Keywords: bridges, seismic, risk, fragility analysis, prioritization.

A new seismic risk prioritization method is proposed for highway bridges in Clark County, Nevada, USA, using (a) a fragility-based vulnerability analysis, (b) an importance analysis, and (c) the latest bridge and site soil data. The analysis methods and data collections are described in the paper; however, the primary focus is on the fragility-based vulnerability analysis used in this project. The results may be used by the department of transportation and by the pre-earthquake planning organizations.

The vulnerability analysis was completed by using the FEMA software HAZUS-MH with a Level II assessment. The analysis used information on the structural systems, combined with geotechnical site characterization, which was accomplished using compiled in situ shear wave velocity measurements and well log databases. The fragility curve for ground shaking is given by a normalized cumulative probability function for a given damage state, median spectral acceleration, and coefficient of dispersion. Fragility curves for ground displacement are developed similar to those for ground shaking and the combined damage probabilities were obtained. To better control the vulnerability scores, relative values were assigned using a two-step process. First, the discrete probabilities for complete and extensive damage states were combined to form the "at least extensive" damage state. The extensive and complete damage states were used since for these two damage states a bridge would not be functional and recovery time is significant. The second step was to assign relative vulnerability scores to the two combined probabilities. A utility function obtained in this manner was used to allow the most vulnerable bridge to have a vulnerability score of 1.0 and the bridge with an average vulnerability to have a vulnerability score of 0.5.

We performed a verification analysis to determine whether selected bridge characteristics used by the program are consistent with those of "typical" Nevada bridges. Bridge models were constructed using the dimensions and material properties specified by the bridge plans. The failure of the bridge bent was assumed to be controlled by the column plastic moment capacity. The pushover analysis verified that the average transverse displacement corresponding to "extensive damage" of the bridges considered was very close to the value provided by HAZUS-MH.

The importance analysis takes into account average daily traffic, detour length, bridge length, pedestrian traffic, railroad traffic and defence route. The final step, the risk prioritization, combines the importance and vulnerability analyses. We used 40 percent weighting factor for vulnerability and 60 percent for importance; however, the user can adjust the percentages. The resulting priority list is compared to the one currently used by the state of Nevada. The risk prioritization does not indicate which bridges will be unusable following an earthquake or will be in need of immediate retrofit. However, it indicates structures that may be candidates for retrofit. The bridges that receive high-risk scores should be analyzed in more detail. Upon further investigation by the bridge owners, bridges with high scores may merit retrofit or replacement.

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