Seismic fragilities for reinforced concrete buildings with consideration of irregularities

Poor seismic performance of non-code conforming RC buildings, mainly designed for gravity loads prior to 1970s, highlights the need for reliable vulnerability assessment and retrofitting. The vulnerability is compounded since the RC buildings are subject to different irregularities such as weak storey, soft storey, plan irregularities, and poor construction quality; and interaction of different irregularities. Fragility based seismic vulnerability of structures with consideration of soft storey (SS) and quality of construction (CQ) is demonstrated on three-, five-, and nine-storey RC frames designed prior to 1970s. Probabilistic seismic demand model (PSDM) for those gravity load designed structures is developed, using the nonlinear finite element analysis, considering the interactions between SS and CQ. The response surface method is used to develop a predictive equation for PSDM parameters as a function of SS and CQ. Result of the analysis shows the sensitivity of the model parameter to the interaction of SS and CQ. The accuracy of the predictive equations is checked for randomly selected SS values and three levels of CQ. Further, the fragility curves are developed for the three structures considering SS, CQ and of their interactions. Finally, confidence bounds on the fragilities are also presented as a measure of their accuracy for risk-informed decision-making.