Performance Seismic Assessment of Steel Frame Structures Having Torsional Irregularity Subjected to Mainshock-Aftershock

The earthquake is a phenomenon that is likely to occur all over the world and so far it has caused a lot of human and financial losses. During earthquake events, it is not uncommon to observe after-shocks following a mainshock, aftershocks have the potential to cause severe damage to buildings and threaten life safety even when only minor damage is present from the mainshock, but in the current regulations, the effect of aftershocks is not considered.Another factor affecting the behavior of the building during the earthquake is a torsional irregularity that may be due to architectural constraints in the building and, if accompanied by aftershock, results in further damage to the structure.In this article, for assessing the effects of torsional irregularity and also the aftershocks, 3,5 and 8 story models with steel moment frame seismic system, first regular in plan and then with torsional irregularity are carried out according to the Iranian seismic code and then 3-dimensional analytical models analyzed based on Incremental Dynamic Analysis (IDA) in OpenSees. The maximum inter-story drift ratio is obtained for 20 sets of ground motion records with aftershocks compatible with the conditions of the region and the capacity is determined according to HAZUS-MH limit states and finally, the corresponding fragility curves for seismic performance levels of slight, moderate, extensive and complete are developed.The resulting seismic fragility curves revealed the destructive effect of torsional irregularity in 5 and 8 story is more than 3 story and the aftershock outputs illustrated that the effect of aftershock on the damage level increase when building have torsional irregularity.