Seismic fragility estimates for reinforced concrete framed buildings

Abstract

Gravity load designed (GLD) reinforced concrete (RC) buildings represent a common type of construction in the Mid-America Region. These buildings have limited lateral resistance and are susceptible to story mechanisms during earthquake loading. Fragility estimates are developed to assess the seismic vulnerability of GLD RC buildings in the Mid-America Region. Fragility is defined as the conditional probability of reaching or exceeding a performance level for a given earthquake intensity measure. Five sample buildings of various story heights (1, 2, 3, 6, and 10 stories) are used to represent generic RC frame buildings of 1 to 10 stories tall. A Bayesian methodology is used to develop probabilistic demand models to predict the maximum inter story drift given the spectral acceleration at the fundamental period of the building. The unknown parameters of the demand models are estimated using the simulated response data obtained from nonlinear time history analyses of the structural models for a suite of synthetic ground motions, developed for Memphis, Tennessee. Seismic structural capacity values are selected corresponding to the performance levels or damage states as specified in FEMA-356 and as computed by nonlinear pushover analyses. For the sample buildings, fragility estimates are developed using the predicted drift demands and structural capacity values. Confidence bounds are developed to represent the epistemic uncertainty inherent in the fragility estimates. In addition, 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 GLD RC frame buildings of 1 to 10 stories. Using the Bayesian approach, a framework is developed to update the analytical fragility estimates using observed damage data or experimental test data. As an illustration of the updating framework, the analytical bivariate fragility estimates for the sample buildings in the Mid-America Region are updated using the damage data obtained from 1994 Northridge, California earthquake. Furthermore, to investigate and demonstrate the increase in seismic performance of the GLD RC frame buildings, the columns of the 2 and 3 story buildings are retrofitted by column strengthening. Fragility estimates developed for the retrofitted buildings show the effectiveness of the retrofit technique by the improved seismic performance of GLD RC frame buildings.