Simplified analysis of the dynamic soil–structure interaction of a portal frame railway bridge

A qualitative analysis of the dynamic soil–structure interaction (SSI) of a portal frame railway bridge based on the linear theory of elasticity is presented. The influence of SSI on the dynamic properties of the structure and its response due to the high-speed load model (HSLM) of the Eurocode is analyzed by simple concepts from the finite element theory. The dynamic behavior of the foundations of the structure is introduced by means of dynamic stiffness functions, describing the stiffness and damping of the foundation–soil interface. These frequency dependent functions are used as boundary conditions on a two-dimensional Euler–Bernoulli model of the structure. The equations of motion are solved in the frequency domain and the time domain solution is obtained by the fast Fourier transform algorithm. It is shown that the radiation and material damping of the foundation–soil interface may give a substantial contribution to the modal damping ratio of the structure. A comparison of the dynamic response of the structure, subjected to the HSLM assuming different SSI models shows that fixing the vertical degree of freedom may grossly underestimate the vertical acceleration in the bridge deck.