Optimizing Bridge Seismic Retrofit Strategy: Implementing Bridge Fragility Curves

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Whether an old bridge needs to be brought up-to-date on the seismic design provision, or it deteriorates below the seismic resistant demand it requires a seismic retrofit. A bridge seismic retrofit project often costs a lot and yet the price and the effect of the retrofit are not always proportional. Bridge owners are less likely to experience obvious cases such as described in the Federal Highway Administration Report that once the rehabilitation cost for a given structure begins to exceed 60 percent of the cost of a new structure, the economics begin to favor a replacement. Most of the time, it is ambiguous to owners as to what extend does the retrofit become less cost-effective if the cost of the retrofit does not exceed 60 percent of the cost of a new structure. The aim of this paper is to present a generic decision support system for selecting a cost-effective bridge seismic retrofit strategy implementing analytical fragility curves. The seismic vulnerability of bridges is usually expressed in form of fragility curves, which display the conditional probability that the structural demand caused by various levels of ground shaking exceeds the structural capacity defined by a damage state. Empirical fragility curves are usually developed based on the damage reports from past earthquakes, while analytical fragility curves are developed from seismic response analysis of bridge structures. An Economic Index (EI) is proposed to identify the most cost-effective solution when decision-makers face multiple alternatives. Bridge fragility curves corresponding to various cost levels of retrofit are constructed to compare their effectiveness. The proposed methodology provides engineers and owners with a quantifiable solution for selecting retrofit alternatives.