The design of prestressed composite steel bridges

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Virginia Polytechnic Institute


In recent years there has been a constant search for better and more economical structures in the field of structural engineering. Within the past ten to twenty years this search has resulted in the introduction of two new structural systems: prestressed concrete and composite design. Each of these new construction methods has advantages and limitations. A new idea of combining these two structural systems into one could result in more economical structures particularly suited for long span bridges.

The slab-and-stringer bridge is one of the most common types in highway construction. Such a bridge is composed of two principal load-carrying elements: the steel beams which transfer the loads in the direction along the bridge axis, and the concrete slab which distributes the loads in the transverse direction to the steel beams. If some appropriate mechanical device is used to connect the steel beams end concrete slab together, the concrete slab can act as a cover plate for the beams and assist the beams in carrying the load in the longitudinal direction. Such a structure is known as a Conventional Composite Structure.

If such a structure is prestressed with high-strength steel cables, it acquires additional qualities. The principle of prestressing in steel structures is not used to overcome tensile deficiencies of the material, as ls the case for concrete, but to build opposing stresses into members in order to counteract the stresses caused by external forces. When favorable residual stresses have been induced in such structures they will be capable of carrying greater loads than their conventional counterpart.

It is the objective of this thesis: (1) to investigate the physical properties of the materials used in prestressed composite structures, (2) to discuss the methods for construction, (3) to develop a usable design technique for simply supported and continuous beams, (4) to discuss the layout of prestressing cables in continuous prestressed composite beams, (5) to show the use of equations for selecting the steel beam cf the prestressed composite structure, and (6) to illustrate the design of prestressed composite structures with typical problems.