable attention has been focused on pin and hanger connections. Concave/convex surfaces work the lubricating lead sheet out by constant rotation and then the surfaces rust and freeze. Many times maintenance problems can be avoided if loading and environmental conditions can be predicted and properly accounted for in the bridge design. This also makes them easy to mislocate. These are often under leaky deck joints and are subject to rust build-up and freezing. except for the short ones pinned through the webs of the I-beams. Pins were replaced on one of the non-redundant structures because of an ultrasonic discontinuity in one of the pins. so too does the amount of wax bloom. B. Paraffin and microcrystalline waxes are mixed in varying proportions depending on the fabricator’s order. and VI(Mod) prestressed concrete beams: Prestressed concrete U-beams: Steel I-beam spans: Curved and tangent closely spaced steel girder units: Widely spaced steel girder units and steel trapezoidal box girder units: Simple and continuous reinforced concrete slab spans Laminated elastomeric bearings Laminated elastomeric bearings Laminated elastomeric bearings Laminated elastomeric bearings Laminated elastomeric bearings for interior reactions Sliding elastomeric bearings for end reactions Pot or disc bearings. and C prestressed concrete beams: Types 54. laminated elastomeric bearings 1/8 in. 72. In the middle 1980s. TxDOT experienced a multiplicity of incidents of bearings migrating out from under prestressed concrete beams. but the bearings were supposedly designed to not slip due to the extra movement. ♦ All types of bearings have been punished by excessive substructure movement. Piers sometimes move along with the bank as it migrates toward the river. This is the reason that natural rubber is now prohibited in TxDOT elastomeric bearings. It was also a time when revised design specifications were casting doubt on the wisdom of beveling the top of the bearing without providing a positive connection to the beam. only to return to the same condition as the movement continues. Current Status The following types of highway bridge bearings are recommended: Types A. Moving skewed abutments causes lateral distortion of bearings that cannot be accommodated by steel bolster shoes. standard bearings were conservatively redesigned. Failure to anticipate the magnitude of horizontal movement has caused elastomeric bearings to become sliding bearings. was prevalent. Plain bearings over one inch thick are subject to splitting along the bulging edges. Slick bearings have contributed to bearing migration. Interior bents have been displaced by movement of the riprap at the bridge end. asphalt board with oil and graphite at expansion Bridge Design Manual 9-78 TxDOT 12/2001 . it was determined that the primary cause of the problem was a paraffin wax bloom on the surfaces of natural rubber bearings making them very slick. As an added precaution. and as the amount of the less expensive paraffin component increases. After much study and testing and after reinstalling many bearings. but no bearings have been replaced in the Texas highway system because of splitting. This multiplicity of hardships makes it difficult for bridge bearings to survive. Abutments regularly move toward the bridge due to fill settlement. No one knew that bearings were being manufactured from natural rubber. after almost 30 years use of elastomeric bearing pads and several years successful use of laminated bearings with their tops beveled to the slope of the beam.Chapter 9 — Special Designs Section 11 — Bearings ♦ Problems with elastomeric bearings have often been caused by design deficiencies. End bearings can get completely extended and connections broken because of this. This wax is a necessary part of the formulation to satisfy current ozone resistance tests. nor had they attached any significance to the wax bloom until the migration problem was epidemic. IV. This came at a time when the use of continuous units of as long as 400 ft. Some have been freed and straightened. with cap corner protection Doweled at fixed ends.Chapter 9 — Special Designs and pan form girder spans: Section 11 — Bearings ends. with cap corner protection (Refer to Figure 7-3 and Figure 7-4 for examples. with cap corner protection Doweled at fixed joints with neoprene ring column corner protection (Refer to Figure 7-12 for an example.) Prestressed concrete slab units: TxDOT box beam spans: Concrete double tee spans: Cast-in-place concrete box girder: Precast segmental concrete box girders: Steel box girder bent caps: Integral steel girder bent caps: Pin and hanger connections should not be used in non-redundant structures. and graphite at expansion end. oil.) Laminated elastomeric bearings. Bridge Design Manual 9-79 TxDOT 12/2001 .) Asphalt board. two at one end and one at the other Laminated elastomeric bearing Rigidly fixed to interior supports Sliding elastomeric bearings at expansion ends Preformed fabric bearings at fixed supports High load sliding elastomeric bearings at expansion supports Preformed fabric bearings Separate shoes with upright convex plate bearing in concave depression in a sole plate (Refer to Figure 9-38 and Figure 9-39. ) Bridge Design Manual 9-80 TxDOT 12/2001 .Chapter 9 — Special Designs Section 11 — Bearings Figure 9-38. : Example of Bearings for Integral Steel Girder Bent Caps – View 1 (Online users can click here to view this illustration in PDF. polytetrafluoroethylene surfaces. Steel Components Bridge Design Manual 9-81 TxDOT 12/2001 .Chapter 9 — Special Designs Section 11 — Bearings Figure 9-39. elastomeric bearings. and pot and disc bearings.) Design Recommendations Suggestions will be given for steel components. : Example of Bearings for Integral Steel Girder Bent Caps – View 2 (Online users can click here to view this illustration in PDF. preformed fabric pads. Pins may conform to ASTM A108 (4 in. Pin and hanger connections should conform to the AASHTO Specification for “Links and Hangers” and the following: ♦ ♦ ♦ Hangers should be constant width plates of A709. or less in diameter) or ASTM A668 Class C. per ft. Plate thickness should be based on service load design.02 radians (0. D. ♦ Rotation of 0. tallow.02 ft. Grade 36. Base plates and sole plates bearing on concrete should be sized for a maximum pressure of 0. Bridge Design Manual 9-82 TxDOT 12/2001 . or G. If weathering steel is used. and linseed oil. Grade 50 steel with maximum width of eight times their thickness. Pins get a special coating of zinc oxide.4 times the calculated reaction due to all loads. Allowable bearing stress on rockers. Hanger plate area should have a tensile working strength. and it should be painted if exposed. of at least 1. Pins and pin holes are finished to ANSI 125 roughness or better. on the net section through the pin. ♦ ♦ Rockers are finished to ANSI 250 roughness. Allowable bending stress in pins is 0. Where: d = diameter of rocker or roller in inches Fy = minimum yield point in tension of steel in the roller or bearing plate. Steel plate should usually conform to American Society of Testing and Materials (ASTM) A709. ≤ d ≤ 125 in. Whichever is the smaller.Chapter 9 — Special Designs Section 11 — Bearings Steel components occur in most types of bearings. or Grade 50.8 Fy. in pounds per linear inch. Pins should be ASTM A668. Concrete bearing areas should be reinforced to resist bursting stresses. as required by AASHTO and the TxDOT construction specifications. is: Fy − 13000 ç ÷ 600d 20000 OR æ Fy − 13000ö ç ÷ 3000 è 20000 d . increased by the square root of the plate area divided into the area of concentric concrete.) may be assumed for dead load plus live load deflections. not to exceed 2. for 25 in. Unless otherwise required. Class F or G with recessed pin nuts. ♦ ♦ ♦ ♦ Allowable bearing stress on pins is 0. the components should be capable of transmitting a horizontal force of one-tenth of the vertical capacity from superstructure to substructure at working stresses.3 f 'c. details that promote a continuous moist condition must be avoided.4 Fy (yield stress of pin or bearing plate). using the bearing pressure as load with supports as defined by the connecting components. F. stability. There is an amount of creep equal to 25 percent +/− of initial dead load deflection. thick. square should not be specified without prior consultation with a reputable bearing manufacturer. Each laminated bearing must be molded and vulcanized separately. Initial deflection under dead load is usually around 4 percent for 50 durometer constant thickness laminated pads with shape factors of approximately 10. Plates reinforcing the girder web for bearing stress should be ASTM A709. usually 12 gauge 0. Details can be found in the TxDOT standard detail sheet Elastomeric Bearing and Beam End Details. symmetrical either side of the web and fully developed by fillet welds and possibly plug welds in shear. under sustained load. Mold cost is significant if only a few bearings of that size will be required. However.005 radians is allowed for deviation of the bearing surface from the theoretical plane.105 in. B. this fatigue damage is a rare occurrence. for tapers approaching 0. TxDOT elastomeric bearing design practice departs from the AASHTO design specification in several key areas. Bridge Design Manual 9-83 TxDOT 12/2001 . rotation. a sizable compressive strain would be induced in an ordinary bearing thickness. A tolerance of 0. Use of AASHTO methodology alone will result in usage of bearings from TxDOT standard sheets that is in conflict with intended applicability. Sizes greater than 42 in. Taper. The elastomer is a proprietary mixture of ingredients that must be loaded into the mold with extreme care and cured under high temperature and pressure to attain a measure of integrity. If all of these tolerances were to accumulate on one corner. Plug welds should be used when the distance between attachment would otherwise exceed 12 times the thickness of reinforcing plate. Controversy over the latest design specification is understandable. Standardization of sizes is preferred. ♦ Elastomeric Bearings Elastomeric bearings shall be laminated neoprene (polychloroprene). Steel “sole” plates may be vulcanized to the bearing for attachment. depending on the size of press available for containing the mold and elastomer during vulcanization. The beam surface that rests on the bearing has another tolerance in addition to its variation due to camber and deflection. which significantly influence elastomer performance. IV. Accurate design of elastomeric bearings is virtually impossible. 54. Grade 50. and 72 beams and utilizing various beam angles have been standardized. but mold costs should not preclude designing new shapes if the situation demands. Tapered bearings deflect up to 60 percent more than constant thickness pads. Elastomeric bearings for Type A. bearings are subjected to fluctuation in vertical and horizontal movement and rotation at extremes of ambient temperature. and testing procedures for material properties are all areas that TxDOT has chosen to base its design philosophy on research results as well as on extensive field experience. The bearing area on the bent has a tolerance for flatness.Chapter 9 — Special Designs Section 11 — Bearings ♦ ♦ The allowable shearing stress in pins and the allowable bearing stress of pins on hangers and reinforced girder webs should be 20 ksi. Buckling of the web plates should be investigated.06 radians. In the structure. allowable compressive stress. There is evidence that fluctuation of vertical load and horizontal movement can cause fatigue damage to the bearing. cold temperature shear modulus.0. There is a limit to the size of bearings that can be vulcanized. C. Solid steel sheet or plate is used for laminates. narrow beam spacings. special bearings should be designed according to the following provisions for prestressed concrete beams. Units made up of Type A or Type B beams may use the standard pad on up to four equal length spans. Note that Types IV and 72 require special consideration of the bent cap width. and severe grades can make the standard bearing pad unacceptable for use on the ends of multiple span units. The 70° F value is used for allowable compressive stress. level beam bearing surfaces. etc. the provisions of Method B of NCHRP Report 325 and now the AASHTO Specifications are recommended. the thickness of the outer layers of elastomer shall be no more than three-fourths of the thickness of the inner layers.. including round bearings. Note: This allows a shear strain of 50 percent. Standard bearing pads to be used as end bearings on any unit with three or more spans should be checked for compliance to slip prevention equations. with clarifications and modifications given below: Note: The shearing modulus of elasticity (“G”) may be taken as follows: Shearing Modulus of Elasticity Hardness At 70° F At 0° F ° ° 50 Durometer 110 psi 175 psi ♦ ♦ Note: Test results are inconsistent.Chapter 9 — Special Designs Section 11 — Bearings Type VI(Mod) beams require laminated 50 durometer bearings of special design. Bridge Design Manual 9-84 TxDOT 12/2001 . The 0° F value is used for slip calculations. These bearings are considered adequate for use with three-span units made up of equal length spans of a given beam type. as previously noted. Note: There is a wide variation in these properties among available sources of information. ♦ For roadway grades equal to 5 1/2 percent or less. For roadway grades greater than 5 1/2 percent. The total elastomer thickness (all layers) shall be no less than twice the calculated translation in one direction based on an assumed temperature fluctuation of 70° F from the installation temperature. tapered pads will deflect more than constant thickness pads. These graphs were taken from two sources. Figure 9-40 gives deflection characteristics for constant thickness pads. Short end spans. ♦ Compressive stress/strain performance of elastomeric bearings may be taken from Figure 9-40. Examples of special anchorage include fixed beam ends at low end. these bearings should have the top surface beveled to match the slope of a line between the bottom of the beam elevations at each end of the beam.11 Also. Various modifications to the plan configurations. At the ends of longer units with continuous prestressed concrete beams. For other elastomeric bearings. special anchorage details may be required. Strain requirements will be satisfied by virtue of restricting the unit length to less than 400 ft. ♦ ♦ For elastomeric layers thicker than 3/8 in. These values are considered sufficiently adequate for TxDOT bearing design. sole plates. to fit standard bents at different skew angles are shown on TxDOT standard drawing IBB. 0E-06 x 70 ° F x 12 6.010 L' (Concrete) (Steel) Minimum T = 0. and creep need not be taken into account.) F = Dead load reduction factor due to beam slope or grade = (0./ft. shrinkage./ft./ft. the pad will accommodate thermally induced shear translation in either direction for the duration of the structure’s life.0050(L')(175)(A)/[(0.) t = Interior layer thickness (in.0002 x 12 0.) L' = Expanding length (ft. = 0.011 L' (Steel) Minimum T = 0. = 0.2)(Rd)(F)] 9-85 TxDOT 12/2001 For No Slippage (without anchorage): Bridge Design Manual ./ft. Concrete Temperature. From this equilibrium position./ft.) W = Bearing length along beam (in. Steel Shrinkage. leaving a very slick interface. = 0.) L = Bearing length across beam (in.0024 