„ New“concepts for prestressed concrete bridges - Segmental box girder bridges with external prestressing Prof. Dr.-Ing. G.Rombach, Technical University , Hamburg-Harburg, Germany Konferenz „ Aktuelle Probleme im Bauwesen“ St. Petersburg, Juli 1997 , „ New“concepts for prestressed concrete bridges - Segmental box girder bridges with external prestressing Prof. Dr.-Ing. G. Rombach, Technical University , Hamburg-Harburg, Germany Konferenz „ Aktuelle Probleme im Bauwesen“ St. Petersburg, Juli 1997 , „New“ concepts for prestressed concrete bridges - Segmental box girder bridges with external prestressing Prof. Dr.-Ing. G. Rombach Technical University, Hamburg-Harburg, Germany Table 2: Advantages and disadvantages of segmental bridge construction Disadvantages • Safety (e.g. in case of fire) • Extra cost (more prestressing required, single spans, truss) • High construction loading (overslunk truss) • New construction method - technology (e.g. geometry control of segments, design) Advantages • prefabrication high quality of concrete • quality control • short construction time segments are prefabricated while the substructure is being built rapid erection of superstructure no in-situ concrete required • construction not sensitive to weather conditions (dry joints) • recycling • economy (less mild reinforcement required) • no falsework required erection from top • less shrinkage effects as segments are stored before erection • enhanced durability as the whole cross section must be under compression under serviceability loads Summary Bridges have been built during thousands of years already. Nevertheless due to demands of the market new, innovative construction methods are still required. Precast segmental box girder bridges externally post tensioned is one of the „ new“ technology. The great advantages of these type of bridges had made them the preferred construction method for many great elevated highway projects, especially in south east asia. In the following some design aspects of this bridge type are discussed. 1 Introduction Segmental bridge construction is not a new technology. The first segmental bridge was built in 1941 (bridges over the river Marne). But intensive research work and more than 3 decades were needed for the refinement of the construction process and to develop adequate design models. As a result segmental bridge construction has gained great acceptance and the number of this bridge type increased rapidly in the last years. What are the main reasons for this development? What are the main demands of the market? The answer to this question is summarized in table 1. Table 1: Requirements of the market on bridges Requirements • cost saving (construction and maintenance) • time saving (short construction time) • high, controlled quality • Construction without interference with ground level (e.g. existing traffic in urban areas) • handling of prefabricated elements • flexibility of the structure • Inspection and exchange of tendons possible Solution (concrete bridges) prefabrication (U-, I-, V- beams, segments) Table 3: Advantages and disadvantages of external prestressing Disadvantages • additional mild reinforcement required (∆σp) • additional Cost for ducts, anchorage, etc. • only straight tendon layout • Diffusion of post-tensioning forces Advantages • replacement of tendons possible • inspection of tendons possible • easier Installation of longitudinal tendons • good corrosion protection of p.t. cables • less dead load (thin webs) • pouring is facilitated (no p.t. ducts) • less friction (no wobble losses) • prestress forces can be modified after construction (spare ducts) • greater permissible p.t. stresses South east asia is one of the main regions, where many segmental bridges are built. The longest segmental bridge which is part of the Second Stage Expressway System in Bangkok (total length: more than 35km) is located in Thailand (Rombach, 1996). The figures shown is this paper are taken from this mega project. prefabricated segments external posttensioning Time (and money) has become the most important factor in all major bridge projects. Precast segmental box girder bridges externally post-tensioned seem to be the best answer to these requirements. Nevertheless problems with segmental construction should not be ignored. Table 2 and 3 give a brief summary of the advantages and disadvantages of segmental bridges and external prestressing. page 1 page 2 „ New“concepts for prestressed concrete bridges - Segmental box girder bridges with external prestressing Prof. Dr.-Ing. G. Rombach, Technical University , Hamburg-Harburg, Germany Konferenz „ Aktuelle Probleme im Bauwesen“ St. Petersburg, Juli 1997 , „ New“concepts for prestressed concrete bridges - Segmental box girder bridges with external prestressing Prof. Dr.-Ing. G. Rombach, Technical University , Hamburg-Harburg, Germany Konferenz „ Aktuelle Probleme im Bauwesen“ St. Petersburg, Juli 1997 , 2 Structural elements of segmental bridges Segmental bridges are mainly built as single span structures to avoid coupling of post tensioning cables. Furthermore in single spans the greatest shear force is not located at the same region as the greatest bending moment. A typical span is shown in figure 1. Dry joints are used for this project (no epoxy glue). No continuous reinforcement is provided across the match cast joints between the segments. Due to the external post tensioning (figure 2) 3 different segments are needed (figure 3): • Pier segment: • Deviator segment: • Standard segment: heavy end diaphgram required to stiffen the box section and for anchorage of p.t. cables required to deviate tendons thin webs (35cm) Figure 1 Typical span Figure 2 Tendon layout Figure 3: Segments page 3 page 4 „ New“concepts for prestressed concrete bridges - Segmental box girder bridges with external prestressing Prof. Dr.-Ing. G. Rombach, Technical University , Hamburg-Harburg, Germany Konferenz „ Aktuelle Probleme im Bauwesen“ St. Petersburg, Juli 1997 , „ New“concepts for prestressed concrete bridges - Segmental box girder bridges with external prestressing Prof. Dr.-Ing. G. Rombach, Technical University , Hamburg-Harburg, Germany Konferenz „ Aktuelle Probleme im Bauwesen“ St. Petersburg, Juli 1997 , 3 Design As all structures segmental bridges have to designed for serviceability and ultimate limit state. Under service condition the concrete compression in the dry joints has to be greater than 1,4MPa. Therefore the whole structure is under compression during normal loading. As there is no tension within the concrete, forces and moments can be calculated based on a linear elastic behaviour of the structure. In addition to monolithic bridges, all joints have to be designed for shear loads. The shear force is carried by shear keys (figure 3) and by friction between the joint surfaces. Under ultimate loads the joints between the segments will open. The resulting decrease of the structural stiffness has to be considered in the design. This can be done by analytical (moment-curvature relationship) or numerical methods (finite element methods). References AASHTO: Guide Specifications for Design and Construction of Segmental Concrete Bridges - Interim Specifications, Washington DC, 1994 Precast segmental box girder bridge manual. Post-Tensioning Institute, Chicago, 1978 P.T.I.: G. Rombach: Bangkok Expressway - Segmentbrückenbau contra Verkehrschaos. In: „ Aus dem Massivbau und seinem Umfeld „ (Hilsdorf H.; Kobler G. ed.), Karlsruhe 1996, page 645-656 Figure 4: Finite element model of standard span Critical areas • midth of span greatest bending moment • first joint after support greatest shear force but prestress force not uniformly distributed in cross sections • diaphragms high concentrated loads due to anchorage of tendons p.t. • deviators page 5 page 6