Quasi-isolated highway bridges: influence of bearing anchorage strength on seismic performance

The Illinois Department of Transportation (IDOT) commonly uses elastomeric bearings to accommodate thermal deformations in bridges, and these bearings also have potential utility in seismic events. IDOT has developed an earthquake resisting system (ERS) using the displacement capacity of typical bearings to achieve a structural response similar to isolation. Earlier research at the University of Illinois comprised full-scale laboratory tests of bearings, and computational models capturing full-bridge seismic response. This prior research validated the quasi-isolated ERS, demonstrated that most bridges in Illinois would not experience severe damage during a 75-year design life, and indicated calibration of fuse capacities might improve overall bridge response. Current research focused on the potential for bearing anchorage capacity to calibrate bridge response in the longitudinal and transverse directions. A suite of 24 bridges was created with variables that included superstructure type, pier height, and anchorage strength. A set of ten synthetic ground motions from the New Madrid Seismic Zone were scaled to match the AASHTO seismic design spectra for Cairo, Illinois, and applied in the longitudinal and transverse directions during a total of 480 nonlinear dynamic analyses conducted in OpenSees. Based on the sensitivity study, longitudinal response was largely insensitive to anchor bolt diameter, but transverse response displayed sensitivity and offered potential for performance calibration. Bridges with short piers and light superstructures were most responsive to anchor bolt calibration, and the use of smaller anchor bolts was effective at reducing column damage. The bearing sliding that followed anchor bolt fracture led to larger superstructure displacements, but never led to unseating or a risk of span loss