Experimental testing of rolling seismic isolators using rubber spheres or grout-filled tennis balls

This study presents the experimental testing of two low-cost, high-performance seismic isolators based on rolling. The different systems comprise: a) deformable spheres made of polyurethane and b) grout-filled tennis balls. In both cases, the spheres roll on concrete plates. When using multiple closely spaced isolators, the systems can isolate lightweight structures. The isolators were initially tested in compression to quantify their bearing capacity under gravity loading. Subsequently, the isolators were tested in lateral cyclic loading under constant compressive load. Parameters of investigation included the sphere type (polyurethane or grout-filled tennis ball), the diameter of the polyurethane spheres, the geometry of the concrete plates (flat or spherical), the testing velocity, and the compressive load. Results showed that the bearing capacity of the proposed isolators is substantially larger than the design compressive load. The polyurethane spheres deform under compression, and their shape becomes oval-like. This influences their restoring force, and their rolling response is flag-shaped. Polyurethane is velocity-dependent, with higher velocities leading to higher energy dissipation. Grout-filled tennis balls have a rigid-like response, and their rolling response is bilinear and velocity-independent. In both systems, the rolling friction coefficient ranges between 3% and 12% and increases with increasing compressive loads. Moreover, the use of spherical (concave) concrete plates increases their restoring force. The low cost of the systems (below 30 USD) can allow for their widespread use in a range of applications.