Soil–structure interaction effects on base-isolated buildings founded on soil stratum

The present study investigates the effects of soil–structure interaction (SSI) on the response of base-isolated multistory buildings founded on an elastic soil layer overlying rigid bedrock and subjected to a harmonic ground motion. Initially a four-degree-of-freedom system (4-DOF) is developed and the equations of motion are formulated in the frequency domain. Frequency independent expressions are used to determine the stiffness and damping coefficients for the rigid surface foundation on the soil-stratum underlined by bedrock at shallow depth. Assuming the foundation mass to be negligible, an equivalent two-degree-of-freedom (2-DOF) system is derived. The first mode of motion of the equivalent 2-DOF system appears to be sufficient to describe the response of the overall system for all ranges of stiffness and inertia properties of the structure and its isolation. An extensive parametric study demonstrates that SSI effects are significant, primarily for squat, light structures, founded on soil-stratum of low stiffness. The methodology could serve as a means to perform a preliminary seismic design of base-isolated building structures founded on homogenous soil-stratum over bedrock.