Fragility relationships for torsionally-imbalanced buildings using three-dimensional damage characterization

In this paper, a methodology for the derivation of fragility relationships for three-dimensional (3D) structures with plan irregularities is developed. To illustrate the procedure, fragility curves are derived for an irregular reinforced concrete (RC) building under bi-directional earthquake loadings. In order to represent the damage state of irregular structures, a spatial (3D) damage index is employed as the salient response parameter. The feasibility of using a lognormal distribution for the bounded response variables, as in the case of structural fragility analysis, is investigated. Through the comparison between the fragility curves derived using the spatial and the previously-existing damage indices, it is shown that the proposed method provides realistic results and is therefore recommended for fragility analysis of buildings with significant torsional and bi-directional responses.