A Preliminary Study of Shake Table Response of A Two-Column Bridge Bent on Flexible Footings

Report No. CCEER 02-3The main objective of this study was to determine the effect of footing flexibility on the dynamic response of large-scale two-column bridge bents subjected to earthquake loads simulated on a shake table. A quarter-scale bent with two square columns supported on spread footings was the subject of the study. The beam and the columns incorporated several innovative features with respect to the plastic hinge location and application of carbon fiber reinforced plastics. However, these issues were not the focus of the study presented in this report. Of particular relevance to this study was simulation of translational and rocking flexibility of soil through elastomeric bearing pads that were placed underneath and on the sides of the footings. Three footing configurations were studied. One was with substandard footings resting on flexible supports, another with retrofitted footings resting on flexible supports, and the third with footings that were fixed to the shake table. The testing of cases with flexible footings was essentially in the linear range so that the frame above the footings could be saved for the fixed base study, in which loading continued until failure. Results showed a substantial reduction in the steel bar strains near the base of the columns as a result of footing flexibility. Reduction in strains elsewhere in the bent and the footings was also significant, but to a lesser extent. The results strongly suggest that retrofit requirements at column bases to address lap splices and low shear and confinement reinforcement may be reduced when the beneficial effects of soil flexibility is accounted for