Hazard-consistent intensity measure conversion of fragility curves

In seismic risk assessment of structures, fragility functions are the typical representation of seismic vulnerability, expressing the probability of exceedance of a given performance level as a function of a ground motion intensity measure (IM). Fragility curves, in general, are structure- and site-specific, thus a comparison of fragility curves is not straightforward across multiple structures and/or sites. The study presented in this paper discusses possible strategies to convert a fragility curve from an original IM to a target IM for a given site. In particular, three conversion cases, under different assumptions on the explanatory power with respect to structural failure of the involved IMs, are considered: (i) a vector-valued IM consisting of two different IMs (to say, original and target), magnitude, and source-to-site distance, (ii) a vector-valued IM consisting of the original and target IMs, and (iii) the original IM only, supposed to be a sufficient one; i.e., the structural response given IM statistically-independent of the other ground motion characteristics. The original fragility functions are supposed to be obtained through the state-of-the-art methods, then the fragility functions in terms of the target IM are obtained via applications of the probability calculus rules, which ensure consistency with the seismic hazard at the site of interest. The considered cases are illustrated via an example referring to an Italian code-conforming RC building designed for a site in L'Aquila. As far as the case-study is concerned, all conversion cases show agreement, likely because of the hazard-consistent record selection and to the explanatory power of the original IM with respect to structural failure.