CONTENTS1 Design 2 Roadbed types 3 Truss types used in bridges 3.1 Allan truss 3.2 Bailey bridge 3.3 Baltimore truss 3.4 Bollman truss 3.5 Bowstring arch truss (Tied arch bridge) 3.6 Brown truss 3.7 Brunel Truss 3.8 Burr Arch Truss 3.9 Cantilevered truss 3.10 Fink truss 3.11 Howe truss 3.12 K truss 3.13 Kingpost truss 3.14 Lattice truss (Town's lattice truss) 3.15 Lenticular truss 3.16 Long truss 3.17 Parker (Camelback) truss 3.18 Pegram truss 3.19 Pennsylvania (Petit) truss 3.20 Post truss 3.21 Pratt truss 3.22 Queenpost truss 3.23 Truss arch 3.24 Waddell truss 3.25 Warren (non-polar) truss 3.26 Whipple Pratt truss 3.27 Vierendeel truss 4 Statics of trusses 5 Analysis of trusses 6.1 Design of members 6.2 Design of joints 7. Force acting on truss bridges 8. Reference A truss bridge is a bridge composed of connected elements (typically straight) which may be stressed from tension,compression, or sometimes both in response to dynamic loads. Truss bridges are one of the oldest types of modern bridges. The basic types of truss bridges shown here have simple designs which could be easily analyzed by nineteenth and early twentieth century engineers. A truss bridge is economical to construct owing to its efficient use of materials. and where the sides extend above the roadbed but are not connected. which will only act in compression or tension. The members of a truss are the chords or horizontals. Roadbed types The truss may carry its roadbed on top.Design of Truss Bridges A truss is constructed of a simple supporting lattice-work of triangles that support the beam or the span by adding rigidity. This can be used to separate rail from road traffic or to separate the two directions of automobile traffic and so avoiding the likelihood of head-on collisions. in the middle. or at the bottom of the truss. forming a box truss. This allows for some movement of the members without snapping. Bridges with the roadbed at the top or the bottom are the most common as this allows both the top and bottom to be stiffened. Sometimes both the upper and lower chords support roadbeds. forming a double-decked truss. verticals and diagonals. It increases the beam's ability to dissipate the compression and tension forces. When the roadbed is atop the truss it is called a deck truss (an example of this was the I-35W Mississippi River bridge). Deck truss railroad The four span through Pony truss bridge of Sky Gate Bridge R . Piles or pilings are the vertical members of a truss bridge. The members are pinned together at the nodes where the straight members meet. it is called a through truss (an example of this application is the Pulaski Skyway). a pony truss or half-through truss. when the truss members are both above and below the roadbed. They are long columns of wood or steel driven into the ground to support vertical loads. New South Wales. TheHampden Bridge in Wagga Wagga. Allan used Australian ironbark for its strength. New York trussGeneral Hertzog Bridge over theOrange River at Aliwal Northcarries vehicular traffic. In his design. Australia.[2] A similar bridge also designed by Percy Allen is the Victoria Bridge . There are many types of designs. is partly based on the Howe truss. the first of the Allan truss bridges. It was constructed with timber to reduce cost. Below are some of the more common types and designs. many dating back hundreds of years. was originally designed as a steel bridge. reinforced concrete at Kansai International Airport. Truss types used in bridges Bridges are many times the best visible examples of truss use to the common person. designed by Percy Allan. It carries three lanes of automobile traffic on top and two of rail below over nine truss spans. is the longest doubledecked truss bridge in the world. Allan truss Allan Truss illustrated Hampden Bridge showing the Allan truss design The Allan Truss.Japan.bridge over the Erie Canal in Lockport. Osaka. New South Wales. A Baltimore truss has additional bracing in the lower section of the truss to prevent buckling in the compression members and to control deflection. Also constructed of ironbark. It is mainly used for train bridges. Designed for military use. note the use of doubled prefabrications to adapt to the span and load requirements. Bollman truss . the prefabricated and standardized truss elements may be easily combined in various configurations to adapt to the needs at the site.on Prince Street Picton.[citation needed] Bailey bridge Bailey bridge over the Meurthe River. Baltimore truss The Baltimore truss is a subclass of the Pratt truss. In the image at right. the bridge is still in use today for pedestrian and light traffic. France. boasting a simple and very strong design. In other applications the trusses may be stacked vertically. Maryland is the only surviving example of a revolutionary design in the history of American bridge engineering. but it is a Warren truss configuration.42 N 76°49 30. The Wells Creek Bollman Bridge is the only other bridge designed by Wendel Bollman still in existence. The design employswrought iron tension members and cast iron compression members. It is still in use today as a pedestrian bridge. moved to Savage in 1887. Built in 1869. It was the first successful all-metal bridge design (patented in 1852) to be adopted and consistently used on a railroad. The structure was also easy to assemble. Wendel Bollman. The use of multiple independent tension elements reduces the likelihood of catastrophic failure. Bowstring arch truss (Tied arch bridge) . Maryland.33 W The Bollman Truss Railroad Bridge at Savage.Bollman truss in Savage. 