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This paper presents the effect of soil-structure interaction (SSI) on seismic inelastic displacement ratios of SDOF systems. Existing methods used in the past assumes the soil is rigid. A simplified equivalent fixed-base method is proposed herein to achieve more accurate estimations for the inelastic displacement demand of a structure.
A well-defined degrading model was used to conduct the dynamic analyses. A total of 300 earthquake motions recorded on firm sites, including recent ones from Japan and New Zealand, with magnitudes greater than 5 and peak ground acceleration (PGA) values greater than 0.08g, were selected and scaled to the same hazard level. These earthquake records were applied on five reinforced concrete (RC) columns that were chosen among 255 tested columns based on their beam-column element parameters reported by the Pacific Earthquake Engineering Research Centre. A total of 384,000 dynamic analyses were conducted to derive the required inelastic ratios. Different strength reduction factors and foundation aspect ratios h/r values were assumed for a range of NEHRP soil types C and D properties in the study. The results show that inelastic displacements are relatively greater for slender columns, particularly for high foundation aspect ratios. 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 new rigorous approach based on fuzzy logic techniques is derived to properly account for the large uncertainty present in the system. The performance evaluation of this approach is evaluated using a series of independent data sets. Accurate results were predicted using the new fuzzy logic model.