FRACAS: a capacity spectrum approach for seismic fragility assessment including record-to-record variability

This paper presents a new approach for the derivation of fragility curves, named FRAgility through Capacity spectrum ASsessment (FRACAS). FRACAS adapts the capacity spectrum assessment method and uses inelastic response spectra derived from earthquake ground motion accelerograms to construct fragility curves. Following a description of the FRACAS approach, the paper compares the predicted maximum interstory drift (MIDR) response obtained from FRACAS and nonlinear time history analyses (NLTHA) for two case-study buildings subjected to 150 natural accelerograms. FRACAS is seen to represent well the response of both case-study structures when compared to NLTHA. Observations are made as to the sensitivity of the derived fragility curves to assumptions in the capacity spectrum assessment and fragility curve statistical model fitting. The paper also demonstrates the ability of FRACAS to capture inelastic record-to-record variability and properly translate this into resulting fragility curves. In particular, through an example application, it is shown that the variability in spectral ordinates for periods beyond the natural period of the undamaged structure is directly correlated to the standard deviations of the fragility curves.