Effect of soil-structure interaction on the dynamics of portal frame railway bridges

Critical resonance phenomena on railway bridges caused by high-speed trains lead to large deformations, accelerations and stresses on the structure, which are significantly influenced by the natural frequencies of the structure and only limited by damping. Particularly portal frame structures with short spans are expected to have a strong soil-structure interaction because they are integrated into the subsoil, which leads to a high geometric damping due to wave propagation in the ground. In the present study the influence of soil-structure interaction on the dynamics of frame-type railway bridges is assessed by numerical simulations, based on natural frequencies, natural vibration modes and equivalent damping coefficients. The basis are simple plane finite element (FE) models, which consist of the frame structure and the surrounding soil. From the free vibration response induced by a pulse load in the center of the beam or column, the fundamental frequency in vertical or horizontal direction is determined. The decay of this response is used to identify effective damping. To obtain a more comprehensive insight into the dynamics of these structures, the geometry of the frame model and the soil parameters are varied in a parameter study.