Effect of Mainshock-Aftershock Sequences on the Inelastic Displacement Ratios

Past studies have shown that aftershocks can increase structural damage, especially for structures damaged in the mainshock and experienced nonlinear behavior. This damage is generally the result of increased lateral displacement of the structure and thus, it is necessary to estimate the lateral displacement of the structure under seismic evaluation accurately. Although the target displacement in the nonlinear static analysis method is calculated by means of relationships presented in performance-based design codes, the effect of the aftershock is not taken into account. In this study, the impact of aftershock on inelastic displacement ratio is considered as the most effective factor in determining the target displacement. The results showed that this ratio increases significantly due to aftershocks. The dispersion of the results led to the investigation of the hazard levels of the input earthquakes and subsequently using the scaling method of the accelerograms. For this purpose, a target aftershock spectrum was generated by a statistical study on the maximum acceleration ratio of the mainshock and its corresponding aftershocks, thereby the aftershocks scale to an independent target acceleration spectrum. Comparison of the responses from scaling revealed that dispersion in results are an inherent issue and cannot be resolved. Finally, by modifying in the current relationship of the code, a new formula has been proposed for the inelastic displacement ratio such that the mean error values were reduced as much as possible.