Optimal Bridge Retrofit Strategy to Enhance Disaster Resilience of Highway Transportation Systems

16. Abstract This study evaluated the resilience of highway bridges under the multihazard scenario of earthquake in the presence of flood-induced scour. To mitigate losses incurred from bridge damage during extreme events, bridge retrofit strategies are selected such that the retrofit not only enhances bridge performance, but also improves resilience of the system consisting of these bridges. The first part of the report focuses on the enhancement of seismic resilience of bridges through retrofit. To obtain results specific to a bridge, a reinforced concrete bridge in the Los Angeles region was analyzed. This bridge was severely damaged during the Northridge earthquake due to shear failure of one bridge pier. A seismic vulnerability model of the bridge was developed through finite element analysis under a suite of time histories that represent regional seismic hazard. The obtained bridge vulnerability model was combined with appropriate loss and recovery models to calculate the seismic resilience of the bridge. The impact of retrofit on seismic resilience was observed by applying a suitable retrofit strategy to the bridge, assuming its undamaged condition prior to the Northridge event. A difference in resilience observed before and after bridge retrofit signified the effectiveness of seismic retrofit. The applied retrofit technique was also found to be cost effective through a cost-benefit analysis. A first-order, second-moment reliability analysis was performed and a tornado diagram developed to identify major uncertain input parameters to which seismic resilience is most sensitive. Statistical analysis of resilience obtained through random sampling of major uncertain input parameters revealed that the uncertain nature of seismic resilience can be characterized with a normal distribution, the standard deviation of which represents the uncertainty in seismic resilience. An optimal (with respect to cost and resilience) bridge retrofit strategy under multihazard was obtained in the second phase of this study. A multi-objective evolutionary algorithm, namely Non-dominated Sorting Genetic Algorithm II, was used. Application of this algorithm was demonstrated by retrofitting a bridge with column jackets and evaluating bridge resilience under the multihazard effect of earthquake and flood-induced scour. Three different retrofit materials—steel, carbon fiber, and glass fiber composites—were used. Required jacket thickness and cost of jacketing for each material differed to achieve the same level of resilience. Results from the optimization, called Pareto-optimal set, include solutions that are distinct from each other in terms of associated cost, contribution to resilience enhancement, and values of design parameters. This optimal set offers the best …