Influence of stiffness degradation on strength demands of structures built on soft soil sites

The effect of stiffness degradation on the lateral strength demands of inelastic single-degree-of-freedom systems subjected to soft soil records is investigated. The modified-Clough model is used to represent structures that exhibit significant stiffness degradation when subjected to reverse cyclic loading and the elastic-perfectly-plastic model is used to represent non-degrading structures. The study is based on two sets of ground motions. The first set includes 100 accelerograms recorded in various recent earthquakes on soft soil sites of Mexico City. The second set includes 16 ground motions recorded on bay mud sites in the San Francisco Bay Area during the 1989 Loma Prieta earthquake. A special emphasis is placed on the effect of stiffness degradation on strength reduction factors that permit the estimation of inelastic strength demands from elastic strength demands. Mean ratios of lateral strength demands of stiffness degrading systems to lateral strength demands of non-degrading systems for both sets of ground motions are presented. It is concluded that structures with stiffness degradation, and with periods of vibration shorter than the predominant period of vibration of the ground motion, can experience lateral strength demands larger than those of non-degrading structures in the same period range. Whereas for structures with periods of vibration equal or longer than the predominant period of the ground motion, the lateral strength demands are typically smaller for stiffness-degrading structures than those of non-degrading structures. (C) 2002 Published by Elsevier Science Ltd.