A Case Study on the Effect of Multiple Earthquakes on Mid-rise RC Buildings with Mass and Stiffness Irregularity in Height

Irregularities present in buildings play a crucial role when subjected to earthquake loads. In this study, an existing Reinforced Concrete (RC) structure is considered, which is designed and constructed without considering irregularity and seismic load effect. Due to functional requirements and usage of the floor for other occupancy purposes, columns were removed, which resulted in stiffness irregularity, and the imposed load difference resulted in mass irregularity at the same storey level. Three types of structures were analyzed and compared to understand the seismic effects and collapse probability of induced irregularity in existing RC structures. The main goal of this study is to find out how multiple earthquakes affect the induced irregularity in structures. To assess the performance of the structures, Incremental Dynamic Analysis (IDA) is performed, and fragility relationships are generated with and without irregularities for both single and multiple earthquakes. The results are compared in terms of Inter-storey Drift Ratio (IDR), roof displacement, and probability of collapse for three framed structures. The analytical study identifies that the effect of multiple earthquakes has a crucial impact on irregular structures. The irregular structure’s median storey drift has been observed to have attained the maximum drift threshold of 4% as per FEMA 273.The findings highlight the importance of analyzing the structures subjected to multiple earthquakes. The results indicate that multiple earthquakes are significant and should be considered in the analysis phase to avoid the collapse of regular and irregular structures. Introducing irregularity into structures already constructed should be avoided.