Seismic Evaluation of Vertically Irregular Steel Moment-Resisting Frames Under Mainshock–Aftershock Vibration.

Purpose: This study investigates the seismic performance of vertical irregular steel moment-resisting frames subjected to mainshock–aftershock (MS–AS) seismic sequence. The building structures situated in a seismically active region experience sequence of earthquakes during their lifetime. These seismic sequences have the potential to increase the damage state of the structures. The irregular building structures are present due to availability of space, functional and architectural requirements etc. behave differently as compared to the regular buildings during earthquakes. Methods: In this paper, three steel-moment resisting frames having different configuration i.e. 3, 6, and 9-storey were considered as a reference regular frame. Three vertical irregularities-stiffness, strength and combined stiffness and strength are introduced in three different locations (bottom, mid-height, and topmost storey) along the height of each building frames. These regular and vertically irregular frames are subjected to mainshocks alone and artificial MS–AS seismic sequences that are generated by using the repeated and randomized approach. The nonlinear time-history analysis is performed using computer software SAP 2000 on regular and irregular building frames subjected to mainshocks and MS–AS seismic sequence. Results: The comparison between the seismic performance of vertically irregular and regular frames subjected to mainshocks alone and MS–AS seismic sequences shows that the vertical irregular frames have a significant effect on the distribution of mean interstorey drift demands along the height of building frames. Conclusion: The effect of MS–AS seismic sequence on seismic demands of vertical irregular and regular frames have been increased compared to mainshocks. However, the repeated seismic sequences show smaller structural responses relative to the randomized seismic sequences. [ABSTRACT FROM AUTHOR]