in. Prestressed Concrete 6. ♦ For Translation (maximum shear strain): • • ♦ • • • Minimum T = 0.0005 x 12 = 0./ft.) Rt = Total reaction (lbs. Prestressed Concrete Creep.2 x G x SF.0055 in. other than that caused by thermal characteristics. elastic shortening.2) Rd = Dead load reaction (lbs.0050 in.) SF = Shape factor: Rectangular — L x W/(2(L+W)t) Round — D/4 A = Area of bearing (in.0060 in. The provisions for simultaneous compression and rotation have been rejected because of undue complication. Note: Bearings have been observed to migrate because adhesives are broken by flexing of the elastomer.5E-06 x 70 ° F x 12 0.0024 in.0050(2)(L') = 0. Note: For the typical prestressed concrete beam pad design.2 Gr = Beam slope (ft.Chapter 9 — Special Designs Section 11 — Bearings ♦ Translation of bearings (from installation position) may be calculated from the following strains: Temperature.) D = Diameter of round bearings (in.2-Gr)/0.500 psi Note: This is a simplification of portions of Method B of the AASHTO Specifications in view of the demonstrated ability of laminated elastomeric bearings to sustain much higher loads. A “one slip” philosophy.8 L' A / [(Rd)(F)] (Concrete) Minimum T = 0. or 1. whereby all movement that occurs after beam erection. Concrete Elastic Shortening. ♦ For precast prestressed concrete beams and steel girders these requirements may be summarized as follows: T = Total elastomer thickness (in. = 0.005 (L')(175)(A)/[(0.0055(2)(L') = 0. whichever is less Maximum Rt/A = 1.200 psi.) G = Shear modulus Hardness = 50 Durometer Minimum shape factor = 6 Target shape factor = 10 to 12 Maximum Rd/A = 1. ♦ The use of adhesives between the bearing and beams or bearing seats is not recommended. may be disregarded due to the beam slipping on the pad one time and thereby restoring the pad to an equilibrium position.2)(Rd)(F)] (Steel) Minimum T = 4. ♦ If upper and lower bearing surfaces are cast against the in-place bearing. ♦ If upper and lower bearing surfaces are precast. the estimated compressive deflection (as determined from the performance curves) should be increased by 10 percent for every 0. and therefore rarely a consideration where differential deck surface elevations are concerned. the provisions of Method B have been rejected because of their unnecessary complication. If total deflections are needed.4 L' A / [(Rd)(F)] Maximum T = the lesser of L/3 or W/3. These numbers are a judgmental recognition of the problem. as defined in the construction specification. Note: To keep the beam from lifting off the pad by more than 20 percent.25 for creep. for the element that allows rotation. Transverse non-parallelism was eliminated from consideration in the Method B Specification. a deflection due to rotation equal to 0. Note: Research has indicated that the pad will continue to function properly with up to 20 percent of lift off (a departure from AASHTO design philosophy for rotation). multiply the above by 1.. This may be taken as 0. Note: This includes a nominal amount for construction tolerances and live load rotation. Note: Higher shape factors call for increased thickness if this control is to be observed. This will increase the allowable rotation in certain cases. The specification should also reference military specification MIL-C-882D.8) / 2 should not exceed the compressive deflection due to total load. ♦ Sliding Elastomeric Bearings Sliding elastomeric bearings contain preformed fabric. non-parallelism may be assumed to be 0.8” factor applied to the pad “W” value. ergo the “0. Typical bearings are shown in the TxDOT standard Bridge Design Manual 9-86 TxDOT 12/2001 . Caution: There are no construction tolerances for this method.02 x (W x 0. Thicker than standard pads with smaller shape factors and moderate to extreme tapers. This strain is then multiplied by T to obtain the estimated deflection.01 radians.Chapter 9 — Special Designs Section 11 — Bearings • • • (Concrete) Minimum T = 4. may merit checking for maximum compressive deflection by the designer. the only nonparallelism that needs to be considered is longitudinal rotation due to the dead load and live load deflection. for rectangular bearings Maximum T = D/4. the compressive strain on that corner would be severe and there might be daylight between beam and bearing on the other side. The compressive deflection can be estimated by using the performance curves in Figure 9-40 with Rt /A and SF to read compressive strain. ♦ For tapered pads.02 radians longitudinally. the total deflection is usually less than 3/16 in. If all of the allowable construction tolerances were to occur in the right direction to effect one corner of the bearing. for round bearings Note: For stability.01 radians of beam slope. For typical prestressed concrete beam spans. Minimum thickness of plate beneath pot or disc is 0. Bearing seats should be level.Chapter 9 — Special Designs Section 11 — Bearings detail sheet Elastomeric Bearing Details (For Steel Girders & Beams). Salient features of the design are: ♦ ♦ ♦ The preformed fabric should be 2 in. thick. The average bearing pressure on the preformed fabric due to dead load and live load without impact should not exceed 2. As currently written these parameters are as follows: ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ Minimum horizontal capacity is 10 percent of vertical capacity. The average bearing pressure due to dead load and live load without impact should not exceed 1. Anchorage to the upper and lower concrete surfaces is with end welded studs.06 D. Preformed fabric pads should be laterally restrained on the bearing seat as shown in the TxDOT standard detail sheet Elastomeric Bearing Details (For Steel Girders & Beams) or by other mechanical means. Allowable stress for steel and concrete is as prescribed by AASHTO.0 in. Minimum depth of limiting ring for disc bearing is 0. The average bearing pressure on the PTFE surface should not exceed 3.07 D.015 D + 0. Concrete or epoxy grout should be cast against the in-place bearing. Minimum rotation capacity is 0.014 D.100 psi. Adhesive should not be used.000 psi. Top and bottom surfaces should be level.500 psi. sole plates should be beveled to the slope of the beam seat adjusted for dead load rotation. ♦ High load sliding elastomeric bearings have been specified for concrete box girders. Pot and Disc Bearings Pot and disc bearings will usually have design parameters defined in the construction specification.000 psi. the following controls are recommended: ♦ ♦ ♦ ♦ The preformed fabric should be 2. The top is stainless steel on a steel backing plate. Bearings are to be designed so that all rotational and sliding elements can be replaced with a minimum of jacking. Minimum thickness of elastomeric disc is 0.02 radians. Under these conditions. Preformed fabric is bonded to a steel plate that is recessed to receive a PTFE sliding surface. Minimum pot wall thickness is 0.75 in. 9-87 TxDOT 12/2001 Bridge Design Manual .12 in. Allowable average compressive stress on elastomeric discs and PTFE surfaces is 3. Top of pot disc to top of pot walls is 0. thick. : Elastomeric Bearing Performance (Online users can click here to view this illustration in PDF.) Bridge Design Manual 9-88 TxDOT 12/2001 .Chapter 9 — Special Designs Section 11 — Bearings Figure 9-40. girders. Complicated framing of steel cap beams required for geometrically restricted urban freeways often placed severe theoretical stresses in anchor bolts embedded in concrete columns. The advent of large roadside signs and overhead sign bridges created a need for serious consideration of tension in anchor bolts. Texas’ earliest research13 was directed to this problem. Frank (CFHR.A. Report 1126-4F. Report 172-1. in conjunction with this problem. 1979) reported that the frequency of deflection was very high but the stress range induced was well below the threshold level for threaded bolts. and shear requirements are small. and trusses. This instigated the research reported in Strength and Behavior of Anchor Bolts Embedded Near Edges of Concrete Piers by Hasselwander. Stress range in anchor bolts for cantilever sign bridges became a concern because of the noticeable traffic-induced deflections. preformed fabric washers were installed to reduce the stress ranges to insignificant values. often requiring anchor bolts to be installed in existing concrete. For the usual beam bearing. Shoes are usually painted but anchor bolts have vacillated between painting and galvanizing depending on the construction specifications of the period. amid considerable controversy regarding materials and embedment addressed by numerous research projects over a 20 year period. confirmed the findings of Axial Tension Fatigue Strength of Anchor Bolts by Fischer.B and others (CFHR. B.H. Anchor bolts for steel bearings are nominally designed.L. R. 1977). Bridge Design Manual 9-89 TxDOT 12/2001 . and K. and others (CFHR. 1990) provides means of determining strength of several types of retrofit anchors. Report 209-1F.H. The new specification for bridge railing called for smaller high-strength bolts with different anchorage conditions.12 The AASHTO Specifications offer little guidance. F. Fatigue Loading in Sign Structures by Creamer. Final Report 29-2F. anchor bolt tension is impossible. 1977) that even highstrength threaded anchor bolts have low fatigue strengths. and others (CTR. Anchorage was usually sufficiently strong to break the concrete slab or parapet when severely impacted. swedged bolts. short of a catastrophe. High theoretical tensile stress ranges discovered in one interchange after construction prompted some undocumented load testing and stress measurement. by Cook. or threaded rods with bent ends. other than a few antique rules for minimum size and embedment for bearings under steel beams. Laboratory tests. G.Chapter 9 — Special Designs Section 12 — Anchor Bolts Section 12 Anchor Bolts Background Many anchor bolts have been used in Texas for various purposes. Anchorage has been obtained by using headed bolts. Although the theoretical stress ranges were not apparent during the testing. Galvanized A321 threaded rods with nuts or A325 headed bolts were used almost exclusively for this. Design Guide for Steel to Concrete Connections. Some of these railing systems and many older railings are now being replaced. which created concern about strength of the threads.L. (CFHR. as for sign supports and tall poles. The anchor bolts had fractures that indicated fatigue. Maintenance Issues Maintenance has been largely confined to rusty bolts in shoes for steel beam units. 1978) reported some fatigue benefit from local bolt prestressing within the thickness of the base plate provided by double nuts top and bottom of the plate. Because of fatigue concerns. Attempts to prestress A193 anchor bolts for tall light poles were complicated. Fatigue of Anchor Bolts by Frank. In spite of all the concern over embedment and fatigue. called A36M55. and A193 material. Rolled threads were required for awhile.H. A321 threaded rods. 1977) reported little difference. Templates are a virtual necessity for anchor bolt groups. Bridge Design Manual 9-90 TxDOT 12/2001 . Axial Tension Fatigue Strength of Anchor Bolts by Fischer. Coordination between traffic signal pole design and anchor bolt sizes was a big problem before standards for these poles were completed. This works well when regular prestressing materials are used. A search for the best available high-strength bolt material identified A193. no problems have been manifest. High-strength bolts required for sign supports were A325 or A321 threaded rods. with cut threads for anchor bolts. for which availability was questionable for the larger sizes. F. K.Chapter 9 — Special Designs Section 12 — Anchor Bolts Early anchor bolts were A307 or A36 threaded rods. and have been abandoned.H. Prestressed anchor bolts present an added construction difficulty. the fatigue was probably caused by a small stress range from a resident tension approaching yield stress. but debate ensued over the relative fatigue strength of rolled threads and cut threads. Construction Issues Construction problems are caused primarily by mislocated anchor bolts. the allowable stress in anchor bolts for sign support and tall light poles was limited to the specification factor times 55 ksi yield stress. Frank (CFHR. but anchorage strength now appears to limit the usefulness of higher strength steels. a chrome-molybdenum steel developed for pressure vessels. For bridge superstructures. This limited the effectiveness of Grade 75 reinforcing bars. Grade B7. had questionable results. in response to this requirement. Concern over fatigue occasionally precipitates a prestressed anchor bolt design. Tall illumination poles introduced the Grade 75 deformed reinforcing bar. Thus. and K. size 18S. but the structure had been modified in a manner that increased the theoretical bolt stresses considerably beyond specification allowables. except for one cantilever sign bridge that fell onto an interstate freeway in Houston. Final Report 172-2F. anchor bolts are often set in preformed holes in the concrete to allow for variation in bent locations or beam length. Manufacturers have developed a 55 ksi bolt material. Threading did not remove all of the deformations. as being readily available at reasonable cost. Report 172-1. Scratch gauge strain measurements made by the Bridge Design Section several years ago revealed no significant stress ranges in tall light poles. Chapter 9 — Special Designs Section 12 — Anchor Bolts Current Status Current usage of anchor bolt materials is as follows: ♦ ASTM A307. as follows: ♦ ♦ ♦ ♦ Mild steel anchor bolts: • • • • ASTM A307 Grade A or ASTM A36 ASTM A36 or ASTM A572 with 55 ksi minimum yield stress. mild steel anchor bolts: High strength anchor bolts: Alloy steel anchor bolts: Suitable nuts and threads are specified for each type. A325: ASTM A193-B7. Design Recommendations Recommended service load stress allowables for the four types of anchor bolts are as follows: ♦ Mild steel (Minimum Fy = 36 ksi) Tension 18 ksi Bridge Design Manual 9-91 TxDOT 12/2001 . A36: • • ♦ • • • • ♦ ♦ • • • • • Bearings for redundant superstructure beams Pedestrian and bicycle railing and T6 traffic railing Traffic signal poles Overhead sign bridges High mast illumination poles Luminaire poles Posts for traffic and combination railing Overhead sign bridges High mast illumination poles Luminaire poles Steel girder bent caps A36M55. A687: Anchor bolts are covered under a separate item in the current TxDOT construction specifications. called A36M55 ASTM A325 or A321 ASTM A193-B7 or ASTM A687 Medium strength. Four types are identified. F1554 Grade 55: ASTM A321. a reasonable factor of safety should be provided. The stress area. Research recommendations were as follows: Bridge Design Manual 9-92 TxDOT 12/2001 . Dia. Several reports14 explored several factors that affect anchorage strength. should be used in all cases. Specifications are unclear regarding the combination of tension and shear in anchor bolts. Dia. Clear concrete cover and anchor bolt spacing are limiting factors. Bending stresses due to unsupported anchor bolt length may be ignored if projection is no more than five times the bolt diameter and double nuts are securely tightened.000.5 ksi. and bridge railing. light poles. redundant for 2.6 è 2 + (tension ) 2 ≤ Allowable Tension Allowable tension should be commensurate with anchorage strength. Preferably.Chapter 9 — Special Designs Shear Fatigue 11 ksi 8 ksi 27 ksi 16 ksi 8 ksi 36 ksi 21 ksi 8 ksi 50 ksi 30 ksi 8 ksi Section 12 — Anchor Bolts ♦ Medium strength mild steel (Minimum Fy = 55 ksi) Tension Shear Fatigue ♦ High strength (Minimum Fy = 70 ksi to 2-1/2 in. Certainly. Shear due to torsion in the anchored member should be added to the shear caused by transverse forces. Most authorities emphasize the desirability of allowing “ductile” failure by sufficient anchorage strength to develop the yield or even the ultimate strength of the bolts.000 cycles. for these reasons. through the threads.) Tension Shear Fatigue ♦ Alloy steel (Minimum Fy = 105 ksi to 2-1/2 in. It appears prudent to utilize a method similar to the requirements for connection bolts so that: æ shear ö ç ç 0. Allowables may be increased for certain load groups as allowed by AASHTO bridge or sign bridge specifications. Nuts and washers (or plates) were found to be the most effective means of anchorage. the anchorage should develop the service load tension in the bolts. Fatigue allowables are based on Category E. This is a virtual impossibility for anchor bolts in columns or parapet walls such as commonly required for sign bridges.) Tension Shear Fatigue The allowable tension for high-strength and alloy steel bolts will probably be unachievable due to fatigue or anchorage limitations.5 ksi and shear of 16. although A36M55 or ASTM A193 bolts are required. Sign bridge and light pole anchor bolts have been limited to a design tension of 27. psi Embedment must be at least 12(Dw − D) to ensure wedge splitting failure. For severe tension or fatigue conditions..02S + 0. The researchers recommended a resistance factor of 0. consideration should be given to prestressed anchorage. The designer is left to individual judgment in the application of load and resistance factors. ç 4è D Dw = = C Ks S f 'c = = = = and not greater than 4D2 bolt diameter. Clear cover to bolt. Confinement reinforcing is beneficial. in. in. of the washer or anchor plate. Nuts should be tack welded to the template to prevent floating during concrete placement. 