Experimental investigation of spherical rubber seismic isolation bearings

Seismic isolation is a mature and effective method of reducing earthquake-induced building damage. However, applications of this technology are limited to special and important structures, predominantly in the developed world, due to the associated high costs. An experimental study of a spherical rubber seismic isolator is presented herein. The cost of these devices is sufficiently low that their wide-spread application in low-income countries seems economically viable. Using a closely spaced grid of such spheres may require only a thin, lightly-reinforced diaphragm slab above the isolation level, further reducing construction costs. Avoiding the cost of this extra slab is crucial to make seismically isolated low-rise buildings economically feasible in poor regions of the globe. The examined seismic isolation bearings are based on rolling, with rubber spheres rolling on concave (spherical) or flat concrete surfaces. Concave concrete surfaces provide restoring force to the isolated structure through gravity. In contrast, in the case of flat concrete surfaces, no restoring force is applied. Rubber spheres offer increased damping and better stress distribution in the contact areas than spheres made using stiffer materials. This work investigated the effects of the geometry of the rolling surface (i.e., flat or concave), the diameter of the rolling sphere (i.e., 50 or 100 mm), and the applied compressive load on the seismic behavior of these isolation bearings. Initially, the rubber isolators were subjected to monotonic uniaxial compression to examine their behavior under vertical loading. Subsequently, cyclic tests were performed to obtain the lateral force-displacement diagram of the isolation system. It was found that the coefficient of rolling friction depends on the axial load, and the diameter of the spheres. The governing parameter for the design of the rubber spheres is not material failure, but excessive compressive deformation that leads to undesirably high rolling friction. Overall, experimental results proved the efficiency of the investigated system in terms of decreasing the inertia forces transmitted to the superstructure.