SEISMIC SOIL-STRUCTURE INTERACTION: BENEFICIAL OR DETRIMENTAL?

TLDR

Soil‑structure interaction (SSI) is often assumed to reduce seismic forces by lengthening periods and increasing damping, but in practice its effects can differ from code predictions. The study examines a common structural model used to evaluate SSI effects on inelastic bridge piers. The analysis shows that SSI can lengthen a structure’s period enough to increase seismic demand, and that careless application of ductility and geometry assumptions can misrepresent performance, as illustrated by numerical examples.

Abstract

The role of soil-structure interaction (SSI) in the seismic response of structures is reex-plored using recorded motions and theoretical considerations. Firstly, the way current seismic provisions treat SSI effects is briefly discussed. The idealised design spectra of the codes along with the increased fundamental period and effective damping due to SSI lead invariably to reduced forces in the structure. Reality, however, often differs from this view. It is shown that, in certain seismic and soil environments, an increase in the fundamental natural period of a moderately flexible structure due to SSI may have a detrimental effect on the imposed seismic demand. Secondly, a widely used structural model for assessing SSI effects on inelastic bridge piers is examined. Using theoretical arguments and rigorous numerical analyses it is shown that indiscriminate use of ductility concepts and geometric relations may lead to erroneous conclusions in the assessment of seismic performance. Numerical examples are presented which highlight critical issues of the problem.