1984) appears to offer a more comprehensive method to design the anchorage to develop ultimate strength of the bolts. Anchorage zone reinforcing should be given careful attention. in. Vertical reinforcing must have sufficient development length to resist stresses delivered by the anchorage plate. It is comfortable to note that there have been no anchor bolt failures in Texas except for rust and vehicle impact. or template. J. and others (CTR. Bridge Design Manual 9-93 TxDOT 12/2001 . Regular post-tensioned systems should be used. be anchored with embedded nuts and plates that can serve as a template as well as increase the pullout resistance. It is recommended that all anchor bolts with calculated tension. Additionally. the diameter. in. Strength and Behavior of Bolt Installations Anchored in Concrete Piers by Jirsa. Dw shall not be taken grater than eight times the thickness of the washer. where a continuous template or anchor plate is used for a group of anchor bolts.Chapter 9 — Special Designs Section 12 — Anchor Bolts Tn = 140 Ab f 'c ê0.0 Center-to-center bolt spacing. Spacing reduction factor = (0. Impact makes bad anchorage conditions worse.7 + lnç ê ë é 2C ÷ ú Ks Dw − D ú where: Tn Ab = = nominal tensile capacity of a bolt net bearing area.40) < 1. The strength of railing anchor bolts is usually controlled by anchorage failure. Anchor bolts for bearings that resist no uplift may be anchored by 12 in. Prestressing anchor bolts by turning the nut is of doubtful value.75 times Tn as defined above. in. right angle bend.O. Final Report 305-1F. except for railing anchorage. calculates as π æ Dw 2 − D 2 ö . embedment and a 2 in. Concrete compressive strength. the washer diameter may be taken as the diameter of a circle concentric with the bolt and inscribed within the template or anchor plate. and transverse reinforcing can improve the shear strength of the anchorage. plate. Designers should consider the findings of three other reports15 and strive for the most ductile connection possible under the conditions imposed. Supporting members must be able to resist the forces delivered by the anchor bolts. local practice requires a minimum embedment of 20 bolt diameters. These will be recognized as stringent limits on anchor bolt effectiveness.2. Chapter 9 — Special Designs Section 12 — Anchor Bolts Typical anchorage details currently in use are shown in Figure 9-41. and Figure 9-43.) Bridge Design Manual 9-94 TxDOT 12/2001 . : Example of Sign Bridge Anchorage (Online users can click here to view this illustration in PDF. Figure 9-42. Figure 9-41. : Example of High Mast Illumination Pole Anchorage (Online users can click here to view this illustration in PDF.Chapter 9 — Special Designs Section 12 — Anchor Bolts Figure 9-42.) Bridge Design Manual 9-95 TxDOT 12/2001 . Chapter 9 — Special Designs Section 12 — Anchor Bolts Figure 9-43. : Example of Integral Steel Bent Cap Anchorage (Online users can click here to view this illustration in PDF.) Bridge Design Manual 9-96 TxDOT 12/2001 . finally.” This drain is still used. back to straight slopes either way from the center. except in de-icing salt areas where small curbs were added to limit corrosive runoff. many bridges were constructed with no curbs and open railing. apart with a note to “omit over header banks and railroad tracks. but 0. Since most bridges had curbs originally and now have concrete wall railing. Bridge Design Manual 9-97 TxDOT 12/2001 . bridge designers abandoned any claim to hydraulic expertise with a plan note that drains shall be “spaced as directed by the Engineer. Cast iron soil pipe was used on many steel beam and concrete girder bridges.05 in. Cross-slope magnitude has varied considerably. The number of drains stabilized at two per span or two per concrete placement on continuous units with drains to be omitted over header banks. was the solution.” For awhile. went briefly to parabolic crowns. triangular depressions. but in the 1960s. but some drainage provisions have always been shown on the details. no bridge was allowed to be constructed to a flat gradeline because of drainage considerations.02 ft. in the form of 3/4 in. in a typical tangent section of roadway. or water quality issues. some type of deck drain has usually been provided. round holes at 4 ft. then simulated parabolas and. is the current preference. This provides sufficient water storage to reduce the hydroplaning film to acceptable thickness. per ft. was used for several years on concrete girders. open at the lower curb face. This eliminated the drainage problem. Drip beads. Roadway crowns started as straight slopes either way from the center.Chapter 9 — Special Designs Section 13 — Deck Drainage Section 13 Deck Drainage Background Most bridge engineers agree that some provision must be made to get rainwater off the bridge. protection of traffic below. Closed systems usually required the services of a hydraulic engineer. Recommended drain design has changed significantly since 1988. Hydraulic analysis has probably been avoided until recently. Texturing concrete surfaces to an average depth of 0.. One of the earliest was 2 in. There is an increasing demand for closed drainage systems for bridges. By the early 1940s the accepted drain detail became a 4 by 6 in. This is still a good idea. were provided around each drain outlet. At one time. Continuous drip beads were provided beneath the overhanging slab. A sheet metal drain box. FHWA publications in 198416 and 198617 contain valuable guidance for the design of bridge deck drainage. formed opening in the deck slab. because of aesthetics. spacing. Hydroplaning became an issue affecting bridge decks as well as approach pavement. Design was by intuition for most open drainage systems. These were spaced approximately 20 ft. Current Status Bridge deck drainage is the responsibility of district highway engineers. Grate inlets used for closed systems are highly susceptible to clogging by roadway debris. Note: TxDOT standards include a guidance detail sheet Bridge Drain Details that shows the latest recommended cast steel grate inlet for use on bridges with conventional decks. Be sure that the approach facilities are capable of handling the bridge runoff. Bridge Design Manual 9-98 TxDOT 12/2001 . Open drains usually let runoff get to the superstructure.Chapter 9 — Special Designs Section 13 — Deck Drainage Maintenance problems can be acute. This sheet is not to be used as a standard. Staining from open drainage is common. ♦ ♦ ♦ Open deck drains located “as directed by the Engineer” should be avoided. Closed systems require downspouts. and CG-MD. Note: Discharge may be into a storm sewer. In de-icing areas open deck drains should empty below the superstructure as shown on bridge standards IBMS. Design Recommendations The Hydraulics Section should be consulted if there is any doubt as to the deck drainage requirements or if a closed system must be designed. A few suggestions are offered. or close to the ground if water quality issues are not a major concern. stilling well. Aesthetics can be compromised by either type of system. A cleanout plug should be provided. SBMS. as follows: ♦ ♦ ♦ For various design parameters and coefficients see the Hydraulic Design Manual. Closed deck drainage systems should have grate inlets and PVC downspouts encased in the substructure insofar as practical. causing corrosion in de-icing areas. Do not use deck drains unless absolutely necessary. assisted by the Hydraulics Section and the Bridge Design Section. Note: One report18 offers a quick method for determining the maximum undrained bridge length. which may direct the entire gutter flow across the roadway. Improper provision for drainage of the approach roadway has caused erosion of the embankment. or areas with water quality issues. Instructions on the sheet require the designer to modify the sheet for specific applications. which are often handled in an unsightly manner. Caution: Beware of cross-slope reversals. Open or slotted railing is generally recommended for stream crossings except in the deicing areas. : Example of Grate Inlet and Installation for a Segmental Superstructure (Online users can click here to view this illustration in PDF. structural integrity of the bridge deck should be maintained by 3 ft.) Bridge Design Manual 9-99 TxDOT 12/2001 . respectively. minimum drain spacing. Figure 9-44. Requirement: If multiple drains are required in sag areas.Chapter 9 — Special Designs Section 13 — Deck Drainage Examples: A welded steel grate inlet system and drainage installation used on segmental box girder bridges are shown in Figure 9-44 and Figure 9-45. Chapter 9 — Special Designs Section 13 — Deck Drainage Figure 9-45. : Example of Drainage Installation for a Segmental Superstructure (Online users can click here to view this illustration in PDF.) Bridge Design Manual 9-100 TxDOT 12/2001 . Early Designs From the late 1940s to the middle 1990s. Types of culverts identified are: ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ Laminated timber Patented creosoted timber Stone walls with stone slab Stone walls with precast reinforced concrete slab Vitrified clay segmental block. or modified and used for higher fill installation. Used directly.Chapter 9 — Special Designs Section 14 — Reinforced Concrete Box Culverts Section 14 Reinforced Concrete Box Culverts Background Texas standard box culvert detail sheets can be found dating from 1918. they were the basis for many millions of dollars worth of culvert construction. or flat bottom arch Masonry arch Interlocking precast concrete u-shaped sections Concrete wall with footing and reinforced concrete simple slab Cast-in-place single boxes. reinforced for positive moment only Cast-in-place single boxes. round. reinforced as a frame Precast single box sections Precast two-piece single box sections The most widely used culverts are the reinforced concrete single and multiple boxes. TxDOT maintained an extensive set of culvert and wing wall standard detail sheets. reinforced as a frame Cast-in-place multiple boxes. The original 1948 standard designs used service load methods and were based on these assumptions: ♦ ♦ ♦ ♦ ♦ Vertical earth pressure 120 pcf times 0. Design procedures for these boxes did not exactly conform to AASHTO requirements.” March 1930 Two feet of surcharge and full lateral pressure used for corner moments 9-101 TxDOT 12/2001 Bridge Design Manual .7 Lateral earth pressure 30 pcf equivalent fluid Live load was a 12 kip wheel Live load distribution according to a Westergaard article in “Public Roads. the modifications were not exactly accurate for all sections.000 psi and steel. AASHTO acceptance was slow because ASTM C789. Another fabricator. 1. instead of cubic yard. Culverts constructed by these designs have proven adequate by virtue of their performance under traffic. in Harlingen. because of the large number of design combinations and time constraints. Outside forms were also removed soon. 18. actually gave the live load and fill height that each section could sustain. This was one reason for the reluctance by TxDOT to redesign according to the latest specifications. lengths. In spite of their somewhat antiquated design. For Texas the industry began in Beaumont in the early 1970s with a fabricator making standard cast-in-place sections vertically in 8 ft. the details were modified but. no significant malfunctions have been observed in the many culverts constructed to these details. Significant to advancement of culvert technology was acceptance of the precast box culvert. Virtually all culverts are now bid this way. This usurpation of the bridge engineer’s field of authority plus loading research in progress and questionable shear transfer across the joint in direct traffic situations. This makes it difficult to verify these designs according to strict AASHTO requirements. Several TxDOT standard detail sheets were developed to allow the use of precast products. The solution involved installing pipe runners across the opening. ASTM C850 was published to cover direct traffic precast box culverts. High-strength dry concrete was required because the inside form was a mandrel that was retracted as soon as concrete placement was complete.Chapter 9 — Special Designs Section 14 — Reinforced Concrete Box Culverts ♦ ♦ ♦ ♦ ♦ No lateral pressure used for positive moments Live load in one span only for positive moment Allowable stresses in concrete. but established allowable fill heights according to local practice. This allowed the contractor to decide whether to build precast or cast-in-place culverts. Texas allowed the industry standard section prior to acceptance. the specifications allowed culverts to be bid by the linear foot. Another advance in culvert design began in the early 1980s when the Federal Highway Administration insisted action be taken to protect errant motorists from plunging into the space at the end of cross drainage culverts or running into the headwalls of parallel drainage culverts. which created hydraulic concerns because of negative effect on the ability of the culvert to carry storm Bridge Design Manual 9-102 TxDOT 12/2001 . also provided TxDOT with standard precast box culvert details before acceptance of an industry standard covered by ASTM C789. as shown in the "Reinforced Concrete Box Culvert Usage" table.000 psi Shear in slabs not considered Moment distribution by hand calculations As changes were made to the AASHTO Specifications and local practice. made it difficult for the industry to gain formal acceptance from AASHTO. Recent Changes In the early 1980s. rather than the usual material specification. 