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UBC Theses and Dissertations

Optimal retrofit strategy determination for bridges using decision analysis Khan, Saqib A.

Abstract

With our increased knowledge about seismicity and risks related to earthquakes, there is a greater need for retrofitting deficient structures to ensure the functioning of a transportation network and to minimize the life and economic loss associated with catastrophic events. Given the scarcity of funds, the decision maker should try to determine the optimal level of retrofit for a structure and the priorities among various candidates. A decision analysis methodology is thus proposed and demonstrated through two example bridges in order to determine the most preferable level of retrofit and the retrofit order for the two candidates. The decision alternative minimizing the total cost of the structure over its life is the best retrofit strategy while the bridge with the lower cost-to- benefit ratio should be retrofitted first. Seismic assessment o f the Colquitz river south structure was performed in terms of three different levels of seismicity. Various earthquake records were scaled to match the site-specific spectra corresponding to each level of seismicity. These records were then used to drive the non-linear dynamic analysis of the bridge pier. Overall damage states for the bridge were determined based on damage index values for the pier and expert judgement for other bridge components corresponding to various pre-defined levels of retrofit. The structural damage was then translated into dollar damage and this information was used in the decision analysis algorithm as consequence costs. An expected annual cost of future damage was then calculated and converted to present worth for each retrofit level. The estimated retrofit costs and the present value of future damages were then added to find the total expected cost for each decision alternative. Sensitivity analyses were carried out to examine the variations in decision outcome due to changes in different input parameters. Also, the effectiveness of decision analysis techniques not employing probability and risk attributes was briefly examined. For the Interurban overpass, it was assumed that the safety level retrofit would be the optimal strategy. Cost-to-benefit ratios for the two structures were then calculated corresponding to the optimal retrofit decision for each, thus determining the order of retrofit.

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