Seismic fragility and confidence bounds for gravity load designed reinforced concrete frames of varying height

Fragility estimates are developed to assess the seismic vulnerability of reinforced concrete (RC) frame buildings, which were designed primarily for gravity loads. Five buildings of various heights (one-, two-, three-, six-, and ten-stories) are used to represent generic RC frame buildings of one- to ten-stories tall. Using a Bayesian methodology, probabilistic demand models are developed to predict the seismic structural demand using the peak interstory drift response for an imposed earthquake. Seismic structural capacity values are selected corresponding to the performance levels or damage state as specified in FEMA 356 or as computed by nonlinear pushover analyses. For each building, fragility estimates are obtained by assessing the conditional probability that the drift demand reaches or exceeds the drift capacity for a given earthquake spectral acceleration. Confidence bounds are developed to represent the epistemic uncertainty inherent in the demand models used in the fragility estimates. Bivariate fragility estimates, formulated as a function of spectral acceleration and the fundamental building period, are developed from the fragility estimates of the individual buildings. The bivariate fragilities can be used to quantify the seismic vulnerability of gravity load designed RC frame buildings one- to ten-stories tall.