A new hybrid method for seismic fragility analysis of reinforced concrete buildings

The development of accurate seismic fragility curves is a key to effective seismic risk management. This study proposes a new hybrid method to derive the seismic fragility curves of reinforced concrete (RC) buildings. In the first step, nonlinear time-history analyses are conducted to analyze the inter-story drift (ISD) ratios. Based on the analysis results, analytical fragility curves are derived for three damage states of light, moderate, and severe with varying levels of peak ground acceleration (PGA). Subsequently, empirical damage data of RC buildings collected for an actual earthquake is converted to ISD ratios using empirical equations. Lastly, based on the empirical damage data, hybrid fragility curves are derived by updating the median of the ISD ratios for a reference PGA level using Bayesian updating. In addition, to update the median values at other PGA levels including no empirical data, a new scaling factor, which is the ratio of the posterior and prior median values at the reference PGA level, is proposed. To test the proposed method, it is applied to RC buildings in Pohang city, South Korea, which were subjected to a magnitude-5.4 earthquake in 2017. From the comparison of analytical fragility curves with hybrid fragility curves, it is observed that the analytical curves overestimate the seismic vulnerability of the RC buildings, and the proposed method provides hybrid fragility curves successfully