Seismic assessment of structures with mass and stiffness irregularities under multiple earthquakes

Multiple earthquakes are reported in many locations around the world, recently on February 6, 2023, in Türkiye two earthquakes of magnitude 7.7 and 7.6 struck with a time interval of approximately 9 h resulted in huge destruction and collapse of structures. The lesser time interval between seismic events leads to the collapse of the structures which were damaged by the first earthquake. Most of the seismic codes do not consider the influence of multiple earthquakes, which makes the structures vulnerable to multiple earthquake phenomena. Bidirectional Incremental Dynamic Analysis (IDA) is performed on regular (S1R) and irregular (S2I) structures that are considered to access the effects of single and multiple earthquakes. The response of the structures is accessed in terms of roof displacement, storey drift ratio, IDA curves, and fragility curves generated for the suite of eleven ground motions. Results obtained from the bidirectional IDA analysis showed the effect of multiple earthquakes is significant and also the irregular structures are of greater risk under multiple earthquakes. It is noticed that from fragility curves in comparison with regular structures irregular structures are 11.11% and 27.78% more susceptible to multiple earthquakes along X and Y-direction. For S1R and S2I when subjected to multiple earthquakes, the average increase in maximum drift ratio is 29.30% and 36.44% along the X-direction, and for multiple earthquakes along the Y-direction, it is 36.42% and 40.34%. It is preferable to avoid adding irregularity into constructed structures, incorporating multiple earthquake occurrences into modern seismic regulations is advantageous.