Approaches for seismic soil-structure interaction analysis of a piled railway bridge abutment with fixed bearings

: Seismic response analyses of bridge abutments rigidly connected to the deck should account for the interaction between the soil, the abutment and the inertial forces acting on the structural masses participating in the seismic motion. When a specific soil-structure interaction (SSI) analysis is not performed, the Italian Building Code permits a simplified analysis in which pseudo-static forces are applied to the abutment on the basis of a design response spectrum. This approach, which is widely used in current practice, often leads to an overestimation of seismic actions and a very conservative design. In this paper, for a High-Speed Railway bridge, results from the simplified approach are compared with those obtained from two- and three- dimensional time-history Finite Element (FE) analyses employing a complete numerical FE model that includes the superstructure, its foundation and the surrounding soil. Such analyses, that belong to the category of “direct methods”, explicitly account for all key aspects of SSI: kinematic and inertial effects, input motion propagation and local site response. Soil nonlinear behavior is coupled and described by elastoplastic stress-dependent constitutive law and the seismic input motion is applied to the model boundaries as an appropriate time-history force excitation. Advantages and disadvantages of each approach are finally discussed, with a focus on the abutment pile foundations.