Torsional Behaviour in Beam and Slab Decks

The paper deals with the torsional behaviour of bridge composite girders, built with plated I beam and top concrete slab. This typology has different variants in the way transverse bracing (diaphragms) are designed, either strut-and-tie type or flexural, and in the optional presence of a bottom horizontal torsional bracing. The torsion stiffness for these girders is made of different contribution: a) the “equivalent Bredt” torsion stiffness of the girder cross-section; b) the concrete slab own torsion stiffness; c) the differential bending in both beams and slab (warping). Depending on the cross section bracing arrangement and overall bridge geometry, the importance and the necessity for the three different contributions varies significantly. Curved bridges, also discussed in the paper, poses further questions strictly connected to torsion. The paper clarifies and compares the different solutions with the support of F.E. analyses and with the direct experience of designing and building few recent examples along Italian motorways and railway lines. The F.E. study shows how to account for the different torsional contributions when simplified grillage types of meshes are used.