Nonlinear seismic analysis of quasi-isolation systems for earthquake protection of bridges

Quasi-isolation is a modern bridge seismic design philosophy where nonlinearity is permitted to occur in specific bearing components such that forces transferred into the substructure are reduced and isolation is achieved by sliding of the bearings. The system is a pragmatic approach for providing earthquake resilient bridges in locations such as the eastern and central United States, as well as in many locations around the world where there is significant earthquake risk at long recurrence periods. Such a seismic risk does not typically justify the design of a rigorous classical isolation system, but instead, a low-complexity, low-cost quasi-isolation approach could provide significant mitigation of earthquake effects. The proposed system employs a set of fixed bearings at one intermediate substructure, and all other substructures are instrumented with isolation bearings that permit thermal expansion such as elastomeric bearings with an elastomer-concrete sliding interface or elastomeric bearings with a PTFE (Teflon) to stainless steel sliding interface. L-shaped steel side retainers are placed in the transverse direction of the elastomeric bearings, and along with the low-profile fixed bearings, these components prevent bridge movement during service