Performance of a Seismically Isolated Bridge under Near-Fault Earthquake Ground Motions

A bridge with lead-rubber seismic isolation bearings was field-tested to evaluate the assumptions and uncertainties in the design and construction. A numerical model was established based on satisfactory predictions of the dynamic characteristics under ambient vibration, free vibration tests, and the seismic performance under the 1022 Gia-Yi earthquake. Parametric studies of this model, under simulated near-fault ground motions, were carried out to investigate the near-fault effect. Two types of velocity pulse that characterize the near-fault effect were investigated. Results revealed amplified seismic response when the pulse period was close to the effective period. The response amplification by a longer period pulse was not consistent for the two pulse types. Additional observations included the ratio of dissipated energy by the lead-rubber bearings to the total input energy, which could be influenced by the near-fault effect. Variation of the near-fault effect due to a change of structural parameters was studied to provide information on possible mitigation strategy.