Characterization of ground motion intensity for the seismic fragility assessment of plan-irregular RC buildings

The identification of ground motion intensity measures (IMs) that are characterized by virtues such as efficiency, sufficiency, scaling robustness and hazard computability, has been the subject of extensive research over the past several years. In the process, there have evolved, numerous non-structure specific as well as structure specific descriptors of ground shaking severity. Most of the studies that investigated the optimality of such descriptors were, however, restricted to the seismic response of single or multi-storey planar frame buildings under unidirectional excitation. The study herein aims to examine, the efficiency and sufficiency of a wide spectrum of IMs, in predicting the maximum interstorey drift demands on plan-irregular RC moment frame buildings subjected to bi-directional ground motion. This is accomplished through a cloud analysis-based framework that integrates nonlinear response history analysis and statistical regression analysis. Further, the use of scalar bidirectional shaking intensity measures (BSIMs) derived from a vector of intensities corresponding to each horizontal component of ground motion using popular combination schemes such as geometric averaging, arithmetic averaging, SRSS and use of envelope values, is examined from the perspective of their ability to predict drift demands. Observations from the study reveal that IMs based on the geometric averaging of spectral ordinates over a range of periods are superior to the other descriptors considered, in terms of the aforementioned attributes and that combining the intensities of the two orthogonal components by geometric averaging leads to an optimal BSIM that could be used in place of vector IMs. The heteroscedasticity in the maximum interstorey drift response under different levels of the BSIM is further investigated using a Multiple Stripe Analysis framework. The optimal BSIM is observed to perform reasonably well at all target intensity levels, making it an excellent candidate to base fragility and vulnerability functions upon.