Performance of grout-filled tennis balls as seismic isolation bearings

This paper investigates the behavior of a low-cost seismic isolator comprising a grout-filled tennis ball rolling on concrete plates. The isolator can be used in lightweight structures in resource-limited areas. The study first characterized the compressive response of the system. Subsequently, full-scale tests with combined compression and lateral cyclic loading were performed. The key parameters examined include the velocity (frequency) of lateral displacements, the compressive load on the isolator, potential degradation of the isolators under repeated loading, the influence of temperature on the response, and specimen-to-specimen variability when different grout-filled spheres from the same mix are used in nominally identical tests. Results show that the lateral cyclic response of the isolators is bilinear and can be approximated by rigid-body equations. The experimentally obtained rolling friction coefficient is suitable for seismic isolation applications. Specimen-to-specimen variability was minimal. Unlike sliding isolation systems, increased compressive load results in a higher rolling friction coefficient and analytical equations are provided to describe this effect. The rolling friction coefficient is independent of velocity (frequency) and temperature, and the isolators exhibited no deterioration under consecutive loading.