Soil-pile-superstructure interaction effects in seismically isolated bridges under combined vertical and horizontal strong ground motions

Abstract

This paper presents a deterministic analysis carried out to study the influence of coupled vertical and horizontal ground motions on the response of an isolated bridge (or soil-pile-bridge system) considering soil-pile-superstructure effects. Towards this end, an advanced 3D finite element model is developed to evaluate the effects of combined Vertical Component (VC) and Horizontal Component (HC) of ground motion on the structural elements of the bridge including Soil Structure Interaction (SSI). Non-linear time-history analysis is carried out under strong ground motions using an advanced soil plasticity model for non-liquefiable soil. Three different bridge configurations consisting of (i) simply-supported girders (SSERB), (ii) continuous girder with fixed bearing (CFB) and (iii) continuous girder with elastomeric bearing (CERB) are considered. Ground motion selection procedure was applied among 21,000 records to select suitable ground motions for nonlinear time-history analysis. Critical damage parameters in the structural elements under the combined HC and VC of the ground motion are compared with those under only the HC of the ground excitation. Results indicate that the vertical components produce significant amplification in the pier's axial load with different lower and upper bounds for different seismic isolation systems. The Influence of higher modes on results of Incremental Dynamic Analysis (IDA) are presented for the different structural elements in a deterministic framework. The variations of shear capacity and demand are presented by IDA and the combined spectral acceleration is implemented as an Intensity Measure (IM) due to the inclusion of higher order modes. The simply-supported and continuous girder with elastomeric bearings have a trend for a higher drift ratio than the continuous girder with the fixed-bearings system under vertical excitation. Furthermore, the variation of axial loads and pile-cap displacements are highlighted with and without VC for piles at piers and abutments. In addition, pile-cap displacements, influence of bridge higher modes and VC effects on Soil-Pile-Superstructure Interaction (SPSI) relations are investigated with respect to frequency content of the ground motion. The kinematic and inertial SPSI effects on seismic isolation systems including VC are also explored comparatively. The influence of elastomeric bearing on SSI reduction was investigated by comparing the relative response of the superstructure, Free-Field and Foundation Input Motion (FIM) at the pile head in the frequency domain. The new information presented in this paper will be useful for realistic design of bridges in seismic areas considering coupled HC and VC of the seismic excitation.