Seismic Reliability of the Non-Code-Conforming RC Building Due to Vertical Mass Irregularity Effect

Recent studies showed that the inelastic seismic response of irregular structures can significantly differ from regular structures. Irregular distribution of mass in elevation is regarded as a structural irregularity by which the modes with high energy levels are excited and occasionally prevents the structure from developing nonlinear deformations and causes some unpredictable damages in structural elements. In this study, seismic reliability and risk assessment of a non-code-conforming concrete building reinforced by plain bars is investigated with consideration of the vertical mass irregularity effect. The framework of this study is based on the determination of fragility via incremental dynamic analysis (IDA). The analyses are carried out on a reference 3-story multi-bay 3D structure modeled in Opensees software. Seismic risk assessment for the complete collapse limit state is evaluated by integrating the site hazard and the structural fragility curves. Also, a relatively simple and efficient nonlinear model based on the experimental behavior of substructures reinforced by plain bars is used to simulate pre- and post-elastic behavior buildings. The results indicated that the effects of vertical mass irregularity of the building have almost significant effects on the represented building's fragility curve parameters and seismic reliability of the represented buildings. Probabilities of occurrence for the irregular bottom and median story are about 1.51 and 1.6 times of the building with regular mass distribution.