Approaches for seismic soil-structure interaction analysis of a railway bridge pier on pile foundations

Soil-Structure Interaction (SSI) effects are often neglected in the current design practice of bridge superstructures and foundations. This simplification generally results in overestimating seismic actions, although underestimations may also occur in some cases. This study investigates SSI effects on a bridge pier supported by pile foundations, using a high-speed railway viaduct as a case study. Two approaches are carried out: a “direct” approach and a “substructure” approach, both capable of simulating SSI. The direct approach is based on a direct-integration time-history analysis of a three-dimensional finite element model (FEM) of the global system (i.e. soil-foundation-structure), in which the nonlinear behavior of the soil is described using an elastoplastic constitutive law with hardening and hysteretic properties. The substructure approach, on the other hand, is based on a visco-elastic lumped parameters model (LPM) described in the literature. This model consists of an assemblage of masses, elastic springs and dampers capable of reproducing the dynamic impedances of the soil-foundation system. The results from these two approaches, carried out using site-specific ground motions, are finally compared with those of a simplified design method compliant with the Italian building code, which neglects the seismic interaction and is based on linear dynamic (response spectrum) analysis of the fixed-base single-degree-of-freedom system.