Influence of soil-structure interaction on the dynamic characteristics of railroad frame bridges

Abstract

Explicit dynamic calculations are required in various standards for bridges on high-speed lines to ensure normative limits. This dynamic analysis is based on the resonance phenomena between the train crossing and the frame's natural frequencies in bending. It is obvious that, especially here, an exact determination of the natural frequencies in the numerical models is important for determining the resonance speed, as there is no conservative consideration in dynamics. Furthermore, considering radiation damping based on soil-dynamic approaches can noticeably reduce the maximum amplitudes at the resonance point when simulating train crossings. However, due to the massive dimensions of structural members and a large number of constraints, the dynamic system of railroad frame bridges is usually not clearly to be identified. Hence, a prediction of natural frequency and radiation damping are currently large uncertainties for the design of frame bridges, making dynamically good-natured behaviour, characterised by large damping, particularly desirable.This paper examines the dynamic behaviour of embedded frame bridges, including the effect of soil-structure interaction, under 2D and 3D numerical approaches. Different modelling methods with respect to the unbounded domain and the satisfaction of the Sommerfeld's radiation condition are compared: (1) direct modelling using tuned damper elements, (2) coupled BEM-FEM, and (3) simplified substructure method. It is shown that the coupled FEM-BEM approach reproduces the relationships of three-dimensional dynamic soil-structure interaction (SSI) more accurately than a planar FEM approach since the SSI does not result independently of the chosen modelling method, which is reflected in divergent radiation damping. Furthermore, the governing rigid body modes can be identified using a simplified substructure method.The following parameters influencing the SSI are investigated and discussed in detail: (a) influence of the soil stiffness; mainly described by the ratio between the natural frequencies of the frame and the soil-abutment system, (b) layering of the underlying soil, (c) degree of superstructure clamping, and (d) material properties of the backfill. It is shown that the natural frequencies react robustly to the SSI and are thus mainly influenced by the structural stiffness of the frame. In contrast, radiation damping exhibits various dependencies. In particular, the superstructure slab's clamping ratio and the vertical rigid-body mode frequency (direct relation to soil stiffness) should be mentioned here, significantly influencing the vibrating system. Finally, based on these results, a recommendation for the dynamic design is given.