000 psi Slab thickness based on allowable shear Bridge Design Manual 9-103 TxDOT 12/2001 . to 20 ft. max. load is considered a point load. some design parameters are as follows: ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ Vertical earth pressure 120 pcf Lateral earth pressure 40 pcf Live load is a 16 kip wheel with impact per AASHTO Distribution of a wheel is a square of 1. fy = 60. Culvert standard detail sheets are available on the TxDOT web site for the following box culverts and appurtenances: ♦ ♦ ♦ ♦ ♦ Cast-in-place single boxes (30 ft. fill height depending on span) Wing walls (straight. and the entire culvert series was redesigned. Distribution and design per AASHTO slab design requirements. fill height) Cast-in-place multiple boxes (23 ft. max. Research was conducted at CTR (Report 301-1F) on the hydraulic aspects and at TTI (Report 280-1 and 280-2F) on the structural aspects. At that time significant changes were made to the wing wall and safety end treatment details. using load factor design and the current specifications. if a special design is warranted. for fills ≥ 2 ft.7 x fill depth. Current Status The current TxDOT box culvert standard detail sheets can account for almost any box culvert design need. For fill heights less than 2 ft.Chapter 9 — Special Designs Section 14 — Reinforced Concrete Box Culverts water. flared. However. The structural results were fine-tuned and safety end treatment standard detail sheets were prepared. f 'c = 3. and parallel) Safety end treatments Design Recommendations The TxDOT box culvert standard detail sheets will significantly reduce the need for special designs for culverts and end treatments. Two feet of surcharge and full lateral pressure used for corner moments Half lateral pressure used for positive moments Spans loaded according to influence lines for moments Class C concrete.600 psi. In the late 1990s it became necessary to produce box culvert standard detail sheets in metric units. Grade 60 reinforcing steel. These metric box culvert standard detail sheets were then converted to English units in 2000. max. fill height) Precast single boxes (12 ft.. 3.Y. 1.000 300 250 11.000 100 19.000  23.) 1 2 3 Class A Class C Class S 148.000 3.600 psi Special deck concrete for direct traffic culverts Alternate precast or cast-in-place.500 2.000 psi Six sack 3.000 2. including when and where to use culverts.Chapter 9 — Special Designs Section 14 — Reinforced Concrete Box Culverts Additional information.F.000 2.000  39. 2. A discussion on the hydraulic requirements of culverts can be found in the Hydraulic Design Manual.000 100 210 548 56 4 Box Culvert (L.000  7. Calendar Year 1966 1971 1976 1981 1983 1986 1988 1997 Five sack 3.700 187.000  730 130.)     49.000 246. linear feet of single barrel Reinforced Concrete Box Culvert Usage Contract Bid Quantities Concrete for Culverts (C.612 Bridge Design Manual 9-104 TxDOT 12/2001 .700 226. 4. can be found in the Roadway Design Manual.000 10. as well as recognized classes of bedding can be found in the Hydraulic Design Manual. regardless of the application. To do away with some of the confusion with specifying reinforced concrete pipe for a project. bedding. are specified under one item number as well. reinforced concrete pipe could be bid as “culvert” or “sewer. The change in specifications allowed all reinforced concrete pipe. Excavation for sewers was specified in more detail in the “Excavation and Backfill for Sewers” item. Virtually all of it is round pipe and most is from 12 to 48 in. Monolithic concrete pipe and corrugated metal pipe could also be bid under the “sewer” specification. In the Texas Standard Specifications (1982). There are four common types of concrete pipe installation conditions: ♦ ♦ ♦ ♦ Trench Positive projecting Negative projecting Imperfect trench Descriptions and illustrations of these installation conditions. Appurtenances are many and vary in detail. Excavation for culverts was specified in the Structural Excavation item. Excavation. and backfill. diameter. The current TxDOT culvert standard detail sheets can account for many these needs. changes to specification concerning reinforced concrete pipe were included in Texas Standard Specifications (1993). Moreover. used as culvert or sewer. to be specified under one item number.Chapter 9 — Special Designs Section 15 — Reinforced Concrete Pipe Section 15 Reinforced Concrete Pipe Background Specifying reinforced concrete pipe for use in projects was somewhat disorganized. Current Status As indicated on the "Reinforced Concrete Pipe Usage" table reinforced concrete pipe is used extensively in Texas highway construction.” Bedding for both was specified under the “culvert” item. Culvert standard detail sheets are available on the TxDOT web site for the following applications related to reinforced concrete pipe: ♦ ♦ ♦ ♦ Headwalls with flared or parallel wings Manholes Inlets Slotted drains 9-105 TxDOT 12/2001 Bridge Design Manual . the implication that open-ended cross drainage structures are “culverts” and partially closed drainage systems with inlets and manholes are “sewers” was not always followed. When pipe is specified using one ASTM C76 class for a project. Item 466 “Headwalls and Wing walls” and the current Special Provisions thereto. can be found in the Hydraulic Design Manual. The smaller the incremental magnitude of specified D-Loads. Specification by D-Loads required by ASTM C76 is acceptable. When pipe is specified using D-Loads. values of the D-Load for the various runs of pipe must be given in the plans. Class III is the most often used.Chapter 9 — Special Designs Section 15 — Reinforced Concrete Pipe ♦ Safety end treatments Pipe strength may be specified as one of the classes described in ASTM C76 or as “DLoad. Bridge Design Manual 9-106 TxDOT 12/2001 . Specification by ASTM Class in the bid item is also acceptable if the advantages of manufacturer design appear remote.” When specifying pipe some guidelines are as follows: ♦ ♦ ♦ ♦ When pipe is specified using ASTM C76 class. type of bedding. and equivalent recommended minimum D-Load increments for each class. Structural design consideration is required to insure adequate strength for the installation conditions. and fill height anticipated. Headwalls and wing walls must be Class C concrete as required by Texas Standard Specifications (1993). the location of each class must be shown on the plans. Specification of concrete pipe by D-Load allows the manufacturer to determine the most economical combination of wall thickness and reinforcing steel. a brief note on the plans verifying the class required will be sufficient. ASTM C76 classes. greater will be the problem of pipe location on the project. 615 9.068 27 39  30 3.683 33 866 36 2. ♦ Bridge Design Manual 9-107 TxDOT 12/2001 .660 25.173  66 1.F.806  Elliptical 40  ♦ D-Load specified for all pipe culverts.928  15 23 20 18 54. pipe. Design Recommendations The design of reinforced concrete pipe shall be governed by the AASHTO Specifications. Earth loads vary with properties of the fill.F. The relationship of actual support conditions to the three-point bearing test is established by empirical equations based on the class of bedding.296 54 525 8.248 39   42 2. Some design notes are as follows: ♦ ♦ ♦ Vertical loads are calculated with empirical equations developed from analytical and experimental observations of soil-structure interaction.033 60 5.899 48 3. Copious table and charts are provided to assist in this. Live load is calculated according to AASHTO distribution of an HS20 truck.Chapter 9 — Special Designs Section 15 — Reinforced Concrete Pipe Reinforced Concrete Pipe Usage Quantity Let to Contract 12 Months Ending November 1998 Round Pipe Diameter Pipe Culvert Pipe Sewer (in. and type of installation.723 28.052 41.949 Total Arch 1. subgrade.664  21   24 7.171 37.) 12 16.296  ♦ 36.736 222.705  78 656  84   88 1. ASTM C76 class specified for 94 percent of the remainder.342  72 8.) (L.) (L. Bridge Design Manual 9-108 TxDOT 12/2001 . D-Load required is an expression of the calculations explained above in pounds per foot of length per foot of inside pipe diameter. Variations of trench width.01 in. Alternatively. D-Load provided is the load at which 0. crack forms in a representative section of pipe under the three-point bearing test described in ASTM C497. Use of this method by TxDOT is unlikely. direct design of concrete and reinforcing in the pipe may be performed with the aid of a finite element soil-structure interaction program. backfill type. Required D-Loads greater than 3000D should be submitted to the TxDOT Bridge Division for special design. or bedding may be more economical than the increase in pipe strength. D-Loads required for various fill heights and diameters of pipe. Use of pipe certified for a D-Load equal to or greater than required by the plans or ASTM C76 insures serviceability of the pipe. The AASHTO Specifications and the ACPA Concrete Pipe Manual will guide such designs. Conduit durability design criteria can be found in the Hydraulic Design Manual. Additional industry guidance may also be solicited. Allowable fill heights for concrete arch pipe and horizontal elliptical pipe can be found in the Texas Standard Specifications (1993). based on installation and bedding conditions. can be found in the Hydraulic Design Manual. Hydraulic design criteria can be found in the Hydraulic Design Manual.Chapter 9 — Special Designs Section 15 — Reinforced Concrete Pipe Additional design guidance can be found in the American Concrete Pipe Association (ACPA) Concrete Pipe Design Manual. Chapter 9 — Special Designs Section 16 — Corrugated Metal Pipe Section 16 Corrugated Metal Pipe Current Status Corrugated metal pipe usage is shown in the "Corrugated Metal Pipe Usage" table. aligned either annularly or helically with riveted. Virtually all is steel pipe. Allowable fill heights and wall thicknesses for full circle corrugated metal pipe can be found in the Hydraulic Design Manual. Design Recommendations The design of corrugated metal pipe shall be governed by the AASHTO Specifications. Steel and aluminum pipe are available in several different corrugation configurations. and 67 percent is pipe arch. Conduit durability design criteria can be found in the Hydraulic Design Manual. resistance welded. Allowable fill heights and wall thicknesses for steel and aluminum pipe arches can be found in the Texas Standard Specifications (1993) Item 460 “Corrugated Metal Pipe. Bedding and backfill are critical. Additional design guidance can be found in the American Iron and Steel Institute (AISI) book Modern Sewer Design.” Hydraulic design criteria can be found in the Hydraulic Design Manual. The current TxDOT culvert standard detail sheets can account for many of these needs. Bridge Design Manual 9-109 TxDOT 12/2001 . or mechanically locked seams. Appurtenances are many and vary in detail. Culvert standard detail sheets are available for the following applications related to corrugated metal pipe: ♦ ♦ ♦ ♦ ♦ Headwalls Manholes Inlets Slotted drains Safety end treatment Pipe strength is specified as a wall thickness for a given size and maximum fill height. ) (L.Chapter 9 — Special Designs Section 16 — Corrugated Metal Pipe Corrugated Metal Pipe Usage Quantity Let to Contract 12 Months Ending November 1998 Round Pipe Pipe Arch Diameter Bid Quantity Design Bid Quantity (in.549 21 36   24 5.) 6    8    12 640   15 473 1 12 18 5.766 42 984 6 1.399 36 1.125 3 26.3 percent had bituminous coating.559 2 8.470 17.F.653 4 4.) Size (L.F. Only 48 linear feet had paved invert.048 30 1.938 48 482 7 797 54 8 735  60 95 9  66 10   72 201 11  78 12   84 13   90 886 14  102    126 19 226  ♦ 44. Bridge Design Manual 9-110 TxDOT 12/2001 .078 5 1.212 Total  ♦ Only 0. Aluminum plate has 9 in. Design Recommendations Designs are usually initiated by industry and checked by the TxDOT Bridge Division. x 2 1/2 in. prior to construction. Galvanized steel plate is available with 6 in. corrugations. nor are there any structural design guides. annular corrugations. x 2 in. Seams are bolted. The structures may be partially prefabricated or field fabricated.Chapter 9 — Special Designs Section 17 — Structural Plate Structures Section 17 Structural Plate Structures Current Status Structural plate structures include the following: ♦ ♦ ♦ ♦ ♦ Structural plate pipe Pipe arches Underpasses Box culverts Special shapes They may be aluminum or steel and may or may not be elongated prior to backfill. There are no standard detail sheets available from the Bridge Division for end treatment or footings. Bridge Design Manual 9-111 TxDOT 12/2001 . using the AASHTO Specification and American Iron and Steel Institute book Modern Sewer Design as guides. The usage of structural plate structures by TxDOT is very small. low profile arch. this experience. the design will probably be initiated by industry and should be checked by the TxDOT Bridge Division using the AASHTO Specification as a guide. and a few others nationwide. incurred so many problems as to discourage most districts from further use. let under the Item 461 Structural Plate Structures of the 1993 TxDOT Standard Specifications. This item was dropped from later construction specifications. 415 x 136 in. The 1982 TxDOT Standard Specifications covered long span structural plate structures. established the delicate reputation of long span plate structures. Recently additional deformations have appeared in the bottom. high profile arch. such as horizontal ellipse.Chapter 9 — Special Designs Section 18 — Long Span Structural Plate Structures Section 18 Long Span Structural Plate Structures Background Long span structural plate structures were constructed with the same corrugated plate used for structural plate structures. various manufacturers of corrugated metal plate actively marketed long span structures in Texas. and pipe arch. Several different shapes were available. but they required special stiffening attachments to limit deflection and stress. 