Seismic response of bearings for quasi-isolated bridges- Testing and component modeling

In areas of moderate seismicity, such as Mid-America, there is interest in developing alternate bridge design and construction methodologies to the typical approaches that are employed in high-seismic regions. One such method is to design so that various bearing components will exceed their ultimate capacities during a major seismic event in a specific sequence, leading to a global bridge structure response similar to a seismically-isolated system. Because the bridge would typically exhibit a stiff response, only developing an isolated response during major earthquakes, the bridge superstructure response may be described as quasi-isolated. This paper presents results for elastomeric bearing components obtained for an experimental program performed to investigate and characterize various bearing types that are being used for quasi-isolated designs. The elastomeric bearings investigated displayed approximately linear elastic response before sliding with a friction coefficient in the range of 0.3 - 0.45, at a shear strain in the range of 150 - 250%. The experiments also investigated the response of anchored retainer brackets used to prevent transverse motion for service loads and small earthquakes to characterize the combined effects of shear and overturning up to and including rupture of the concrete anchor. The test data indicate that the required horizontal force delivered by a bearing to a retainer to cause a failure of the anchor is slightly higher than the ultimate pure tensile strength of the anchor.