Influence of dynamic soil-structure interaction and bridge length on the response of continuous, multi-span railway bridges

This paper focuses on the effect of dynamic soil-structure interaction (SSI) on the free and forced vibration response of continuous, multi-span box girder bridges with periodic geometry. The influence of the bridge length (number of spans) on the response is also investigated. In order to compute the dynamic response of finite, periodic railway bridges, an efficient model based on the wave finite element method (WFEM) is presented. The computational domain is restricted to a reference cell (one bridge span) and Floquet theory is used to describe wave propagation in the periodic structure. Additionally, we take advantage of the periodicity within the reference cell to maximize computational efficiency. The dynamic stiffness and damping of a piled foundation embedded in layered soil is computed a priori using a finite element-boundary element (FE-BE) model. The foundation stiffness and damping are subsequently incorporated into the bridge model using frequency-dependent springs and dashpots. The response of an infinitely long bridge is characterized by dispersion curves of torsional and bending waves. The natural frequencies of bridges of finite length are situated within the pass bands of these waves. Dynamic SSI mainly reduces the natural frequencies of torsional and bending modes with in-phase motion in neighboring spans. When dynamic SSI is taken into account, the mid-span receptance is not much affected by the bridge length. Hence, infinitely long bridge models can be used to predict the response of bridges of finite length, reducing computational complexity. The peak acceleration during train passages is reduced due to dynamic SSI; a slightly higher response is observed for shorter bridges.