State-Dependent Fragility Curves for Aftershock Seismic Risk Assessment of Japanese Steel Frames

Abstract eng:
Probabilistic seismic risk assessment of civil infrastructures has been attracting attention in Japan, especially after recent mega-earthquakes with a long-lasting series of aftershocks capable of accumulating building damage; e.g., the 2011 Tohoku earthquake. To this aim, it is valuable to be able to assess the failure probability of a particular structure and its evolution in time due to sequential earthquake events, which may cause a difficulty for stakeholders to perform consistent decision making to warrant business continuity. This kind of risk analysis may require state-dependent fragility curves, which in the study were developed for a Japanese steel frame. To construct the curves, a numerical model of a three-story steel moment-resisting frame was first constructed and calibrated according to the results of shake table tests for a typical Japanese steel structure. This model was subsequently transformed in an equivalent single degree of freedom (ESDOF) system, based on the results of the nonlinear static (pushover) analysis. The probabilistic damage model was then constructed via nonlinear dynamic analysis of the ESDOF system. The spectral acceleration of the elastic period of the ESDOF system was selected as the ground motion intensity measure while the drift angle was selected as response measure. All the records used in the analysis were selected from the Japanese strong-motion network, K-Net. Finally, the state-dependent fragility curves were developed for five levels of damage: As-New (AN), Immediate Occupancy (IO), Life Safety (LS), Collapse Prevention (CP) and Failure (F). The limit state value for each damage state (DS) was set in compliance with the results of the shake table tests. After computing the damage state probability due to the mainshock, the time-variant aftershock risk of the steel structure was quantified integrating the developed state-dependent fragility curves with the seismic hazard, following a Markov chain model already available in the literature, which makes use of aftershock probabilistic seismic hazard analysis (APSHA). Hazard was computed assuming that the structure was located in Osaka, a site that may be affected by a mega-earthquake at the Nankai Trough subduction-zone. In particular, in the considered exercise, the most probable damage state due to the considered mainshock scenario was found to be IO, followed, in probability terms, by LS, F, AN, and CP. Given the probability distribution of the mainshock-induced damage, the daily evolution of aftershock damage was computed, and it was found that the most likely DS after two months since the mainshock was still IO followed by F, LS, CP and AN.

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