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Optimal seismic retrofitting level for bridges based on benefit-cost analysis Gao, Yulin

Abstract

There are a large number of seismically deficient bridges in British Columbia that need to be strengthened to protect public safety in future earthquakes: Many upgrading options are available for seismic rehabilitation of these bridges, such as No Retrofitting, Safety Level Retrofitting, and Functional Level Retrofitting, etc. The search of the optimal solution among various feasible options is a complicated decision problem. The big amount of money spent for seismic retrofitting needs to be justified based on the economic and safety decisions, and they involve considerations of risk and cost. A reliability-based risk decision model is constructed in the thesis to try to facilitate an answer to the seismic retrofitting of bridges. The methodology and procedures of decision analysis are demonstrated through a case study bridge. The global linear, elastic response spectrum analysis is undertaken to obtain seismic demand and the component capacity/demand ratios are computed to identify the critical structural components. Seismic deficiencies and failure mechanism of the identified critical components are evaluated by local inelastic push over analysis. Two seismic retrofitting schemes are designed to counteract the seismic deficiencies. The effect of seismic retrofitting on the structural behavior during earthquake excitations is evaluated. The retrofitting costs of both schemes are calculated. Structural failure probability during future earthquakes is calculated by the simple FORM/SORM approach. Latin Hypercube Sampling (LHS) is used to generate random variables to obtain seismic demand and seismic capacity, which are fitted to the probability distribution functions. Both the failure probabilities of original bridge and retrofitted bridge are computed. The reduced failure probability due to seismic retrofitting is obtained. Seismic damage analysis is undertaken to compute damage indices of the bridge before and after seismic retrofitting, which are used for mapping out economic losses. Both direct and indirect economic losses are estimated. An expected value of the future earthquake damage costs are calculated and discounted to the present year. Present values of the total costs including retrofitting cost and future seismic financial damages for all retrofitting schemes are calculated. Then a benefit-cost analysis based on the constructed decision model is undertaken to determine the optimal seismic retrofitting level for the bridge. It concludes that for the case study bridge considered in this research, the optimal seismic retrofitting option is the level II retrofitting, which aims to keep normal or a limited traffic flow immediately after an earthquake of 10% exceedence probability in 50 years. Sensitivity analysis is made to explore the effect of change of input variables on the decision outcome.

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