Due to the inherent complexity, the common approach in analysing nonlinear response of structures with soil-structure interaction (SSI) in current seismic provisions is based on equivalent Single Degree-of-Freedom systems (E-SDOF). This paper aims to study the influence of higher modes on the seismic response of SSI systems by performing intensive parametric analyses on more than 6400 linear and non-linear Multi Degree-of-Freedom (MDOF) and E-SDOF systems subjected to 21 earthquake records. An established soil-shallow foundation-structure model with equivalent linear soil behaviour and nonlinear superstructure has been utilized using the concept of cone models. The lateral strength and ductility demands of MDOF soil-structure systems with different number of stories, structure-to-soil stiffness ratio, aspect ratio and level of inelasticity are compared to those of E-SDOF systems. The results indicate that using the common E-SDOF soil-structure systems for estimating the strength and ductility demands of medium and slender MDOF structures can lead to very un-conservative results when SSI effect is significant. This implies the significance of higher mode effects for soil-structure systems in comparison with fixed-based structures, which is more pronounced for the cases of elastic and low level of inelasticity.
