Evaluation of Bridge Damage Data from the Loma Prieta and Northridge, California Earthquakes
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This report studies damage to bridges during the 1989 Loma Prieta and the 1994 Northridge, CA earthquakes. In both earthquakes, less than five percent of the bridges exposed to ground shaking were damaged. Bridges with non-monolithic abutment types, discontinuous spans and single column bents performed poorly. High skew contributed to high damage levels. Bridges designed and built before 1971 fared worse than those designed according to most recent standards. The total estimated repair cost of $150 million for the bridges damaged in the Northridge earthquake was about two thirds of the repair cost estimated from the Loma Prieta earthquake. Columns were the most damaged components in both earthquakes. Data on bridge damage were compiled and analyzed to correlate observed bridge damage to structural characteristics of a bridge, ground motion levels and estimated repair costs. Damage states reported after the earthquakes were investigated and new damage state definitions for concrete bridges were proposed. Bridges were grouped by their structural characteristics and correlation studies were performed to obtain ground motion-damage relationships and ground motion-repair cost ratio relationships. Logistic regression analysis was used to obtain empirical fragility curves. Currently available fragility curves and damage probability matrices were compared to observed damage data and the empirical relationships developed in this study.