39°8 5. The type was named for its inventor. a self-educated Baltimore engineer. useful for bridge sections resting upon high pylons.[3] Thrust arches transform their vertical loads into a thrust along the arc of the arch. Note the stone pier in the background from the Wabash Bridge. Pennsylvania. while in a bowstring arch the thrust is taken horizontally by a chord member to the opposite side of the arch. in Pittsburgh. This type of truss is particularly suited for timber structures that use iron rods as tension members. and a horizontal thrust.A tied arch bridge. In a typical arch this horizontal thrust is taken into the ground. Brown truss Brown truss illustrated. The bowstring arch through truss bridge was patented in 1840 by Squire Whipple. Burr Arch Truss . a vertical thrust equal to a proportion of the weight and load of the bridge section. At the ends of the arch this thrust (at a downward angle away from the center of the bridge) may be resolved into two components. This allows the footings to take only vertical forces. All interior vertical elements are under tension. A covered bridge with a Burr Arch Truss structure This combines an arch with a truss to form a structure both strong and rigid. at least over a portion of the span. The typical cantilever truss bridge is a balanced cantilever. The Appomattox High Bridge on the Norfolk and Western Railroad included 21 Fink deck truss spans from 1869 until their replacement in 1886. Howe truss . which enables the construction to proceed outward from a central vertical spar in each direction. In a cantilever truss the situation is reversed. Cantilevered truss Forth rail bridge Most trusses have the lower chord under tension and the upper chord under compression. Fink truss Fink Truss (half span and cross section) The Fink truss was designed by Albert Fink of Germany in the 1860s. This type of bridge was popular with the Baltimore and Ohio Railroad. Usually these are built in pairs until the outer sections may be anchored to footings. if present. A central gap. can then be filled by lifting a conventional truss into place or by building it in place using a traveling support. the diagonal web members are in compression and the vertical web members are in tension. Kingpost truss One of the simplest truss styles to implement. includes vertical members and diagonals that slope up towards the center. the opposite of the Pratt truss.[4] In contrast to the Pratt Truss. Missouri.the diagonals are under compression under balanced loading King Post Truss A large timber Howe truss in a commercial building K truss A truss in the form of a K due to the orientation of the vertical member and two oblique members in each panel. Howe truss illustrated . the king post consists of two angled supports leaning into a common vertical support. patented in 1840 by Massachusetts millwright William Howe. andSandy Creek Covered Bridge in Jefferson County. Examples include Jay Bridge in Jay. Lattice truss (Town's lattice truss) . New York.Jay Bridge showing the truss design The relatively rare Howe truss. Where the arches extend above and below the roadbed.Plank lattice truss of a covered bridge This type of bridge uses a substantial number of lightweight elements. it is a lenticular pony truss bridge. 1859 A lenticular truss bridge includes a lens-shape truss. Truss elements are usually of wood. or steel. This in turn enables the truss to be fabricated on the ground and then to be raised by jacking as supporting masonry pylons are constructed. An example of a lenticular pony truss bridge that uses regular spans of iron is the Turn-of-River Bridge designed and manufactured by the Berlin Iron Bridge Co. Pennsylvania is another example of this type. iron. As the horizontal tension and compression forces are balanced these horizontal forces are not transferred to the supporting pylons (as is the case with most arch types). This truss has been used in the construction of a stadium[5]. easing the task of construction. with trusses between an upper arch that curves up and then down to end points. Long truss . with the upper chords of parallel trusses supporting a roof that may be rolled back. The Royal Albert Bridge (United Kingdom) uses a single tubular upper chord. One type of Lenticular truss consists of arcuate upper compression chords and lower eyebar chain tension links. TheSmithfield Street Bridge in Pittsburgh.. and a lower arch that curves down and then up to meet at the same end points. Lenticular truss Royal Albert Bridge under construction. Pegram truss Pegram Truss The Pegram truss is a hybrid between the Warren and Parker trusses where the upper chords are all of equal length and the lower chords are longer than the corresponding upper chord. patented this truss design in 1885. Long in 1830. There are ten remaining Pegram span bridges in the United States with seven in Idaho.HAER diagram of a Long Truss Designed by Stephen H. Canada.[7] The Pegram truss consists of a Parker type design with the vertical posts leaning towards the center at an angle between 60 and 75°.