Evaluation of quasi-isolated seismic bridge behavior using nonlinear bearing models

Seismic isolation is a well-accepted bridge design philosophy for providing earthquake resistance, but the design complexity and higher cost of construction can make this approach relatively less attractive in regions of moderate seismic hazard or with the potential for large earthquakes only at long recurrence intervals. Quasi-isolation is an innovative, yet economical and pragmatic, design philosophy that employs typical bridge bearings as fuses to ensure predictable seismic structural response. This paper presents computational models for evaluating quasi-isolated bridge systems, where certain bearing components can slide and limit the forces transferred between the superstructure and substructure. Nonlinear elements have been formulated to capture the local bi-directional stick-slip behaviors in the bridge bearings and the bilinear (and eventual fracture) behavior of steel retainers that limit transverse bearing movement. A bridge prototype is described, with the anticipated nonlinear behaviors in the structural components defined and implemented in a finite element model of the global structure. Static and dynamic pushover analyses are performed in both longitudinal and transverse directions to demonstrate limit states and progression of damage in the bridge structure. Results indicate that the abutment backwalls provide significant force capacity in the longitudinal direction. In the transverse direction, the force capacities of the retainers and fixed bearings have a significant influence on global bridge behavior and therefore should be appropriately proportioned to allow for effective quasi-isolation of such bridge structures.