Probabilistic seismic assessment of as-built and retrofitted old and newly designed skewed multi-frame bridges

This paper presents a probabilistic seismic assessment of older and newly-designed skewed multi-frame reinforced concrete box-girder bridges in California. The overall system and individual components fragility curves were developed for both older and new bridges with different skewness angles across four different damage states. Moreover, the effectiveness of two retrofit strategies: restrainer cables and shear keys at both in-span hinges and abutments was explored for mitigating the seismic fragility of multi-frame bridges and contributing to the nation's effort in maintaining the deteriorating infrastructure. The results indicate that the bridge system and different component fragilities are affected by the design details chronological changes, deck skewness, and the choice of the retrofit components. Deck unseating at the in-span hinge is shown to be the most sensitive component to the retrofit measures and design chronological changes. The retrofit measures are also shown to reduce the vulnerability of older bridges and have a negligible effect on newly designed bridges. On the other hand, the bearing pads damage of both old and new bridges is the most susceptible component to the deck skewness. For completeness, the results of this study are compared to fragility medians suggested by HAZUS-MH, and a significant underestimation of vulnerability for both old and new skewed multi-frame bridge is shown.