[6] Parker (Camelback) truss A Parker truss bridge is a Pratt truss design with a polygonal upper chord. A "camelback" is a subset of the Parker type. The design resembles aHowe truss. George H. The members which would be vertical in a Parker truss vary from near vertical in the center of the span to diagonal near each end (like a Warren truss). An example of a Parker truss is the Traffic Bridge in Saskatoon. Delaware. Pegram. Pennsylvania (Petit) truss . but is entirely made of wood instead of a combination of wood and metal. This design also facilitated reassembly and permitted a bridge to be adjusted to fit different span lengths. The variable post angle and constant chord length allowed steel in existing bridges to be recycled into a new span using the Pegram truss design. where the upper chord consists of exactly five segments. each panel was not square. Because of the difference in upper and lower chord length. while the chief engineer of Edge Moor Iron Company in Wilmington. one surviving example is the Old Blenheim Bridge. They are statically determinate bridges. Pratt truss illustrated . Pratt truss A Pratt truss includes vertical members and diagonals that slope down towards the center. Post. Queenpost truss . This truss is practical for use with spans up to 250 feet and was a common configuration for railroad bridges as truss bridges moved from wood to metal.the interior diagonals are under tension under balanced loading and vertical elements under compression. The Pratt Truss was invented in 1844 by Thomas and Caleb Pratt. the opposite of the Howe truss. Massachusetts.[9] The Ponakin Bridge and the Bell Ford Bridge are two examples of this truss. creating Y. which lends themselves well to long spans. Post truss A Post truss A Post truss is a hybrid between a Warren truss and a double-intersection Pratt truss. it is occasionally referred to as a Post patent truss although he never received a patent for it. Invented in 1863 by Simeon S.[8] An example of this truss type is the Schell Bridge in Northfield.and K-shaped patterns.[4] It can be subdivided.The Pennsylvania (Petit) truss is a variation on the Pratt truss. If pure tension elements are used in the diagonals (such as eyebars) then crossing elements may be needed near the center to accept concentrated live loads as they traverse the span. Waddell truss . The primary difference is the horizontal extension at the center which relies on beam action to provide mechanical stability. The latter form is common when the bridge is constructed as cantilever segments from each side as in the Navajo Bridge. is similar to a king post truss in that the outer supports are angled towards the center of the structure. or of two arcuate sections pinned at the apex.Queen Post Truss The queenpost truss. This truss style is only suitable for relatively short spans.[10] Truss arch A truss arch may contain all horizontal forces within the arch itself. or alternatively may be either a thrust arch consisting of a truss. sometimes queen post or queenspost. beam. but only to tension or compression. It was intended to be used as a railroad bridge. Loads on the diagonals alternate between compression and tension (approaching the center). This configuration combines strength with economy of materials and can therefore be relatively light. forming alternately inverted equilateral triangle-shaped spaces along its length.Waddell "A" truss (1898 bridge) Patented 1894 (U. usually thin. Whipple Pratt truss Bridge L-158 A whipple truss is usually considered a subclass of the Pratt truss because the diagonal members are designed to work in tension.S. It is an improvement over the Neville truss which uses a spacing configuration of isosceles triangles. An example of a Pratt Truss bridge is the Fair Oaks Bridge in Fair Oaks. at a shallow angle and cross two or more bays (rectangular sections defined by the vertical members). Patent 529. while elements near the center must support both tension and compression in response to live loads. The main characteristic of a whipple truss is that the tension members are elongated. with no vertical elements. Warren (non-polar) truss Warren truss illustrated some of the diagonals are under compression and some under tension The Warren truss was patented in 1848 by its designers James Warren and Willoughby Theobald Monzani. California Vierendeel truss . ensuring that no individual strut. and consists of longitudinal members joined only by angled cross-members.220) its simplicity eases erection at the site. or tie is subject to bending or torsional straining forces. which also depends on the truss geometry. we have the following necessary condition forstability: where m is the total number of truss members. unlike common pin-jointed trusses. it is not sufficient for stability. Analysis of these conditions at each node yields the magnitude of the forces in each member of the truss. These may be compression or tension forces. is described as being statically determinate. Given a certain number of joints. it must be entirely composed of triangles. which affects both interior and exterior styling aspects Statics of trusses A truss that is assumed to comprise members that are connected by means of pin joints. this is the minimum number of members. the truss is said to be statically determinate. Trusses that are supported at more than two positions are said to be statically indeterminate. Newton's Laws apply to the structure as a whole. In order for a truss with pin-connected members to be stable. then the truss as a whole fails. In order for any node that may be subject to an external load or force to remain static in space. In mathematical terms. This is advantageous both in allowing flexibility in the use of the building space and freedom in selection of the