Due to the inherent complexity, the common approach in analysing nonlinear response ofudstructures with soil-structure interaction (SSI) in current seismic provisions is based onudequivalent SDOF systems (E-SDOF). This paper aims to study the influence of higher modes onudthe seismic response of SSI systems by performing intensive parametric analyses on more thanud6400 linear and non-linear MDOF and E-SDOF systems subjected to 21 earthquake records. Anudestablished soil-shallow foundation-structure model with equivalent linear soil behaviour andudnonlinear superstructure has been utilized using the concept of cone models. The lateral strengthudand ductility demands of MDOF soil-structure systems with different number of stories,udstructure-to-soil stiffness ratio, aspect ratio and level of inelasticity are compared to those of ESDOFudsystems. The results indicate that using the common E-SDOF soil-structure systems forudestimating the strength and ductility demands of medium and slender MDOF structures can leadudto very un-conservative results when SSI effect is significant. This implies the significance ofudhigher mode effects for soil-structure systems in comparison with fixed-based structures, whichudis more pronounced for the cases of elastic and low level of inelasticity.
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