Risk-based seismic life-cycle cost–benefit (LCC-B) analysis for bridge retrofit assessment

Bridges constitute key elements of the nation’s infrastructure and are subjected to considerable threats from natural hazards including seismic events. A range of potential bridge retrofit measures may be used to mitigate seismic damage in deficient bridges, and help to avoid associated social and economic losses. However, since resources are often limited for investment in seismic upgrade, particularly in regions of large but infrequent events, a risk-based approach for evaluating and comparing the cost effectiveness of different mitigation strategies is warranted. This paper illustrates a method for evaluating the best retrofits for non-seismically designed bridges based on seismic life-cycle costs and cost–benefit analysis. The approach integrates probabilistic seismic hazard models, fragility of as-built and retrofitted bridges for a range of damage states, and associated costs of damage and retrofit. The emphasis on life-time performance and benefits, as opposed to initial retrofit cost alone, not only permits risk-wise investment, but also helps to align upgrade actions with highway agency missions for sustainable infrastructure. An application of the seismic life-cycle cost–benefit analysis is conducted for four representative bridges of different classes, and seven different retrofit options ranging from the use of seat extenders, to isolation bearings, to steel jackets. The same bridges are evaluated located at three sites of varying seismicity: Caruthersville, Missouri; Charleston, South Carolina; and Los Angeles, California. A summary of the proposed optimal retrofit measures for the case-study bridges and locations is presented. The results show that not only do the magnitude of the expected losses and resulting retrofit cost–benefit differ by location, but the most cost-effective retrofit for a particular bridge may vary as well due to local seismic hazard characteristics and the effect of retrofit at different damage levels.