Evaluation of seismic performance of steel frames with stiffness irregularity subjected to mainshock–aftershock using equivalent single-degree-of-freedom system

Many buildings in the present-day scenario have irregularities due to the discontinuity of their mass, stiffness, and geometry through their height. Buildings with vertical irregularities suffer severe damage as observed in past records. Structures situated in earthquake-prone regions are subjected to multiple earthquakes, occurring at a short interval of time, due to the mainshock–aftershock (MS–AS) sequence. This study investigates the effects of stiffness irregularities on the seismic performance of the 9-storey steel moment-resisting frame (SMRF) subjected to mainshocks and MS–AS. Nonlinear time-history analysis (NTHA) was conducted to compare the height-wise variation of maximum inter-storey drift ratio (IDR) demands of the regular and stiffness irregular frames. The results show that stiffness irregularity through the height of the frame influences the height-wise variation of inter-storey drift. Also, the structural responses of regular and irregular frames subjected to MS–AS are increased compared to mainshocks. This study also evaluates the accuracy of previously proposed three different equivalent single-degree-of-freedom systems (ESDOF) for obtaining seismic demands of multi-degree-of-freedom (MDOF) systems of regular and irregular frames subjected to mainshocks and MS–AS. The mean values of the height-wise variation of IDR obtained from MDOF and ESDOF systems are compared. The effectiveness of ESDOF systems for estimation of IDR is examined by mean bias and standard deviation of the ratio of mean values of IDR determined by NTHA of MDOF systems to the values determined by ESDOF systems. The study concludes ESDOF systems are useful for estimating the height-wise variation of IDR for regular and stiffness irregular frames subjected to mainshocks and MS–AS.