VULNERABILITY ASSESSMENT of AN EXISTING HIGHWAY BRIDGE by 3-D PUSH-OVER ANALYSIS and PROPOSING ITS RETROFIT DESIGN
Bridges are the key structures in highway systems which are important from both rescue and relief activities and economical aspects after earthquakes. Shortcomings of the old codes cause that many of existing bridges need to be retrofitted. Furthermore, some bridges need to have a performance level higher than that considered for them in their design. In this paper the seismic vulnerability of an existing two span reinforced concrete slab bridge with round columns as piers has been studied. At first, the bridge design has been checked by the seismic provisions of the new version of AASHTO (2000) code, which has shown that the bridge is seismically weak in bending resistance and particularly shear resistance of its pier columns. Regarding the high Demand over Capacity Ratios (DCRs), in the next step some push over analyses (POA) have been performed. The bridge specifications for POA have been defined based on FEMA 356 regulations by using the AASHTO design spectra and considering the life safety performance level for the bridge. The target displacement for POA has been calculated by shear-displacement relationship for the center of gravity of the bridge deck. By performing the POA the ultimate resistance as well as the rotations of plastic hinges in columns have been obtained which in comparison with the FEMA acceptance levels show that the bridge is highly vulnerable. On this basis, two retrofit designs by adding some diagonal bracings between pier columns and beside abutments have been proposed, whose adequacy has been shown by another POA.