building's outer curtain wall. imposes significant bending forces upon its members but this in turn allows the elimination of many diagonal elements. and which is supported at both ends by means of hinged joints or rollers. When m = 2j 3. While the relation (a) is necessary. and the application of Newton's Laws alone is not sufficient to determine the member forces.A Vierendeel bridge The Vierendeel truss. because the (m+3) internal member forces and support reactions can then be completely determined by 2jequilibrium equations. as well as all moments acting about the node equal zero. While rare as a bridge type this truss is commonly employed in modern building construction as it allows the resolution of gross shear forces against the frame elements while retaining rectangular openings between columns. as well as to each node or joint. once we know the external loads and the geometry of the truss. . j is the total number of joints and r is the number of reactions (equal to 3 generally) in a 2-dimensional structure. support conditions and the load carrying capacity of the members. the following conditions must hold: the sums of all (horizontal and vertical) forces. in the sense that if any member is taken out (or fails). Those structures may survive even when some of the members fail. in addition to the equilibrium condition described. Analysis of trusses . Their member forces depend on the relative stiffness of the members.Some structures are built with more than this minimum number of truss members. theflexibility method or the finite element method. bending moment. This also makes trusses physically stronger than other ways of arranging material because nearly every material can hold a much larger load in tension and compression than in shear. Provided the members are long and slender. The analysis of trusses often assumes that loads are applied to joints only and not at intermediate points along the members. and other more complex stresses are all practically zero. the moments transmitted through the joints are negligible and they can be treated as "hinges" or 'pin-joints'. a truss is usually modelled as a two-dimensional plane frame. The weight of the members is often insignificant compared to the applied loads and so is often omitted. If required. If there are significant out-of-plane forces. half of the weight of each member may be applied to its two end joints. Every member of the truss is then in pure compression or pure tension shear. the structure must be modelled as a three-dimensional space.Cremona diagram for a plane truss Because the forces in each of its two main girders are essentially planar. bending. Structural analysis of trusses of any type can readily be carried out using a matrix method such as the direct stiffness method. torsion. or other kinds of force. Forces in members . This makes trusses easier to analyze. the vertical members are in tension. In the truss shown above right. The diagonal and vertical members form the truss web. vertical and half the total load. External loads are concentrated in the outer joints. In addition to carrying the static forces. and the top chord in compression. they are also in tension and compression. A building under construction in Shanghai.On the right is a simple. Design of members A truss can be thought of as a beam where the web consists of a series of separate members instead of a continuous plate. . the lower horizontal member (thebottom chord) and the upper horizontal member (the top chord) carry tension and compression. it is clear to see that the reactions at A and B are equal. the bottom chord is in tension. statically determinate flat truss with 9 joints and (2 x 9) 3 = 15 members. fulfilling the same function as the flanges of an I-beam. The internal forces in the members of the truss can be calculated in a variety of ways including the graphical methods: Cremona diagram Culmann diagram the analytical Ritter method (method of sections). The truss sections stabilize the building and will house mechanical floors. In the picture to the right. preventing buckling. the members serve additional functions of stabilizing each other. In the truss pictured above right. and the diagonals are in compression. and carry the shear force. the top chord is prevented from buckling by the presence of bracing and by the stiffness of the web members. Individually. In the truss. the exact arrangement of forces is depending on the type of truss and again on the direction of bending. Which chord carries tension and which carries compression depends on the overall direction of bending. Since this is a symmetrical truss with symmetrical vertical loads. off-site fabrication. e. . see also shear stress. The members under compression also have to be designed to be safe against buckling. For members under tension the cross-sectional area A can be found using A = F × / y. On the other hand. while tension is the directly opposing force that lengthens and stretches the object. In other cases the appearance of the structure may take on greater importance and so influence the design decisions beyond mere matters of economics. Giving one member a larger cross section than on a previous iteration requires giving other members a larger cross section as well. on-site erection. Sometimes the designer goes through several iterations of the design process to converge on the "right" cross section for each member. Modern materials such as prestressed concrete and fabrication methods. involving