Seismic Performance of Isolated Bridges Under Beyond Design Basis Shaking

Abstract

Seismically isolated highway bridges are expected to provide limited service under a safety evaluation-level ground shaking with minimal to moderate damage. The behavior under shaking beyond design considerations, corresponding to a large return period seismic hazard, is not well understood and could induce significant damage. In these rare events, the seismic isolation system can be subjected to displacement demands beyond its design capacity, resulting in failure of the bearings, exceeding the clearance and pounding against the abutment backwalls, or damage propagating to other primary structural components. To better understand the seismic performance of simple highway bridges subjected to earthquakes beyond design considerations, this study simulates the response of a prototype bridge structure and examines the lateral displacement demands, the transfer of forces to the substructure, and potential failure modes of seismically isolated bridges. Advanced modeling approaches are considered to capture bearing characteristics, such as hardening at large strains, and a pounding macro-element to capture the effects of impact. Results show that for beyond design shaking, the bearings can reach the maximum shear strain capacity, significant residual deformation of the abutment can result from pounding, and the columns can experience moderate damage. The progression of damage is identified in an effort toward the development of models suitable for assessing the overall seismic risk, repairability, and downtime of seismically isolated bridges.