Experimental investigation of a spherical rubber isolator for use in low income countries

This paper presents an experimental study of a low-cost seismic isolator that can be used for the protection of low-rise houses in low-income countries. The isolator is based on rolling of a rubber sphere on flat or spherical concrete surfaces. Using a closely spaced grid of such spheres may allow for avoiding of the diaphragm slab at the isolation level, or of reducing its thickness. Avoiding the cost of this extra slab is crucial for building seismically isolated low-rise dwellings economically feasible in low-income regions of the globe. The effects of the geometry of the rolling surface (i.e., flat or concave), of the diameter of the rolling sphere (i.e., 25, 50 or 100 mm), and of the applied compressive load on the seismic behavior of such isolation bearings were investigated. 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. Finally, a slab supported by 4 isolators was subjected to a group of 61 ground motions. Eight different configurations were tested, leading to a total of 488 dynamic tests. It was found that the restoring force of such systems can originate not only from the curvature of the concrete surface, but also from the vertical motion induced by the drastic change of the shape of the rubber spheres, as well. In fact, for the configurations tested, the vertical motion sourcing from shape of the deformed sphere was much larger than the vertical motion sourcing from the curvature of the concrete surfaces. Moreover, equations offering the coefficient of friction as a function of the vertical load, the size and the deformation of the sphere were derived. This is crucial for design, as 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. Dynamic tests proved that the proposed low-cost rolling rubber isolators can substantially reduce the accelerations transmitted to the superstructure The cost of the tested 25 mm, 50 mm and 100 mm diameter natural rubber spheres is 0.6 $, 3 $ and 17$, respectively.ISSN:0141-029