Shake-table testing of low-cost seismic isolation bearings based on rolling rubber spheres

Seismic isolation is a well-established method of seismic protection. However, applications of this technology in low-income countries are limited due to the associated high costs. An experimental study of rubber spheres rolling on concave or flat concrete surfaces is presented herein. The cost of the proposed devices is sufficiently low to make them applicable in low-income countries. The application of the system in a residential building would comprise a large number of closely-spaced rubber spheres placed below the walls of the structure. This would allow for significantly reducing the thickness and the steel reinforcement of the concrete slab at the isolation level. Avoiding the construction of a heavily reinforced isolation diaphragm is necessary to make seismic isolation affordable in low-income countries. The rubber spheres isolators were subjected to monotonic uniaxial compression to examine their behavior under vertical loading. Subsequently, a total of 488 shake-table tests was performed to investigate the behavior of the system under seismic loading. Parameters of investigation were the geometry of the concrete plates (i.e., flat or concave), the diameter of the rolling sphere (i.e., 50 or 100 mm), and the applied compressive load (i.e., 2.08, 3.23, or 4.74 kN/sphere). Results showed that the isolators substantially reduced the acceleration transmitted to the superstructure while sustaining zero residual displacements. The dynamic behavior of the system mainly 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 would reduce the efficiency of the isolators.