Shake-table testing of resilient rolling seismic isolators based on deformable rubber spheres.

This study presents the results of a large-scale experimental investigation of sustainable and low-cost seismic isolators based on deformable rubber spheres, rolling on concrete surfaces. Different types of spheres, with or without a steel core were tested. Both flat and spherical (concave) concrete plates were investigated. A potential application of the proposed isolator could be in low-rise masonry structures in the developing world. The spheres were initially subjected to monotonic uniaxial compression and sustained compression under vertical load to examine their compressive behaviour. Subsequently, lateral cyclic tests under large displacements were performed. Finally, a total of 1170 shake-table tests were performed in 1:2 scale, with various different isolators subjected to a large number of ground motion excitations. Both in the lateral cyclic and in the shake-table tests, different vertical loads were investigated to quantify the influence of the vertical load on the response. Results showed that the compressive strength of the spheres was substantially higher than the estimated design load, whereas the presence of the steel core made the spheres stiffer. The lateral cyclic response differs substantially from the prediction of a rigid body model due to the non-negligible deformability of the spheres. The rolling friction coefficient ranged between 3.7% and 7.1%, with these values being suitable for seismic isolation applications. A higher vertical load leads to a slightly higher value of the rolling friction coefficient and increased energy dissipation. The spherical concrete plates increase the restoring force of the system. When tested in a shake table under 1170 ground motions, the isolators substantially reduced the acceleration transmitted to the superstructure (to approximately 0.15 g) while maintaining reasonable peak and zero residual displacements. Notably, the shake table tests were repeatable, and the isolators did not deteriorate even after subjected to 65 ground motion excitations.

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