Implications of non-synchronous excitation induced by nonlinear site amplification and of soil-structure interaction on the seismic response of multi-span bridges founded on piles

This work investigates the effects of soil-structure interaction and spatial variability of seismic motion due to nonlinear site amplification on the seismic behaviour of long multi-span bridges founded on piles. An analysis framework able to include the spatial variation of ground motion induced by specific geological and geomorphological scenarios in the seismic soil-structure interaction analysis of long bridges is adopted, exploiting advantages of the substructure approach. The methodology is applied to a case study constituted by a pile-supported multi-span bridge founded in a soft clay deposit overlaying a stiff bedrock with three different configurations: horizontal, inclined and wedge-shaped. The reference input motion at the outcropping bedrock is represented by a set of real accelerograms and different seismic response models are used to compute site amplification effects, discussing the contribution to the free-field ground motion of both the two-dimensional configuration of the deposit and the nonlinear soil behaviour. The ground motions obtained from the different models are then used for computing the foundation input motion accounting for the pile-soil kinematic interaction; thereafter, inertial interaction analyses are performed on structural models with either fixed or compliant base, considering the non-synchronous seismic actions at the piers foundation. The results, compared in terms of piers head displacements, ductility demand and deck transverse bending moments, finally show the relative importance of bedrock morphology, soil nonlinearity and soil-structure interaction on the structural response.