shear of the bolt connections used in the joints. to hold the greater weight of the first member one needs to go through another iteration to find exactly how much greater the other members need to be. reducing the size of one member from the previous iteration merely makes the other members have a larger (and more expensive) safety factor than is technically necessary. where F is the force in the member. the next step is to determine the cross section of the individual truss members. component transportation.g. have significantly influenced the design of modern bridges. the availability of machinery and the cost of labor. being a balance between the costs of raw materials.5 but depending on building codes) and y is the yield tensile strength of the steel used. A spring is a good example of a simple mechanism that works with both forces. the last step in the design of a truss would be detailing of the bolted joints. The weight of a truss member depends directly on its cross section that weight partially determines how strong the other members of the truss need to be. Forces Acting on Truss Bridges There are two major forces that act on bridges: compression and tension. The effect of the weight of the individual truss members in a large truss.The inclusion of the elements shown is largely an engineering decision based upon economics.. but doesn't require another iteration to find a buildable truss. Design of joints After determining the minimum cross section of the members. The compression force bears down on an object to shorten or compress it. such as automated welding. such as a bridge. is usually insignificant compared to the force of the external loads. is a safety factor (typically 1. Once the force on each member is known. The compression on the upper side of the decking causes the underside to go into tension and stretch. the soldiers are ordered to "break step" so that the cadence is broken and wave forms cannot set up. stretches and finally breaks. Compression & Tension Forces Compression and tension are seen in a bridge in the road deck. The truss bridge designs ensure that these forces are dissipated or transferred through the members and away from the structure. weakens. Other Forces Acting On Truss Bridges Other forces. pushing down with weight and gravity. Seismic waves caused by earthquakes move through the ground and perturb the surface in all three dimensions. which shortens on top as traffic weight loads bear down directly on it. Fatigue refers to the repeated bending up and down of horizontal members causing them to fail. Torsion is a rotational or twisting force produced during storms with high hurricane or near hurricane strength winds. Waves move back and forth. When an army marches across a bridge. Resonance refers to standing waves that can run back and forth through a bridge causing it to fail through fatigue. such as torsion. If the compression force overcomes the bridge's ability to handle the stress. thus shortening the spring and tension pulls the coils further apart. The simple truss construction disperses forces through the members so that no single member can be overstressed in this manner. Truss bridges are just as vulnerable as any other structure to this kind of failure. At these points. Truss construction allows the winds to blow through both the substructure and the superstructure of the road deck so that torsion is minimized. A simple example is a wire bent up and down repeatedly so that one area heats up due to friction. most of the downward load or weight of compression applied to the structure is dissipated or transferred into the earth. and if the tension forces overwhelm the structure. resonance and seismic forces from earthquakes cause stresses on bridges in different ways. up and down and from side to side and it's very difficult for any structure to resist this kind of punishment. Dissipation means to spread the force out or transfer it to a larger area so that no one point bears the entire concentrated focus of the force. The diagonals and horizontal members of a truss transfer the forces to the pilings. its members may snap. Construction Strength Of Truss Bridges Truss bridges are designed so that compression and tension don't cause the structure to fail by snapping or buckling. lengthening the spring. Railroads have replaced almost all truss bridges in areas prone to seismic disturbance .Compression pushes the coils together. it will buckle. from Pghbridges. Over 17. . Iron and Early Steel Bridges of Ohio . Historic Bridges of Michigan and Elsewhere . "Trusses: A Study by the Historical American Engineering Record" (pdf).com . and invites visitor assistance in identifying extant or demolished bridges. Many of these are truss bridges.A Spotter's Guide to Bridge Design . National Park Service.de The Structurae database on bridges. Matsuo Bridge Company: Bridge Types . Historic Bridges of the U. Historic Bridges of Iowa . Includes maps.Illustrates many of the various types of truss arrangements used in bridges.A comprehensive inventory of all remaining truss bridges in Ohio.Reference y y y y y y Historical American Engineering Record (1976). photos. y y . USA. Retrieved 2008-07-20.Truss structurae.Many photos of truss bridges are available on this informative and mainly truss-focused bridge website.400 truss bridges are listed here.An illustrated list of different architectural bridge types found in Iowa.An enormous database of historic bridges.S. Bridge Basics .