Effect of aftershock characteristics on the fragility curve of post-mainshock RC frames

Abstract

Earthquakes are not isolated events but are often followed by aftershocks. While structures are typically designed to withstand the mainshock, the presence of aftershocks poses additional challenges. In some cases, a damaged structure could collapse under a strong aftershock because of the damage evolution and cumulative damage as well as strength and stiffness degradation. Thus, it is essential to understand the structural behavior for post-mainshock decision-making and seismic assessment. The current study explores the impact of aftershock characteristics on the collapse capacity of buildings in seismic zones in accordance with a probabilistic point of view. In this regard, two non-ductile reinforced concrete (RC) frames with 3 and 6 stories were investigated and analyzed using 62 real aftershock records with varying characteristics. Frames were modeled in OpenSees software using Beam With Hinges Element with appropriate plastic hinge length and fiber section to provide suitable accuracy for dynamic analysis. The incremental dynamic analysis (IDA) was used to generate collapse diagrams and fragility curves. Some important seismic parameters including the mean period (Tm), predominant period (Tp), bandwidth frequency (Ω), significant duration of aftershock (Ds) and site characteristics were considered. The results revealed that the probability of collapse would increase in aftershocks with longer periods. For example, for Sa (T1, 5 %) of 1 g and 3-story frame, the collapse probability is increased ≈25 % in Tm > 0.5s compared to Tm < 0.5s which is remarkable. Also, site characteristics could increase the probability of collapse by ~ 6 % in the 6-story frame. The same results can also be seen for aftershocks with a larger predominant period (Tp > T1), greater significant duration (Ds > 11 s) and lower bandwidth frequency (omega <0.5). The findings emphasize the significance of aftershock characteristics on structural collapse capacity, necessitating careful selection of seismic records for accurate evaluation of seismic behavior.