Development of seismic fragility curves for mainshock-damaged reinforced-concrete structures

This communication presents an original procedure to derive fragility curves for previously damaged structural systems. These damage state-dependent functions constitute an important component in the framework of time-dependent risk assessment. For instance, they are used to estimate the updated vulnerability of structures exposed to potential aftershocks, based on the knowledge of their damage state after the main seismic event. While the state-of-the-art method for deriving such functions relies on incremental dynamic analysis, the present work proposes an alternative approach based on (i) the application of sequences of ground-motion records to the undamaged structure in order to have a representative panel of structures in each damage state, and (ii) an innovative statistical treatment of the permanent residual drift of damaged structures Δ_(p,t0) to derive the fragility functions: the maximum transient additional drift (Δ_(t,max)-Δ_(p,t0)) is correlated to the intensity measure of the ground motion, and the influence of the initial permanent drift is then eliminated by estimating its distribution for a given initial damage state. The developed procedure is applied to a single story reinforced-concrete frame with a concentrated plasticity model and fragility curves are proposed for various initial damage states.