Intensity measure conversion of fragility curves

In seismic risk assessment of structures, fragility functions are the probabilistic characterization of vulnerability at the component and/or structural level, expressing the probability of failure 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, then of vulnerability, is not straightforward across multiple structures. Also, it could be the case that hazard at a site of interest is not available for the IM originally considered in the fragility assessment. These situations require to convert fragility curves from an original IM to a target one. The present study addresses a hazard-consistent probabilistic framework for converting spectral acceleration-based IMs from an original IM to a target IM at a given site. In particular, three conversion cases, under different assumptions on the explanatory power of the involved IMs with respect to structural failure, are discussed: (a) a vector-valued IM consisting of the original and target IMs, magnitude, and source-to-site distance; (b) a vector-valued IM consisting of the original and target IMs; and (c) the original (scalar) IM only, assuming that structural response, given the IM, is statistically independent of the other ground motion variables. In this framework, the original fragility functions are characterized using the state-of-the-art methods in performance-based earthquake engineering, then the fragility curves as a function of the target IM are evaluated through applications of the probability calculus rules, ensuring consistency with the seismic hazard at the site of interest. The conversion strategy is illustrated through the applications to three-, six-, and nine-story Italian code-conforming reinforced concrete buildings designed for a high-hazard site in Italy. The study shows that, in most of the cases, the converted fragility curves have agreement with the reference curves directly developed in terms of the target IM. Cases in which least agreement was found are likely due to the models used to obtain the terms required by the conversion equations.     

Nonlinear behaviour of scoria soil sediments evaluated from borehole records in eastern Shizuoka prefecture,Japan

We evaluate the non-linear behaviour of soil sediments, analysing five weak and four strong motions observed at depths of 1 m and 28 m, in eastern Shizuoka prefecture, Japan. We identify S-wave velocities and frequency-dependent damping factors by minimizing the residual between observed and theoretical spectral ratios, based on a linear one-dimensional model. We find that S-wave velocities identified from strong motions, whose peak ground acceleration are 440, 210, 176, and 140 cm/s², are significantly smaller than those identified from weak motions. The shear modulus reduction ratios estimated from identified S-wave velocities become clear above an effective shear strain of 10^-4 and agree with laboratory test results below an effective shear strain of 8×10^-4. The differences of damping factors between weak and strong motions are not clear below this effective shear strain, as the laboratory test suggested. The equivalent linear one-dimensional model, with frequency-dependent damping factors, is confirmed to be valid to simulate strong motions at least an effective shear strain of less than 4×10^-4. © 1997 John Wiley & Sons, Ltd.