1987 1986 1984 High profile arch (aluminum) Low profile arch (steel) Low profile arch (aluminum) 292 x 178 in. span. 364 x 119 in. It was completed after much delay and additional strengthening but retained some abnormal deformation of the top. Several installations were made. Whatever the cause. Bridge Design Manual 9-112 TxDOT 12/2001 . The structure was a horizontal ellipse with 40 ft. Designs were highly proprietary. Only three such structures were constructed on the state highway system through 1997. They also required careful backfilling to prevent distortion of the shape. All were very critical in the backfilling stage. Current Status The only long span structural plate structure in 1998 was 240 x 120 in. In the early 1970s. Cause of the malfunction had to be related to backfill sequence and/or equipment or design. in particular. inverted pear. One. Design Recommendations If there should be a need for a long span structural plate structure. Overhead sign support usage for the period 1980-1989 is shown on Figure 9-46. Many conditions required directional signs to be mounted above the freeway. to be effective. The standards were revised in 1963 to replace much of the galvanized pipe with wide flange and angle shapes. The first version made extensive use of galvanized pipe. Regulatory and precautionary signs could remain fairly small. Many different concepts emerged from this. one person in the Bridge Design Section coordinated a large research study19 to develop breakaway supports for sign support structures. designed their own sign supports. Bridge Design Manual 9-113 TxDOT 12/2001 . Early roadside sign mounts created alarm from a traffic safety standpoint because of their exposed drilled shaft foundations and strong wide flange supports. which turned out to be feasible but impractical. Mechanisms were developed for small and large roadside signs that have saved many lives. the new specification controls were interpreted in the most economical manner possible. The bridge is now in service near Hearne. or ramp lanes. specifications were revised to require only the standard details. Luminaires. Optional designs began to disappear and in 1984.Chapter 9 — Special Designs Section 19 — Sign Support Structures Section 19 Sign Support Structures Background Construction on the interstate highway system demonstrated a need for prominent traffic signs. even for overhead truss bridges. and the practice of allowing commercial design options continued. Responding to pressure from highway engineers. Signs alongside the freeway were often inadequate. This proved to be expensive. it was decided that the Bridge Design Section should reassume responsibility for overhead sign bridge design. Commercially designed options were also allowed. Simultaneously. In 1966 responsibility for sign support design was transferred to the Traffic Operations section of the Maintenance and Operations Division. Roadside sign supports were constructed from Bridge Design Section standard details. With input from one such fabricator. The AASHTO Specifications for Structural Supports for Highway Signs. if sufficiently large. Some districts and fabricators still dislike them. but overhead sign bridges continued to be constructed mostly according to commercial designs. but were heavily revised to the present condition in 1975. could be mounted alongside the freeway. and Traffic Signals were introduced in 1968. The first Bridge Design Section standard details of sign supports for interstate highways began to be issued in 1960. new standard details were being developed. Some directional signs. One overhead sign bridge was tested with breakaway supports. At about the same time. especially Houston Urban Project. Some of the larger districts. There ensued a hectic period during which new and different specification controls were enforced on fabricators long accustomed to following more liberal rules for optional designs. Chapter 9 — Special Designs Section 19 — Sign Support Structures Figure 9-46. not even the light commercial designs.4 times the yield strength of the material. For cantilevers. begun with little guidance. it is eccentric dead load in combination with wind. Forces from a 50year recurrence interval wind are resisted at 1. since no sign bridges have failed due to wind. The controlling load for sign bridges is wind. was recognized by AASHTO in 1968 and underwent a complete revision in 1975.) Design Issues Design of overhead sign bridges. There is a feeling that this is conservative. : Chart of Highway Accessory Structures Usage (Online users can click here to view this illustration in PDF. Bridge Design Manual 9-114 TxDOT 12/2001 . If located within the limits of a bridge. and Traffic Signals. Questions regarding overhead sign supports or their mounting should be directed to the Bridge Design Section. Luminaires. There is very little provision for adjustment on a sign bridge. Foundations are always drilled shafts. There should be a soil test boring no further than 500 ft. Support bracket of a type previously used for the smaller sign bridge columns is shown on Figure 9-47. Drilled shaft foundation design curves for roadside sign supports.21 are included with the standard details. designed by Bridge Design Section methods. from each foundation for proper design. support brackets are usually preferable to ground mounting. developed on a 1970 research project. and special designs that may be required. Conditions often change during construction and may not be recognized until late in the project. and verified by research. resulting in certain areas being coated with pickling acid rather than zinc.20 Foundation design curves are included with the standards. Current Status Supports for signs are a responsibility of the Maintenance and Operations Division. All overhead sign support structures are constructed from the standard details prepared by the Bridge Design Section. have rusted severely from the inside because of incomplete zinc coverage. A few years of interaction between TxDOT and the hot dip galvanizing industry transpired before details and capabilities were optimized and the appearance was satisfactory with construction engineers.Chapter 9 — Special Designs Section 19 — Sign Support Structures Construction Issues Construction problems centered on quality of galvanizing in the beginning. with a few clarifying local interpretations. A number of tubular support members. The details provided insufficient openings to allow a free flow of zinc within the tubes. Overhead sign support structures should not be mounted on bridges unless absolutely necessary. will conform to the AASHTO Specification for Structural Supports for Highway Signs. Maintenance Issues Maintenance problems have been caused by galvanizing deficiencies. Coordination of foundation spacing and elevation with fabricated dimensions of sign bridge components can be a problem. mostly for cantilever or butterfly sign bridges. Design Recommendations Standard overhead sign support structure designs. Getting the right anchor bolts installed to the proper dimensions on the foundation has also been prone to error. Bridge Design Manual 9-115 TxDOT 12/2001 . ) Bridge Design Manual 9-116 TxDOT 12/2001 .Chapter 9 — Special Designs Section 19 — Sign Support Structures Figure 9-47. : Example of Overhead Sign Bridge Support Bracket (Online users can click here to view this illustration in PDF. agreement was reached whereby both offices would use the same design for high mast poles. Poles were 8-sided. Texas has only one such installation with no evidence of malfunction. and proper placement of lights. Design conformed to the 1968 AASHTO Specification for 50 year recurrence interval wind speed. Agriculture Department recommendations for pole buildings. Gears. They were tapered in diameter and spliced by inserting the top of one section into the bottom of the next with a 1. A circular luminaire mounting ring was connected to a retainer on top of the pole.5 diameter overlap. Soon several fabricators were pursuing the market and it was necessary to fine-tune design procedures in order to referee impartially between them. fit of the lap splices.Chapter 9 — Special Designs Section 20 — High Mast Illumination Poles Section 20 High Mast Illumination Poles Background These structures have been exclusively steel poles since the concept of high level lighting began in Texas in the early 1970s. The only reported failure was collapse of the upper section of a pole in a tornado near Wichita Falls. High-level illumination pole usage for the period 1980-1989 is shown on Figure 9-46. The mounting height was originally 150 ft. Most of the Texas installations were galvanized. Bridge Design Manual 9-117 TxDOT 12/2001 . Other states have reported failure of lap joints due to rust buildup in weathering steel poles. Designs originated with a manufacturer and were checked structurally by the Bridge Design Section. Scratch gauges were mounted on a pole in three different locations in an attempt to evaluate fatigue stresses near the base. Finally. with completion of the Bridge Design Section proposed standard details in 1989. The only measured stresses greater than 5 ksi were a few excursions during a norther near Canyon. The poles have been practically maintenance-free. Most had single drilled shaft foundations. Minor construction problems were encountered with shop welding. pulleys. Design competition was quickly manifest in the Houston Urban Expressway project and. Mechanical features and lights were the responsibility of the Highway Design Section. type of anchor bolt. for several years. and cables were located inside the pole for raising and lowering of the luminaires. This was determined to be cracks in the zinc coating. Foundation design was a compromise between a Broms method22 and U. or round.S. designs from both offices were constructed. 12-sided. There was some cracking reported in the pole-to-base plate weld on some early installations. The Bridge Design Section was responsible for structural design of the pole. New standard drawings HMIP and HMIF. were developed using nonlinear BMCOL7623 with lateral soil resistance based on various research reports. are available but are awaiting a new construction specification before formal use. Foundation design curves.Chapter 9 — Special Designs Section 20 — High Mast Illumination Poles Current Status Bridge Division standard drawings HMIP 80. dated May 1989. 150 and 175 ft. heights designed for 100 mph wind.and 12-sided poles of 100. Soil was allowed to reach half ultimate stress under the action of design wind. 125. The construction specifications will allow optional designs and require galvanized poles. and HMIF. Design Recommendations Standard designs comply with the AASHTO Specifications. The details cover 8. shown on the standard drawings. dated November 1986 and standard construction specification Item 613 are used for high mast illumination poles. Tip deflection at design wind is upward of 10 ft. Linear beam column program BMCOL51 is useful for calculating moments that include the magnification effect of axial load and deflection. HMIP 100. The Houston District uses a modified version of these details. Bridge Design Manual 9-118 TxDOT 12/2001 . were received. covering strain pole and mast arm pole assemblies. contractors. Further delay was caused by shop plan submission procedures. It is practically impossible to determine the number of traffic signal poles used by the department. Arms may be attached to one or two sides of the pole and can extend as much as 40 ft. problems will be minimized. Note that this is seldom used in Texas. some of which are used in districtconstructed signal installations and others furnished to contractors for projects involving signals only. furnished by the contractor or by TxDOT. the specified design load was sometimes found to be unrepresentative of the actual loading conditions. The situation was gradually improved through interaction between the Bridge Design Section. If the districts continue to use these drawings. The Bridge Design Section prepared. Some projects were held up. and fabricators. “Installation of Highway Traffic Signals. and utter confusion ensued. and others had anchor bolts installed before approved shop plans. Bridge support poles: Signals suspended from a beam supported by two poles. They may be classified as follows: ♦ ♦ ♦ ♦ Pedestal poles: Signal mounted directly atop the pole Strain poles: Signals suspended from cables strung between poles Mast arm poles: Signals mounted at the ends of horizontal cantilever arms. Each project may contain one or more poles. steel or timber. The number of signal projects for the period 1980-1989 is shown on Figure 9-46. districts.” by the lump sum for each project. Traffic Operations Section. requiring different anchor bolts. and the Traffic Operations Section issued. Poles were furnished to a procurement or construction specification that prescribed a design load and anchor bolt size but required a design check based on the AASHTO Specification. Many poles have been purchased by the state for stock.Chapter 9 — Special Designs Section 21 — Traffic Signal Poles Section 21 Traffic Signal Poles Background Traffic signal poles present a complicated design problem that was oversimplified by early construction specifications. the specified anchor bolts did not conform to the actual loads and current design practice. Bridge Design Manual 9-119 TxDOT 12/2001 . nine standard drawings in June 1985. The Bridge Design Section assumed responsibility for traffic signal pole design in 1975 along with overhead sign supports. During shop plan review. Often. Poles are erected under the bid item. Luminaries. submission of shop plans to the Bridge Design Section is not required. Design Recommendations For special designs. and Traffic Signals. SP-100. SMA-80. DMA-100. and TS-FD are used for the conditions shown on the drawing. For design purposes. and designs will be checked according to the AASHTO Specification for Structural Supports for Signs. MA-C. the yield stress of anchor bolts will be assumed to be no more than 55 ksi and. Bridge Design Manual 9-120 TxDOT 12/2001 .Chapter 9 — Special Designs Section 21 — Traffic Signal Poles Current Status If standards SP-80. MA-D. shop drawing submittal is required. no more than 50 ksi. SMA-100. DMA-80. for pole material. Drilled shafts have been the preferred foundation for ground-mounted walls. Federal requirements for sound abatement have been promulgated. Foundation design should be referred to the Geotechnical Group. with some sort of masonry or precast concrete panels being the predominant types. sporadically. a majority of sound walls have been constructed in the Houston District. however. Walls have been mounted behind bridge rails to ease this concern. Concern for this problem has led Houston to require extra strength in the lower part of walls. A good reference for masonry walls is the TEK Manual published by the National Concrete Masonry Association. except for one instance where a street sweeper knocked a portion of wall into an empty playground. Bridge Design Manual 9-121 TxDOT 12/2001 . There have been no maintenance problems to date. thick reinforced concrete wall system has recently been successfully crash tested. where they have developed a design system and construction specification. Sound barriers mounted atop bridge rails must be able to resist vehicle impact. 281 freeway in San Antonio was designed in the early 1970s. Current Status The Bridge Design Section will be glad to assist any district in the structural aspects of sound wall design. District environmental coordinators ensure response to these requirements on each project. Design Recommendations The AASHTO Guide Specification for Structural Design of Sound Barriers should be followed. Commercial alternates have predominated. a newly designed combination T501 shape and 7 1/2 in. Since then there has been a growing demand for such treatment.Chapter 9 — Special Designs Section 22 — Sound Walls Section 22 Sound Walls Background When the U. The Bridge Design Section has assisted other districts. extensive and architecturally pleasing walls were required to insulate some sensitive areas from highway noise. Construction specifications usually allow commercial alternates for the appearance portion. Lately. in the structural aspects of sound walls.S. 7 “Elastomeric Bearing Research.batcon. 1986. 6 “Design of Neoprene Bridge Bearing Pads. Grasses. Near cities. 1 2 “Iron and Steel Beams. 1959. Before disturbing nesting swallow colonies. 