Seismic soil-structure interaction response of inelastic structures

Abstract

We analyze the effects of soil-structure interaction (SSI) on the response of yielding single-story structures embedded in an elastic halfspace to a set of accelerograms recorded in California and to a set of records from Mexico City. We find that, for nonlinear hysteretic structures, SSI may lead to larger ductility demands and larger total displacements than if the soil were rigid. This behavior differs from that envisioned in current seismic provisions, which allow designers to ignore SSI altogether or to reduce the base shear force with respect to that of the fixed-base structure. To overcome this deficiency, we examine an approach to incorporate SSI in determining the seismic design coefficient, C˜y, of systems with non-structural degrading elastoplastic behavior and target ductility μt. In this approach, C˜y is obtained as the unreduced seismic coefficient, C, of an equivalent fixed-base structure with natural period T˜, the natural period of the elastic SSI system, and with inelastic reduction factor equal to the λ-root of the fixed-base reduction factor, where λ=T˜/T, and T is the elastic natural period of the structure ignoring SSI. The peak relative displacement of the structure can be evaluated as the yield displacement u˜y times the target ductility μt. The peak total displacement, including SSI, is closely approximated by u˜y(μt+λ2–1).