In this research, the effect of soil-structure interaction (SSI) on seismic inelastic displacement ratios of SDOF systems is investigated for the case of single and repeated earthquakes. To account for the effect of SSI, a simplified equivalent fixed-base method is proposed based on FEMA guidelines. NEHRP soil types C and D were considered for different foundation aspect ratios and different structure strength reduction factors. Different models from the literature are adopted to account for the effect of repeated earthquakes. A well-defined degrading model was used to conduct the dynamic analyses. A total of 100 earthquake records, including recent ones from Japan and New Zealand, with magnitudes greater than 5 and peak ground acceleration (PGA) values greater than 0.08 g, were selected and scaled to the same hazard level. The effect of aftershock and pre-shock was included in the analysis. These earthquake records were applied on five RC columns that were chosen from the Pacific Earthquake Engineering Research Centre database. A large number of dynamic analyses were conducted to derive the required inelastic ratios. Different strength reduction factors, foundation aspect ratios, and repeated earthquake models were assumed for a range of NEHRP soil types C and D properties in the study. The results show that inelastic displacements are greatly affected by both the soil flexibility and the effect of repeated earthquakes. The large collected data was used to derive mathematical expressions for inelastic displacement ratios, suitable for use in performance-based seismic evaluation in a design office. A simple expression was proposed to properly account for the different parameters influencing the behavior. Current performance-based seismic design guidelines of structures do not take into account the effect of soil-structure interaction or that of repeated earthquakes. This research fills this gap through a comprehensive numerical study that considers all relevant parameters.
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