Risk-based cost-benefit analysis for the retrofit of bridges exposed to extreme hydrologic events considering multiple failure modes

Bridges exposed to flooding, hurricanes, tsunamis, and other extreme hydrologic events have been observed to fail due to deck dislodgement, pier failure, or foundation failure. However, the risk assessment and retrofit methodologies for these bridges have typically only been developed around a single failure mode. This paper addresses this gap by integrating the three observed failure modes for bridges vulnerable to extreme hydraulic events into a comprehensive risk assessment framework. Through the use of an event tree, the methodology accounts for the different consequences of failure associated with different failure modes. Bridge management strategies are investigated to determine the effectiveness of the retrofit actions with respect to their benefit (i.e. reduction in risk) and costs. An illustrative example for riverine bridges under various exposure scenarios is presented. The risk assessment and benefit-cost analysis elucidate the need to incorporate all pertinent failure modes of the structure by highlighting the competing nature of different failure modes. The illustrative example shows that the effective management of structures is site-specific and, that the intensity of the hazard at the bridge location affects which management strategy is preferred. The sensitivity to exposure level indicates that the optimal management of the structure should incorporate considerations for potential future changes in the intensity and frequency of the hazard.