Influence of kinematic SSI on foundation input motions for pile-supported bridges
The seismic analysis of bridge structures is often performed using the substructure method in which the foundation is replaced by an equivalent "spring" representing foundation impedance. Ground motions from seismic hazard analyses correspond to a free-field condition, and therefore should be modified to account for kinematic soil-structure interaction effects before being used as input to the springs. This paper presents closed-form analytical solutions for the response of an elastic pile subjected to harmonic seismic excitation in uniform elastic soil. We use these solutions to compute transfer functions relating foundation input motion to free-field ground motion and use the results to verify predictions from a beam-on-Winkler-foundation numerical model. The two approaches show good agreement, indicating that the numerical modeling method is appropriate for investigating more complex effects such as soil and pile nonlinearity. Ground motion deamplification due to kinematic SSI is demonstrated to be significant for stiff foundations in soft ground conditions. Numerical simulations using recorded ground motions demonstrates that transfer functions can be computed only from frequency bands for which the motions contain adequate energy.