Effect of the end cross beams on the railway induced vibrations of short girder bridges

This work is devoted to the analysis of the railway-induced vertical vibrations of simplysupported double track bridges composed by pre-stressed concrete girder decks. Despite the low torsional stiffness that this particular deck configuration exhibits, several structures of this type do exist in both conventional and high-speed railway lines in Spain. Even though railway administrators recommend the construction of transverse or end beams bracing the longitudinal girders at the supports in girder bridges, in several occasions these elements are not built in order to accelerate the construction process. The aim of this study is to evaluate the beneficial effect of installing these transverse beams on the vertical dynamic response of the aforementioned structures and to determine what particular bridges are most affected by the presence of these elements. To this end, a representative ensemble of girder bridges covering a range of span lengths L between 10 m and 25 m has been predimensioned and their dynamic behaviour has been predicted by a finite element model that adopts common assumptions in engineering practice. Conclusions show that installing these elements is particularly relevant in the case of short (10-12.5 m) oblique bridges with a low number of longitudinal girders for a particular deck bending stiffness, leading to an important increase of the first torsion and first transverse bending natural frequencies and to a reduction of the structural response. Finally, experimental measurements on a real bridge belonging to the Madrid-Sevilla high-speed line are included in the final section to illustrate the theoretical derivations