1970.S. Jr. FHWA. 1977. Design Recommendations There is a bat and bridges project report issued by Bat Conservation International (www. Scalzi.” E. NCHRP Report 109. Netting details and suggestions are available from the Bridge Division. Large pigeon populations roosting in bridges over rivers also cause concerns with water quality during low flow periods. efforts have been made to purposefully provide bat habitat in other bridges and culverts in Texas and throughout the U. TxDOT has a long history of wildflower cultivation in the right-of-way.” PTI Ad Hoc Committee on Cable Stayed Bridges.B. 3 “Arch Bridges. 5 “Recommendations for Stay Cable Design and Testing. pigeons perch on concrete and steel bridges alike. Pigeon nests and droppings are reported to be a source of corrosion to steel bridge members.” Structural Engineering Series No.” AASHTO. duPont de Nemours & Co. W. Small pipe underpasses were constructed near Bastrop to let the endangered Houston Toad cross safely. Cliff swallows have nested under bridges near water in rural and suburban settings for a long time.I. 1985.” 1883-1952.. Bridge Design Manual 9-122 TxDOT 12/2001 . In the mid-1980s. and decorative treatments have increasingly been used to make the highways environmentally friendly. For obsolete bridges that are to be replaced. American Institute of Steel Construction. Pigeon excluders have been developed for use in critical areas. determine whether a permit is required. 1976. and J. Since then.Chapter 9 — Special Designs Section 23 — Wildlife Issues Section 23 Wildlife Issues Background With the increase in environmental concerns has come an awareness of the need to consider the impact of highway construction on the flora and fauna of Texas. the Congress Avenue Bridge in downtown Austin became home to one of the largest urban bat colonies in the world. providing roosting space for 1 1/2 million Mexican freetail bats. “Guide Specification for Strength Design of Trusses. John Wiley and Sons. Fifth Printing 1968. plants. it may be advisable to provide bird netting to exclude swallows approximately one year prior to demolition.org) that contains suggestions for providing bat habitat. 4 “Construction and Design of Cable-Stayed Bridges. The Special Projects Branch is the designated contact for wildlife matters.” Podolny. Current Status The Bridge Design Section will work with the Environmental Division to mitigate the effects of bridges on Texas wildlife. 2. 1987. 1966. G. CTR. Report 1126-4F. “Factors Affecting Anchor Bolt Development. Final Report 29-2F. Hirsch. 15 “Bridge Deck Design for Railing Impact. CFHR. 1990. 1982.C. First Edition. J. Bridge Design Manual 9-123 TxDOT 12/2001 . Final Report 172-2F. Breen. CTR. Reese. 1964. G. 1970. CTR. Report No. 14 Development Length for Anchor Bolts. 21 “Design Procedure Compared to Full Scale Tests of Drilled Shaft Footings. 1985.” Jirsa.E. “Response of Highway Barriers to Repeated Impact Loading. CTR.” Roeder. F. 1978.” Hasselwander. 1984. 1983.” DuPont Company. 1977. B. Report 88-1F.” Creamer. and others.” Breen.” Meyer.A. 12. and others.E. and L. Report 209-1F. Report 172-1. FHWA/RD-87-014. 18 “Bridge Deck Drainage Guidelines. and others. 22 “Tapered Steel Poles – Caisson Foundation Design. 12 “Development Length for Anchor Bolts.” American Iron and Steel Institute. 1977. FHWA. 1977. and K. and T. 20 “Analysis of Drilled Shaft Foundation for Overhead Sign Structures. Final Report 295-1F. “Fatigue of Anchor Bolts. Breen. 1990. CTR. J. 1979.E.H. 1983 Est. Report 53-1F.” Fischer. and H. R. and J.W. 11 “Elastomeric Bearing Research.E. and J.W. 1966.” Turner-Fairbank Highway Research Center.” Klingner. J. NCHRP Report 248. Report No. and C. and W. Report 105-3. “Strength and Behavior of Anchor Bolts Embedded Near Edges of Concrete Piers. B.H. Report 1126-4F.” Cook. FHWA/RD-87-014. CFHR.” Frank.E. R. D. 9 “Performance of Elastomeric Bearings.O.H. CFHR. Final Report 305-1F.E.. Construction and Materials. CFHR. 1980.B and others. CFHR.B and others. Final Report 305-1F.F.J. 1984. NCHRP Report 109.W.” Bogard. 1986. “Fatigue Loading in Sign Structures. J. “Axial Tension Fatigue Strength of Anchor Bolts. J. Report 53-1F. and others.” Turner-Fairbank Highway Research Center. 13 “Development Length for Anchor Bolts. R.J. CFHR. TTI.” Lee.Chapter 9 — Special Designs 8 Section 23 — Wildlife Issues “Elastomeric Bearings Design. 1970. CHFR. U. NCHRP Report 298. 1969. K.” Jirsa.S. “Design Guide for Steel to Concrete Connections..” Arnold A. and others. Final Report 29-2F. CTR. “Factors Affecting Anchor Bolt Development. 17 “Bridge Deck Drainage Guidelines. J.W. TS-84-202.L. C. 19 “Modern Sewer Design. Final Report 244-2F.E. and others.” Breen.” Cook. 1980. D. Matlock. 1986. Frank. “Strength and Behavior of Anchor Bolts Embedded Near Edges of Concrete Piers. “Design Guide for Steel to Concrete Connections.L... CFHR.” Hydraulic Engineering Circular No. 1964. 10 “Engineering Properties of Neoprene Bridge Bearings.A. 16 “Drainage of Highway Pavements. 23 “A Computer Program for the Analysis of Beam-Columns Under Static Axial and Lateral Loads.” Hasselwander.” Ivey.” Teng and Associates. Report 53-1F.A. Report 382-1 & 2F. Roeder. “Strength and Behavior of Bolt Installations Anchored in Concrete Piers.” Stanton. 1985.” Klingner. Dunlap. 1984. and others. R. TTI. Report 382-1 & 2F. CHFR. “Engineering Properties of Neoprene Bridge Bearings.” DuPont Company. and others. Steel Corporation Publication. D. D. 1964. “Response of Highway Barriers to Repeated Impact Loading.” Breen.” Lee. Offshore Technology Conference Paper 2953. 1977 “Strength and Behavior of Bolt Installations Anchored in Concrete Piers. CFHR. Report 88-1F. 1983 Est. CFHR.O. ......................10-3 Section 2 — Lateral Loads and Resistance..............................10-6 Section 3 — Retaining Walls..................................................................................................................................................................................................10-7 Section 4 — Slope Stability................................Chapter 10 Foundation Design Contents: Section 1 — Vertical Resistance..............10-10 Bridge Design Manual 10-1 TxDOT 12/2001 ........................................................... Chapter 10 — Foundation Design Section 1 — Vertical Resistance Bridge Design Manual 10-2 TxDOT 12/2001 . bridge foundation matters were coordinated by the Plan Review Section of the Bridge Division. There are many types and sub-types. A few caissons. shear strengths of various types of soil were correlated with the number of blows of a 170 pound free-falling hammer required to drive the cone penetrometer a given distance into the founding material. With cooperation from the Materials and Tests Division and the Equipment and Procurement Division. Drilled shaft technology developed in the late 1940s. Prior to 1940. the Texas cone penetrometer test was developed. Early bridge designers relied heavily on spread footing foundations. were used for larger stream crossings. Mercifully. Bridge Design Manual 10-3 TxDOT 12/2001 . Drilled shaft and prestressed concrete pile foundations now dominate for bridge construction in Texas. One of the first projects of this group was development of a reliable soil test method for use with exploratory drilling rigs. Specialized expertise in general geotechnical engineering resided in the Materials and Tests Division. although timber and concrete piling were available options. pneumatic and open. During the following 15 years.Chapter 10 — Foundation Design Section 1 — Vertical Resistance Section 1 Vertical Resistance Background Variation in foundation material properties makes accurate design extremely complicated. ranging from soft soil to hard rock. simple tests have been developed that allow structural engineers to estimate the vertical capacity of most foundation elements with reasonable accuracy. Steel H piling became popular in the late 1930s. Confidence in this test continues to be justified by lack of foundation failures on Texas bridges. that occur in different layers with properties that vary with moisture content and overburden. A bridge foundation soils group was then formed to insure continuing and consistent handling of increasing foundation problems. Additionally. Beginning in 1962. geotechnical design and foundations issues are the responsibility of the Geotechnical Branch of the Bridge Division Technical Services Section. and length of foundation elements to be used Reviews plans prepared in the districts.Chapter 10 — Foundation Design Section 1 — Vertical Resistance Bridge Foundation Design Issues Bridge foundations are a major factor in bridge design. Construction specifications dating from 1918 cover timber and concrete piling. Foundation and geotechnical issue are handled by the Texas Department of Transportation’s (TxDOT) geotechnical engineers within the Bridge Division. remain in the current construction specifications. advantage may be taken of soil set up tendencies by redriving the test piling after seven days. Around 1980 it became a separate branch in the Bridge Design Section. Beginning in 1965. The majority were performed by the Center for Transportation Research at the University of Texas. The geotechnical group that originated in plan review became part of the Construction Section of the Bridge Division in 1962. The Geotechnical Branch performs the following: ♦ ♦ ♦ ♦ Coordinates preliminary exploration of foundation conditions Consults with bridge designers regarding type. The majority of these studies were performed by the Texas Transportation Institute at Texas A&M University. and soil. under some soil conditions. which was calculated for service loads. to determine safe load capacity. for proper foundation design Consults on construction problems associated with bridge foundations and retaining walls. cushion. Test-driven piling are used more often to supplement the hammer formula or wave equation in verifying safety under the design load. or by consulting engineers. Vertical load resistance was estimated from the last few hammer blows using the Engineering News formula. modified for double acting power hammers. For clayey soils. the Texas Quick Load Test Method was incorporated into the specifications to establish safe load capacity of piling and drilled shafts. One or more test loads are specified in rare instances where analytical design appears undependable and significant savings are possible through more refined methods. dynamic analysis by a wave equation computer program is used. In 1965. The same basic resistance formulas. This program is also useful for computing stresses in the pile for various combinations of hammer. Bridge Design Manual 10-4 TxDOT 12/2001 . Currently. Calculated resistance was required to be equal to or greater than the pile load shown on the plans. 10 research studies generating 24 reports on drilled shaft foundations were sponsored by TxDOT. size. 11 research studies generating 37 reports on piling foundations were sponsored by TxDOT. This approach is said to date back to 1888. Foundation design is accomplished using the information contained in the Geotechnical Manual. and the use of bell footings was virtually eliminated. skin friction was utilized to increase load resistance. Verification of the reliability of the slurry displacement method. design should be completed and pile or drilled shaft loads calculated for service loads. Design Recommendations If there is any doubt as to the type of foundation appropriate for a bridge. all foundation design is finalized by the Geotechnical Branch. As confidence grew from research and test loads. This method also eliminates the need for troublesome casing. should be given to the Geotechnical Branch to establish lengths and recommend special notes or test procedures if needed. with a print of the layouts and boring logs. in the early 1970s.Chapter 10 — Foundation Design Section 1 — Vertical Resistance Drilled shaft usage was promoted vigorously by the industry and supported by TxDOT research. After the type of foundation has been established. Current Status For plans prepared in the Bridge Design Section. This information. Foundation designs prepared by district personnel and by consulting engineers are reviewed by the Geotechnical Branch. Point bearing design using bell footings was predominant. the Geotechnical Branch should be consulted early in the design process. led to increased usage of drilled shafts in previously questionable soil conditions. Bridge Design Manual 10-5 TxDOT 12/2001 . TxDOT attempts at field measurement of lateral pressure and response confirm the high degree of variability. including surcharge. Classical methods of Rankine or Coulomb have been used for active pressure and passive resistance. lateral soil pressure may be assumed to be 40 pounds per square foot per foot of height. see the Geotechnical Manual. Design Recommendations For most conditions. Current Status For most bridge designs. computer programs are available that consider the effects of lateral soil resistance on the structural design. The Federal Highway Administration (FHWA) Handbook on Design of Piles and Drilled Shafts under Lateral Load 1 is probably the most comprehensive treatment of the resistance of soils to lateral load.Chapter 10 — Foundation Design Section 2 — Lateral Loads and Resistance Section 2 Lateral Loads and Resistance Background Lateral loads imposed by soils and response of embedded elements to lateral loads are even more speculative than vertical load response. is considered sufficient. judgmental treatment of lateral load and resistance. For critical situations. based on experience. Bridge Design Manual 10-6 TxDOT 12/2001 . For more complete information. For questionable conditions. computer programs FRAME51 or COM624 may be used with lateral load-deflection values (p-y curves) as recommended by the geotechnical engineer. Detailed plans are prepared by the successful wall supplier. plan and elevation geometry for the walls and a construction specification. as the designer saw fit. Bridge Design Manual 10-7 TxDOT 12/2001 . The latest report2 contains a design procedure for closely spaced drilled shaft walls. For many years. Retaining walls were formerly designed in a Bridge Division or district structural group to resist active pressures calculated according to Rankine or Coulomb. retaining walls are predominantly geotechnically controlled.Chapter 10 — Foundation Design Section 3 — Retaining Walls Section 3 Retaining Walls Background Although not strictly foundations. footings were placed on piling of various types and configurations. Since the mid-1980s proprietary retaining wall designs have proliferated. and review all others. Other proprietary variations of mechanically stabilized earth (MSE) walls soon appeared. In extremely soft soils. TxDOT sponsored five research studies. retaining walls in Texas were mostly cantilever walls on spread footings. Various district standards for cantilever walls have been used. Buttress or counterfort walls were used occasionally. Geotechnical Branch engineers perform virtually all retaining wall designs for which the Bridge Design Section is responsible. In the late 1970s reinforced earth walls began to be accepted and were often the contractor’s choice on alternate bids. or to 30 or 40 pounds per square foot per foot. generating 12 reports relative to retaining wall design. The Bridge Division issued a comprehensive set of cantilever retaining wall standards in 1975 and updated them in 1984. only to have them rendered virtually obsolete by the rise in popularity of MSE walls. Beginning in 1971. Bidding information for MSE walls consists of one sheet of general