A novel approach to improve accuracy in seismic fragility analysis: The modified intensity measure method

Seismic fragilities, i.e., probabilities that structural systems exceed specified limit states, are used in Performance-Based Earthquake Engineering to characterize structural behavior during seismic events. Generally, seismic fragilities are constructed from structural responses to scaled accelerograms described by single/multiple ordinates of the pseudo-acceleration response spectrum, peak ground acceleration or other intensity measures. Recently, it was shown that the resulting fragilities provide limited if any information on the structural seismic performance if the dependence between intensity measures and demand parameters is weak. Yet, the method is used in Performance-Based Earthquake Engineering since the statistical uncertainty caused by the limited numbers of recorded ground accelerations is overcome by scaling these records. This study develops a method for estimating the fragility of linear systems for cases in which intensity measures and demand parameters are weakly correlated, i.e., situations in which the current methodology is inaccurate. The method is based on a linear transformation of samples of a given intensity measure which is designed to improve the correlation between demand and intensity parameters. The effectiveness of the proposed method for linear system is demonstrated by an elementary oscillator, a multi-degree of freedom system and a real complex multi-degree of freedom structural system.