Seismic Performance of Quasi-Isolated Highway Bridges in Illinois

The Illinois Department of Transportation (IDOT) commonly uses elastomeric bearings to accommodate thermal deformations in bridges, and these bearings 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. Project R27-70 was conducted to validate and calibrate the quasi-isolated ERS based on full-scale laboratory tests of bearings and computational models capturing full-bridge seismic response. The overall report is divided into two volumes. Volume 1 discussed the experimental program. This second volume focuses on the analytical program but also contains retainer design recommendations. Results from the experimental testing program were used to develop constitutive bearing models, which were incorporated into the finite element model of a three-span bridge with simply supported abutments and fixed bearings at one pier. A suite of 48 bridges was created to represent the most common highway bridge configurations in Illinois. Variables included superstructure type, pier type, pier height, elastomeric bearing type, and foundation flexibility. Two sets 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. A total of 12,000 nonlinear dynamic analyses were conducted in OpenSees at six scale factors from 0.5 to 1.75 and used to create coarse incremental dynamic analyses. On the basis of the findings of the parametric study, most bridges in Illinois would not experience severe damage during a 75-year design life, and bearing unseating or span loss are not likely to occur in regions with moderate seismic hazard. Piers with fixed bearings commonly yielded for small earthquakes, but future calibration of fuse capacities may improve this behavior.Illinois Department of Transportation R27-70published or submitted for publicationnot peer reviewe