Traditional seismic design of bridges includes ductile details that allow bridges to develop substantial inelastic deformations when subjected to severe earthquakes. While bridges designed in this manner may be safe from collapse foll owing an earthquake, they are susceptible to considerable damage and permanent lateral displacements that can impair traffic flow and necessitate costly and time-consuming inspections and repairs (perhaps even demolition). Nowadays, as an alternative design strategy, bridges with columns supported on rocking foundations are designed to undergo large deformations but sustain far less damage and can re-center after large earthquakes.
The numerical study presented herein investig ates the seismic response of two bridges subjected to two sets of forty ground motions each, one consisting of pulse-type near-fault ground motions and another containing a mix of near- and far-fault ground motions. Three design strategies were considered for each of the two bridges. The first design is based on current common practice, which expects flexural plastic hinging in the columns. The other two designs use rocking shallow and pile foundations, respectively. The columns in the bridge with the rocking foundation are designed to remain elastic while also accounting for the effect of framing between the columns, the deck, and the abut ments. The bridges with rocking foundations consider several different cases in terms of size of columns, bearings, and expansion joints at the abutments.
Each bridge model is subjected to the two sets of ground motions using two horizontal components for each ground motion. The numerical results show that lateral drift similar to that experienced by fixed-base bridges is possible in the bridges with rocking pile foundations, with essentially an elastic response in the columns. A comparison of the seismic performance of the bridges in terms of post-earthquake repair cost is conducted using an existing performance evaluation framework based on the Pacific Ea rthquake Engineering Research Center’s performance-based earthquake engineering method. Existing damage models for the columns, bearings, and shear keys are used, while a new damage model of rocking shallow foundations is developed. The structural components are classified in different performance groups, with discrete damage states and repair methods. Based on an existing methodology developed by other researchers, repair costs are calculated based on the repair quantities and the materials used in the repair methods of every performance group. The post-earthquake repair cost of the rocking bridges is negligible for the range of intensity measures considered in this study.
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