Project # NCTRMP
- Marios Panagiotou, Assistant Professor, UC Berkeley
- Grigorios Antonellis, Graduate Student Researcher, UC Berkeley
Traditional seismic design of bridges includes ductile details that permit bridges to develop substantial inelastic deformations expected when subjected to severe earthquakes. While bridges designed in this manner may be safe from collapse, they are susceptible to considerable damage and permanent lateral displacements that can impair traffic flow and necessitate costly, time consuming, and disruptive inspections and repairs (and perhaps even demolition) following an earthquake. As an alternative design, bridges supported on rocking foundations can undergo large deformations but suffer far less damage, with the added bonus of re-centering after large earthquakes.
In this study the seismic response of two bridges subjected to two sets of forty ground motions each, one consisting of pulse-type near-fault ground motion and another containing a mix of near- and far-fault ground motions, is investigated. Three designs were considered for each of the two bridges. The first design considered conventional fixed-base foundations, including flexural plastic hinging in the columns. The other two designs used rocking shallow and pile foundations and columns designed to remain essentially elastic, but also accounting for the effect of framing between the columns, the deck, and the supports at the abutments. In contrast to the fixed-base design, the designs with rocking foundations considered also the cases of larger diameter columns, larger bearings at the abutments, and expansion joints with greater displacement capacity.
Each bridge model was subjected to the two sets of ground motions using two horizontal components for each ground motion. The numerical seismic response of the bridges is compared in terms of drift ratios and tensile strains of the columns, displacements of the expansion joints, displacements of the bearings, and axial force variation in the columns. A comparison of the seismic performance of the bridges is conducted using an existing performance evaluation framework based on the PEER performance based earthquake engineering method. The seismic performance of the bridges is compared in terms of post-earthquake repair cost. Existing damage models for the columns, the bearings and the shear keys were used, while a new damage model of rocking shallow foundations was developed. The structural components are classified in different performance groups, with discrete damage states and repair methodologies, and repair costs are calculated from the repair quantities of different materials used for the repair methods of every performance group based on an existing methodology developed by other researchers. The numerical results show that lateral drift similar to that experienced by fixed-base bridges can be achieved in the bridges with rocking pile foundations, with essentially elastic response in the columns. The probabilistic seismic performance evaluation demonstrates that the damage of the rocking bridges was negligible for the range of intensity measures considered in this study.