Conclusions
Around 200,000 response-history analyses were carried out using fixed-base and soil-structure interaction models to study the elastic and inelastic response spectra of buildings on soft soil profiles. Based on results for 20 ground motions recorded on very soft soil deposits, it was shown that normalizing the equivalent period of an SSI system Tssi by the corresponding predominant period resulted in more rational spectra for seismic design purposes. In the elastic response spectra, Tssi is normalized by the spectral predominant period TP corresponding to the peak ordinate of a 5% damped elastic acceleration spectrum, while for nonlinear structures Tssi should be normalized by the predominant period of the ground motion Tg at which the relative velocity spectrum reaches its maximum value. It was shown that an actual SSI system could be replaced by an equivalent fixed-base oscillator having a natural period of Tssi, a viscous damping ratio of ξssi, and a ductility ratio of μssi. It was concluded that the absolute acceleration spectra, instead of the pseudo-acceleration spectra, should be used for EFSDOF oscillators in force-based design of SSI systems. The EFSDOF oscillator approach provided an excellent estimate of acceleration and inelastic spectra for lightly-to-moderately damped SSI systems. However, it was shown that the EFSDOF oscillators, in general, overestimate the constant-ductility strength reduction factor Rμ of SSI systems with high initial damping ratio (e.g. squatty structures founded on very soft soil profiles), which consequently leads to an underestimation of inelastic displacement ratio Cμ. Based on the results of this study, a correction factor was proposed to improve the efficiency of the EFSDOF oscillators to predict the Rμ and Cμ spectra of SSI systems having initial effective damping ratios greater than 10%.