design requirements. prestressed ground anchors were added to reduce the number and size of drilled shafts. Closely spaced drilled shafts have been used for crowded conditions since 1970. Later. The most severe loading condition apparently occurs during backfilling. One notable time-related failure of a cantilever/spread footing wall occurred in San Antonio. Copper waterstops had been used between wall and footing.Technical Services Section is responsible for the design or review of all retaining walls on the state highway system. several wall sections fell onto the highway during a rainstorm.Chapter 10 — Foundation Design Section 3 — Retaining Walls Construction Issues Most problems with retaining walls occur during construction. Observed malfunctions have been: ♦ Excessive wall deformation: • • ♦ Excessive toe pressure due to soft foundation or poor backfill material and/or procedures Deformation of the wall itself due to heavy backfill equipment or deficient MSE backfill Insufficient vertical load such as with a short heel on cantilever walls Loss of passive resistance by excavation in front of the toe General embankment stability failure Improper backfill materials for MSE Inadequate compaction of backfill. In a very soggy environment. Current Status The Geotechnical Branch of the Bridge Division . Some 30 years after construction. the reinforcing steel became a sacrificial anode in a corrosion cell and gradually deteriorated. Bridge Design Manual 10-8 TxDOT 12/2001 . Sliding of spread footing walls: • • ♦ ♦ Rotation and sliding of wall and footing: • • • Backfill failure: Maintenance Issues Retaining wall malfunctions are very difficult to repair. refer to the Geotechnical Manual. retaining walls may be designed for a pressure of 40 pounds per square foot per foot of wall height plus surcharge. Closely spaced drilled shaft walls should be designed with the assistance of a lateral load and resistance computer program such as COM624.Chapter 10 — Foundation Design Section 3 — Retaining Walls Design Recommendations For usual conditions. For specific guidance regarding retaining wall selection and design. Current American Association of State Highway and Transportation Officials (AASHTO) Specifications contain a comprehensive section on retaining wall design. Bridge Design Manual 10-9 TxDOT 12/2001 . but this is subservient to the established practice of the Geotechnical Branch. but embankment slopes may also fail because of poor fill material and penetration of surface water. Mitigation of the effects of river bank migration taxes the ingenuity of bridge and geotechnical engineers alike. Design Recommendations Bridge engineers should not hesitate to bring questionable slope conditions to the attention of the Geotechnical Branch. FHWA-IP-84-11. The Geotechnical Branch is responsible for slope stability when bridges or retaining walls are likely to be involved. Beginning in 1971. Migration of river banks is related to slope stability in that clay layers and ground water are usually involved. Experience. For specific guidance regarding slope stability issues. greater cut and fill requirements. Report No. perhaps due to increasing construction intensity. the cost of repair is great. Bridge engineers are not often involved in slope stability design until instability threatens a bridge or retaining wall. Bridge Design Manual 10-10 TxDOT 12/2001 . Current Status Highway design engineers are generally responsible for initial evaluation of slope stability. When such failures occur. 1 “Handbook on Design of Piles and Drilled Shafts Under Lateral Load. computer programs. or when otherwise asked to evaluate potential problem sites. One of these3 contains a survey of slope failures and repair methods in Texas. There have been several instances of bridge piers being severely displaced by movement of the surrounding soil toward the river. Many failures occur within cut slopes. Corpus Christi. Either significant failures did not occur or failures were repaired without fanfare by maintenance forces. Weak natural ground can allow the embankment failure surface to extend below and considerably beyond the toe of the slope. Then.Chapter 10 — Foundation Design Section 4 — Slope Stability Section 4 Slope Stability Background Slope failures apparently received very little attention prior to the interstate highway era. and other analytical methods are available for proper consideration of the problem. 1984. slope failures appeared to increase. or public exposure. Slope failures are almost always associated with plastic clay soils and water. and Beaumont. Bridge engineers are often asked to participate in the design of structures to repair local slope failures not associated with a retaining wall. refer to the Geotechnical Manual. TxDOT sponsored four research studies generating nine reports relative to slope stability. Turner Fairbank Highway Research Center. reaching a peak in the early 1980s with major failures in Houston. S. 1985. 3 “A Survey of Earth Slope Failures and Remedial Measures in Texas. 1972.G.C. Report 161-1. CHFR. T.Chapter 10 — Foundation Design 2 Section 4 — Slope Stability “Study of Design Method for Vertical Drilled Shaft Retaining Walls.G. Bridge Design Manual 10-11 TxDOT 12/2001 .T. and S. Final Report 415-2F.” Wang. Wright. and L. CTR. Reese.” Abrams. 50 16.67 14.110 klf composite dead load (1/3rd of T501 rail ~ 0.08 16. 75% maximum debonding per section 0.9 Maximum debonding length = 0.25 NA 13.67 16.) U40 Beam U54 Beam 16.75 Guidelines in the preceding table assume the following design criteria: ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ ♦ Interior beam design only 8" slab thickness (maximum clear span = 8'-8") Maximum composite width to either side of top flange = 48" Live load distribution factor = S/11 per truck/lane.67 12.00 16. Span Lengths Maximum Beam Spacing (ft.00 16.25 NA 11.330 klf) 50% relative humidity (used only for producing table) 1/2" 270 ksi low-relaxation strand f'ci max = 6500 psi.08 16.Appendix A Supplemental Design Criteria for Prestressed Concrete U Beams U-Beam Spacing Use the following guidelines for the spacing of U beams.67 9.67 11. whichever is less 75% maximum debonding per row. with a minimum value of 0.50 16.67 7.75 16. f'c max = 8500 psi No overlay Span lengths shown are CL to CL Bent with 9 ½" distance to CL Bearing Bridge Design Manual A-1 TxDOT 12/2001 . Span Length (ft.50 15.2L or 15'.67 15.) 75 80 85 90 95 100 105 110 115 120 125 130 U-beam Spacings vs.75 NA 12.00 NA 9. the slope of the bearing seat. This report summarizes the bearing pad taper perpendicular to the centerline of bearing for each beam bearing location. In RDS. All U-beam jobs should have a Bearing Pad Taper Report sheet that contains the various pad tapers for use by the bearing pad fabricator. the following sign convention will be used looking in the direction of increasing station numbers: ♦ ♦ ♦ ♦ ♦ ♦ positive bearing seat slope is up and to the right negative bearing seat slope is down and to the right positive beam grade is up negative beam grade is down positive bearing pad taper is up negative bearing pad taper is down Bridge Design Manual A-2 TxDOT 12/2001 . The amount of bearing pad taper depends on three factors . The bearing pad is oriented along the centerline of bearing. and the beam angle. This configuration for the U-beam bearing allows the pad to taper in only one direction (perpendicular to the centerline of bearing).the grade of the U beam. For purposes of developing the formulas for calculating bearing pad taper. The calculations that follow derive the formula for calculating the bearing pad taper perpendicular to centerline of bearing. the report is titled “Bearing Pad Taper -.Appendix A Supplemental Design Criteria for Prestressed Concrete U Beams Bearing Pad Taper Calculations for U Beams The bearing seat for a U beam is level perpendicular to the centerline of bearing but slopes along the centerline of bearing between the left and right bearing seat elevations.Fabricator’s Report”. and ELEV3 at points 1. Online users can click here to view this illustration in PDF format. the pad taper is only function of the bottom surface of the beam. Using ELEV1. respectively. Bridge Design Manual A-3 TxDOT 12/2001 . at the bottom of U beam. Figure A-1. Plan View of Bearing Seat with Right Forward Beam Angle. Perpendicular to the centerline of bearing. the equation for pad taper is: TAPER = (ELEV2 – ELEV3)/W Where: ELEV2 = ELEV1 + BEAM GRADE × (W/sin θ) ELEV3 = ELEV1 + SLOPE × (W/tan θ) Substituting for ELEV2 and ELEV3 . ELEV2. The bearing seat is level in this direction. the equation for pad taper becomes: TAPER = (BEAM GRADE – SLOPE × cos θ)/sin θ CASE II. Beam Angle is θ < 90° and Right Forward. and 3.Appendix A Supplemental Design Criteria for Prestressed Concrete U Beams CASE I. so the component of pad taper due to the top surface of the bearing seat is zero. 2. Beam Angle is θ < 90° and Left Forward. the equation for pad taper is: TAPER = (ELEV2 – ELEV3)/W Where: ELEV2 = ELEV1 + BEAM GRADE × (W/sin θ) ELEV3 = ELEV1 – SLOPE × (W/tan θ) Substituting for ELEV2 and ELEV3. ELEV2. respectively. the component of bearing pad taper due to the bearing seat is zero. Plan View of Bearing Seat with Left Forward Beam Angle. the equation for pad taper becomes: TAPER = (BEAM GRADE + SLOPE × cos θ)/sin θ CASE III. Using ELEV1. 2. at the bottom of the U beam.Appendix A Supplemental Design Criteria for Prestressed Concrete U Beams Figure A-2. Beam Angle is θ = 90° Since the centerline of the U beam is perpendicular to centerline of bearing. and ELEV3 at points 1. and 3. Online users can click here to view this illustration in PDF format. and the bearing pad taper is simply: TAPER = BEAM GRADE Bridge Design Manual A-4 TxDOT 12/2001 . Plan View of Bearing Seat. Defining β = beam angle as measured counterclockwise from centerline of bearing. Online users can click here to view this illustration in PDF format. the equation for the calculating bearing pad taper for any bearing location is as follows: TAPER = (BEAM GRADE – SLOPE × cos β)/sin β Bridge Design Manual A-5 TxDOT 12/2001 .Appendix A Supplemental Design Criteria for Prestressed Concrete U Beams Summary Figure A-3. U beams will be at some cross-slope other than the cross-slope of the roadway surface. such as varying cross-slope and/or varying overall width. In terms of calculating the required haunch at centerline of bearing for a U beam.Appendix A Supplemental Design Criteria for Prestressed Concrete U Beams Figure A-4. i. the haunch at centerline of bearing for the left edge of the beam may be different than the haunch at right edge of the beam. Haunch Calculations for U Beams U beams are not placed vertical like I beams but at a cross-slope.e. it is due to the geometry of the roadway surface and not necessarily the balancing of the U beam). Once the minimum haunch value is established. For spans with more complicated geometry. Online users can click here to view this illustration in PDF format. Skewed beam end conditions can also contribute to a different haunch at centerline bearing for each edge of the beam (this difference can exist even with a constant cross-slope. U beams will be at the same cross-slope as the roadway surface.. Example Shear Key Detail. the maximum haunch at centerline of bearing on the opposite top edge of the U beam can be calculated as well as the deduct value for computing bearing seat elevations. For spans with constant cross-slope and constant overall width. Thus. The following is a suggested method of calculating the required haunch and the corresponding deduct values for U beams: Bridge Design Manual A-6 TxDOT 12/2001 . Each U beam in a span is balanced in cross-slope from the back bearing to the forward bearing of the beam so that no torsion is introduced into the beam. the haunch for each edge of top flange of the U beam must be calculated. RDS will take into account the skew at that end of the U beam and give the corresponding vertical ordinates at the centerline of bearing for the left and right edges of the top flange (See Figure A-5). Also. Figure A-5. (See Figure A-5). Example Plan View of the Vertical Ordinates for U Beam. Three lines of vertical ordinates will be generated for every U beam. One or both vertical ordinate values at the left and right top edge of the U beam at centerline of bearing will be zero. and right top edge of U beam. centerline. the vertical placement is controlled by these “corners” of the U beam that have vertical ordinates of zero. input the section depth as zero and the pedestal width equal to the top flange width of the U beam (7. This negative vertical ordinate is the difference in haunch at centerline of bearing from the left top edge of beam to the right top edge of beam due to bridge geometry and/or balancing of the U beam. for skewed beam end conditions. This will instruct RDS to keep the top flange width dimension perpendicular to the centerline of U beam. Step 2. A vertical ordinate of zero indicates that the top edge of the U beam is matched with the elevation of the top of slab at that point. On the BRNG card. a negative vertical ordinate value at the dependent corner means that at that point the top edge of U beam is below the elevation at top of slab. Its value depends on the bridge geometry and/or balancing of the U beam. However. respectively. Execute a preliminary RDS run using the beam framing options 20. or 22 in order to calculate the vertical curve component of the haunch.Appendix A Supplemental Design Criteria for Prestressed Concrete U Beams Step 1.e. The first and last columns of each vertical ordinate table are the ordinates at centerline of back bearing and forward bearing. include a VCLR card for each span with the bridge alignment as the specified alignment. Thus.00' for U54 beams). Thus. Bridge Design Manual A-7 TxDOT 12/2001 .42' for U40 beams and 8.. The corner opposite to the controlling corner at the centerline of bearing will either have a zero or negative vertical ordinate. the vertical ordinates along the left top edge. Be sure to add the letter “P” in column 80 of the BRNG card. A zero value for the vertical ordinate at the dependent corner means that at that point the top of the U beam is also matched with the elevation of the top of slab. i. 21. Online users can click here to view this illustration in PDF format. Examine the RDS output. Appendix A Supplemental Design Criteria for Prestressed Concrete U Beams Figure A-6 illustrates the vertical ordinates produced by RDS for the left and right top edges of the U beam when framing a span with a crest vertical curve and a varying cross-slope along the span. It is shown only to help visualize a possible scenario of vertical ordinates produced by RDS. Figure A-6. Example Vertical Profile at Edges of U Beam. Online users can click here to view this illustration in PDF format. Bridge Design Manual A-8 TxDOT 12/2001 Appendix A Supplemental Design Criteria for Prestressed Concrete U Beams Figure A-7. Example of U Beam Geometry as per RDS. Online users can click here to view this illustration in PDF format. When inputting the top flange width of the U beam, bf, on the BRNG card, the VCLR command calculates the vertical ordinates at an offset distance of bf/2 from the RDS beam line (See Figure A-7.). The standard convention for defining the RDS beam line is a vertical line at a point coinciding with the centerline of the bottom of the bearing pad. Thus, for U beams at a cross-slope, the beam rotates