Seismic Vulnerability of Reinforced Concrete Frame Structures: Obtaining Plan or Vertical Mass Irregularity from Structure Use Change

Changes in a structure’s use can have a significant impact on distribution of live loads, both in the plan and elevation of a building. Unevenly distributed live loads can introduce variations in mass, causing irregularities in the structure and making it more susceptible to damage during earthquakes. Previous studies have extensively investigated the seismic vulnerability of structurally irregular buildings. However, due to computational challenges, simplified two-dimensional models are commonly employed, which fail to accurately capture the nonlinear interaction between torsional and lateral responses, thereby limiting their ability to reflect the actual structural behavior. This research focuses on examining the seismic fragility of reinforced concrete frames that exhibit mass irregularities in the elevation or plan caused by the change in structural usage. For accurate vulnerability evaluation, this research employs comprehensive three-dimensional models of reinforced concrete irregular structures. A total of 21 frame models encompassing various degrees of vertical and in-plan irregularities resulting from different scenarios of live-load distribution were studied. Nonlinear dynamic response history analysis was conducted with 10 earthquake input ground motions. To assess the vulnerability of the studied structures against the given earthquakes comprehensively, the concept of a fragility surface was introduced. The fragility surface is derived from fragility curves corresponding to various damage limit states, providing a complete overview of the seismic vulnerability of the structure in terms of peak ground accelerations and allowable drift ratios. In this study, eight allowable drift ratios were defined to establish failure limit-state conditions. The seismic vulnerability of all assumed building usage scenarios ware quantified, and this study highlights that uneven structure use in the regular structure can significantly affect its seismic performance. The findings of this study can contribute to a better understanding of the novel aspects related to the seismic behavior of irregular structures, enhancing our knowledge in this field.