Optimal Seismic Bridge Retrofit Strategy under Budget Constraint

Abstract eng:
Highway transportation networks, an important lifeline system in urban areas, are highly vulnerable to strong seismic ground motions. Particularly due to possible bridge failures resulting from a severe earthquake, network characteristics may change and traffic flow capacity may significantly drop suddenly, causing excessive societal disruption and indirect economic losses. Pre-event seismic retrofitting of bridges therefore is desirable in order to reduce bridge damageability and minimize the extent of post-event societal disruption and economic losses. However, in most cases, funding available for retrofitting is limited and hence, an optimal strategy is needed to prioritize the selection of bridges and the order in which they are to be retrofitted. This paper aims to perform a simulation-based study on retrofit prioritization in order to identify the best strategy. For this purpose, Los Angeles area network spanning Los Angeles and Orange County with more than 3000 bridges is utilized as a testbed and analyzed under several tens of scenario earthquakes that represent the regional seismic hazard consistent with USGS hazard map. The study first analyzes the original network without retrofit under each scenario earthquake. Degradation of transportation network caused by post-event damaged bridges is measured in terms of social cost under the pre-event origin-destination (OD) matrix and is used as baseline measure of the network degradation immediately after an earthquake event. In the second stage, the same network with some retrofitted bridges is analyzed under all scenario earthquakes and under the same pre-event OD matrix. The result is a smaller value of social cost that represents the effect of retrofit by definition under the specific retrofit strategy and under each scenario earthquake. The retrofit strategy that produces the largest expected reduction is the optimal retrofit strategy determined starting from a particular scenario earthquake. In particular, an effort is made on the basis of the principle of optimality to find an analytical solution to the problem of retrofit prioritization under budget constraint.

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