about this point. This rotation of the U beam shifts the top flange of the beam transversely with respect to the RDS beam line. RDS makes no adjustment for the rotation-induced transverse movement and, therefore, yields vertical ordinate calculations that are not exactly at the outside edge of the top flange. However, this error should be negligible as the offset error will only be approximately 1" for a U54 on a 2% cross-slope. Step 3. Calculate the required minimum haunch at centerline bearing that will work for all U beams in a span. Start by calculating the required minimum haunch at centerline of bearing for both the left and right top edges of each U beam in that span. For each side of the beam, work from the controlling corner and use the entire maximum vertical ordinate on that edge in your haunch calculation. Do not be concerned with the vertical ordinate value at the dependent corner for each side because that value affects only the maximum haunch (see Step 4), not the minimum haunch. Also, we typically use 75% of the predicted camber by Prestress 14 for U beams because in the field we have not been consistently getting our predicted cambers. Keeping the sign convention used by RDS [a positive vertical ordinate means the top of beam is above the top of slab at that point, while a negative ordinate means the top of beam is below the top of slab at that point], the required minimum haunch values at centerline of bearing for each U beam will be: Left Top Edge: Min. Haunchreq’d = 0.75C - 0.8∆DL + VOmax L + Min. HaunchCL Span Right Top Edge: Min. Haunchreq’d = 0.75C - 0.8∆DL + VOmax R + Min. HaunchCL Span Where: VOmax L = maximum vertical ordinate, left top edge (usually at mid-span) VOmax R = maximum vertical ordinate, right top edge (usually at mid-span) C = camber of U beam DL = dead load deflection of U beam due to slab only A-9 TxDOT 12/2001 Bridge Design Manual Appendix A Supplemental Design Criteria for Prestressed Concrete U Beams Min. HaunchCL Span = minimum haunch specified at centerline span (usually ½") Using the largest required haunch value for that span: Min. Haunchused = Min. Haunchreq’d (round up to the nearest ¼") This haunch value will be the haunch at all controlling corners for each U beam in that span. Step 4. Calculate the corresponding maximum haunches at centerline of bearing. The maximum haunches at centerline of bearing occur at the dependent corners of each U beam. These maximum haunches may vary between U beams in a span but typically will not vary for the same U beam. The maximum haunches at centerline of bearing for each U beam in a span are: Left Top Edge: Max. Haunchcal’d = Min. Haunchused - VOLt Depdt Corner Right Top Edge: Max. Haunchcal’d = Min. Haunchused - VORt Depdt Corner Where: VOLt Depdt Corner = vertical ordinate value, left top edge, dependent corner VORt Depdt Corner = vertical ordinate value, right top edge, dependent corner Step 5. Calculate the slab dimensions at centerline of bearing, Xmin and Xmax, and the theoretical slab dimensions at mid-span, ZL and ZR, for each U beam in the span (See Figure A-8). The equations are: Xmin = Min. Haunchused + slab thickness Xmax = Max. Haunchcal’d + slab thickness ZL = Min. Haunchused - Min. Haunchreq’d @ left edge + slab thickness + Min. HaunchCL Span ZR = Min. Haunchused - Min. Haunchreq’d @ right edge + slab thickness + Min. HaunchCL Span Again, Xmin is the section depth at all controlling corners of the beam while Xmax is the section depth at all dependent corners of the beam (any difference in Xmax for each dependent corner of an individual beam should be negligible). Bridge Design Manual A-10 TxDOT 12/2001 Appendix A Supplemental Design Criteria for Prestressed Concrete U Beams Figure A-8. Section Depth Information on Production Drawings. Online users can click here to view this illustration in PDF format. Figure A-8: above shows a typical plan view and table that can be used on production drawings to describe the depth and location of the X and Z values. In order to use a single and generic detail, the “min” and “max” designation is changed to an open convention using the letters “A” and “B”. As a result, XA and XB can be either the Xmin or Xmax values. Step 6. Calculate the required deduct at the specific bearing location to use in computing the final bearing seat elevations. The first table in this appendix shows the pedestal widths for the U40 and U54 beams with the standard and dapped end conditions. The pedestal widths listed depend on the beam angle and are adequate for up to two 9" x 19" bearing pads. Also, because you can only input one pedestal width per BRNG card, the pedestal width used must be for the U beam with the smallest beam angle at that bearing location. Be sure to omit the letter “P” in column 80 of the BRNG card so that RDS applies the pedestal width along the centerline of bearing. 75 60 45 Beam Angle Standard End Dapped End Standard Bearing Seat Dimension “D” 4'-6" 5'-0" θ 90 5'-0" 5'-6" θ < 75 5'-6" 6'-0" θ < 60 Bridge Design Manual A-11 TxDOT 12/2001 Appendix A Supplemental Design Criteria for Prestressed Concrete U Beams Figure A-9. Location of Deduct for Final Bearing Seat Elevations. Online users can click here to view this illustration in PDF format. The required deduct for calculating bearing seat elevations needs to be the deduct at the edge of the bearing seat (see Figure A-9.) This deduct can be obtained by interpolating between the values Xmin and Xmax for each U beam using the beam angle, the top flange width of the beam, and the chosen bearing seat width. The largest calculated deduct at that bearing location should be used to compute the final bearing seat elevations for all the U beams at that bearing location. The difference between the calculated deducts at a given bearing location should be negligible, but you may want to check your worst case span initially to see if the difference is large enough to take into account The deduct for the calculation of final bearing seat elevations is: Deduct = (Xmax - Xmin)/(bf /sin θ) x (bf - D)/2 + Xmin + Beam Depth + Brng. Pad Thickness Where: bf = top flange width θ = beam angle D = chosen pedestal width Note: The beam angle can probably be ignored because of negligible difference in the final deduct amount. In addition, this formula does not include the vertical adjustment of the beam depth and bearing pad thickness due to the cross-slope of the beam, which is also negligible. Summary The required haunch at centerline of bearing for the left and right top edges of the U beam should always be calculated working from the controlling corner for that side. This is done because the vertical ordinate for the dependent corner is “built-in” to the geometry for the beam and bridge. We cannot use that value in determining our haunch because the vertical ordinate at the dependent corner is always present, i.e., we cannot adjust the beam vertically to reduce that dimension. Basically, the controlling corners will have the minimum haunch at centerline of bearing while the dependent corners will have the maximum haunch at centerline of bearing, the difference being the vertical ordinate value at the dependent Bridge Design Manual A-12 TxDOT 12/2001 Appendix A Supplemental Design Criteria for Prestressed Concrete U Beams corner. Incidentally, because the U beam is at an average cross-slope, the haunches at centerline of bearing for the back end of the beam for the left and right edges will typically be reversed at the forward end of the beam. At mid-span, the theoretical haunch value for the left and right top edges of each U beam will be the same value if the roadway surface cross-slope is constant or transitions at a constant rate over the entire length of the span and the beam spacing remains constant in the span. For any other case, the theoretical haunch at mid-span may be different for the left and right top edges of each U beam. Bridge Design Manual A-13 TxDOT 12/2001 Appendix B Design Formulas for Inverted Tee Bents Introduction In addition to conventional design requirements for flexure, shear, and side beam reinforcement, the following design recommendations for ledge depth, ledge reinforcement, and web reinforcement are given. This information is adapted from material published in Design of Reinforced and Prestressed Concrete Inverted T-Beams for Bridge Structures.1 Bridge Design Manual B-1 TxDOT 12/2001 Appendix B Design Formulas for Inverted Tee Bents Figure B-1. General Information. Online users can click here to view this illustration in PDF format. Bridge Design Manual B-2 12/2001 5S Note: If C > .Appendix B Design Formulas for Inverted Tee Bents Ledge Depth Figure B-2.2f ′c (distr width)dsf Distribution width equals the lesser of: Interior Beam: B + 4a or S Exterior Beam: 2C or C + . If C ≤ . This limit is imposed because when computing Avf to determine required ledge reinforcing the formulas given would become unconservative for higher concrete strengths. Interior Beam: φPn = φ 4 f ′c (B + 2W + 2Z + 2dpv )dpv c Exterior Beam: φPn = φ 4 f ′ (. However. Online users can click here to view this illustration in PDF format.85 Note: Use a maximum f ′c of 4000 psi. Since punching shear is usually more critical than shear friction in determining ledge depth this limitation seldom affects final design. these equations can generally serve as a basis for determining punching shear capacity for other cases.5(B + 4a) use interior beam criteria Bridge Design Manual B-3 12/2001 .5B + W + Z + dpv Shear Friction: φ = . Punching Shear: φ = . φPn = φ .5B + W + Z + dpv + C )dpv .85 Note: These equations are for rectangular pads with no skew. Punching Shear. Shear Friction Requirement: φ = . Ledge Reinforcing. Online users can click here to view this illustration in PDF format.85 Avf = Pu φ × 1. (C + .Appendix B Design Formulas for Inverted Tee Bents Ledge Reinforcing Figure B-3. but not less than × dist . width φ × fy × 0.5S ).9 Asf = .04f ′c × dsf Pu × a . or (B + 5a) Bridge Design Manual B-4 12/2001 .4fy Distribution width equals the lesser of: Interior Beam: (B + 4a) or S Exterior Beam: 2C . (C + .8dsf fy Distribution width equals the lesser of: Interior Beam: (B + 5a) or S Exterior Beam: 2C . or (B + 4a) Flexure Requirement: φ = .5S ). Appendix B Design Formulas for Inverted Tee Bents Tension Requirement: φ = .2Pu φ × fy Distribution width equals the lesser of: Interior Beam: (B + 5a) or S Exterior Beam: 2C .667 Avf An Asf + An A ≥ + . or s distr . width Second Layer: .333Avf A ≥ s distr . width distr .9 An = Nu . (C + .5S ). or (B + 5a) Top Layer: . where Nu = . width Bridge Design Manual B-5 12/2001 . width distr . s Bridge Design Manual B-6 12/2001 .Appendix B Design Formulas for Inverted Tee Bents Web Reinforcing Hanger Reinforcement: φ = .85 Design for the largest value of Av from the following equations. Torsion Elements. ù é 4 f ′c x 2 y Av æ φTn = φ ê fy (α t x1 y 1 ) .85 Figure B-4. but ≤ 18φ f ′c ç + ç 3 2s ê è ë Bridge Design Manual B-7 x 2y ö 3 12/2001 .Appendix B Design Formulas for Inverted Tee Bents 2Pu − 4 f ′c × bf × dpv Av φ = s fy (B + 2dpv ) S Av 2Pu Av 2Pu = for interior beams or = for exterior beams. Also. Online users can click here to view this illustration in PDF format. if C < s φfyS s φfy × 2C 2 B + 3a Av 3Ps Av 3Ps = for interior beams or = for exterior beams. This allows for a reasonable size and spacing of hanger reinforcement and limits cracking between the flange and stem. Shear: φ = . not depth of cap from column face.85 φVn = φ (2 f ′c × bwdw + Av fydw ) s Note: Take Vu at face of column. additional hangers with anchorage hooks at their ends should be arbitrarily placed along the end face of inverted-T caps. if C < s fy (B + 3a) s fy (2C ) 2 Where: Pu = The largest factored single beam reaction on one side of the cap Ps = The largest service load single beam reaction on one side of the cap Av = Area of both/all stirrup legs s = stirrup spacing Note: It is recommended that the distance from centerline of exterior beam at centerline of bearing to the end of cap (C) should not be less than 24 inches. (See Appendix A reference 56) Torsion: φ = . Stirrup requirements for shear are normally not added to the hanger requirements. These end hangers should be sized to match the stirrups and spaced at 6 inches max. 33 2 2 y1 ≤ 1. PCI Journal.66 + .5 x1 æ Vu ö æ Tu ö ç ≤ 1 . the four corners of the beam ledges. and added to the flexural reinforcing. July – August 1985. 1 “Design of Reinforced and Prestressed Concrete Inverted T-Beams for Bridge Structures.Appendix B Design Formulas for Inverted Tee Bents Where α t = .0 ç φV ÷ + ç φT ÷ ç è n è n If >1.0 add additional stirrups A′v s and additional longitudinal steel A = A′v s (x1 + y 1 ) Note: A should be distributed to the four corners of the web.” Furlong and Mirza. Bridge Design Manual B-8 12/2001 . Appendix C Load vs. Diagrams for 3000 psi and 3600 psi concrete represent commonly used drilled shaft and column concrete strengths. Bridge Design Manual C-1 TxDOT 12/2001 .through 72inch diameter. The AASHTO minimum reinforcement ratio of one percent is normally the controlling design feature. Moment interaction diagrams were developed for 24. Moment Interaction Diagrams Introduction The following Load vs. Using grade 40 allows for considerably shorter lap and embedment requirements. Column design is normally not a strength issue. This is because grade 40 is commonly used in the design of interior bent columns even though grade 60 will be required by the specifications. round columns with common reinforcing patterns used on TxDOT projects. reducing congestion and improving constructibility. Note that the reinforcing is grade 40. Appendix C Load vs. Moment Interaction Diagrams Figure C-1. Bridge Design Manual C-2 TxDOT 12/2001 . Online users can click here to view this illustration in PDF format. Diagrams for 24-Inch Diameter Columns. Online users can click here to view this illustration in PDF format. Bridge Design Manual C-3 TxDOT 12/2001 . Moment Interaction Diagrams Figure C-2. Diagrams for 30-Inch Diameter Columns.Appendix C Load vs. Online users can click here to view this illustration in PDF format. Bridge Design Manual C-4 TxDOT 12/2001 .Appendix C Load vs. Moment Interaction Diagrams Figure C-3. Diagrams for 36-Inch Diameter Columns. Appendix C Load vs. Moment Interaction Diagrams Figure C-4. Diagrams for 42-Inch Diameter Columns. Bridge Design Manual C-5 TxDOT 12/2001 . Online users can click here to view this illustration in PDF format. Online users can click here to view this illustration in PDF format. Diagrams for 48-Inch Diameter Columns. Bridge Design Manual C-6 TxDOT 12/2001 . Moment Interaction Diagrams Figure C-5.Appendix C Load vs. Online users can click here to view this illustration in PDF format. Bridge Design Manual C-7 TxDOT 12/2001 .Appendix C Load vs. Diagrams for 54-Inch Diameter Columns. Moment Interaction Diagrams Figure C-6. Appendix C Load vs. Moment Interaction Diagrams Figure C-7. Diagrams for 60-Inch Diameter Columns. Bridge Design Manual C-8 TxDOT 12/2001 . Online users can click here to view this illustration in PDF format. Bridge Design Manual C-9 TxDOT 12/2001 . Moment Interaction Diagrams Figure C-8. Diagrams for 66-Inch Diameter Columns. Online users can click here to view this illustration in PDF format.Appendix C Load vs. Diagrams for 72-Inch Diameter Columns.Appendix C Load vs. Online users can click here to view this illustration in PDF format Bridge Design Manual C-10 TxDOT 12/2001 . Moment Interaction Diagrams Figure C-9.