Effects of the diaphragm at midspan on static and dynamic behaviour of composite railway bridge: A case study

This study focuses on composite multi-girder railway bridges of moderate span for high-speed trains. In this type of bridge the concrete deck is the primary element of transverse rigidity, so the real impact of cross beams is unknown. These cross beams increase the cost of this type of bridge, because they require handling and welding on site. This research studies the impact of a diaphragm in a typical multi-girder railway bridge used in France. The first objective is to develop appropriate modelling techniques that can be used to evaluate the function and effectiveness of a diaphragm in the transverse distribution of traffic loads. The second objective is to know if a diaphragm is really necessary. To answer this question, a three-dimensional model and nonlinear material constitutive equations are considered. Different static loading conditions (self-weight, UIC load, and TGV) are studied and the bridge deck responses to them with and without transverse diaphragm are analyzed. The TGV is modelled with moving loads in order to take into account the dynamic behaviour of the bridge deck. Several critical speeds of train corresponding to natural frequencies are then studied in dynamic analysis. Our static and dynamic analyses concluded that the diaphragm does not seem to be necessary, except for an accidental lateral load. Therefore, it may be possible to not install diaphragms on these bridges.