Noninvasive Methods to Estimate the Mass Eccentricity of Steel-Cored Elastomeric Spheres for Seismic Isolation

Rolling isolation systems are a promising cost-efficient alternative to conventional seismic isolation (e.g., rubber bearing or frictional devices). Recent studies have suggested a rolling isolation system based on elastomeric spheres with a steel core that can roll along a concave surface. The concave surface provides a restoring force based on gravity (i.e., as the pendular component of frictional pendulum systems), whereas the elastomeric material dissipates energy (from the rolling resistance). A centered steel core benefits the system in two aspects: it reduces vertical deformations while increasing vertical-loading capacity and improves the rolling mechanism’s behavior. However, the centeredness of the steel core is not easily achieved during the manufacturing process, compromising the performance of the whole isolation system. An accurate and fast estimation of the core eccentricity will allow an early assessment of spheres to be applied as a seismic isolation device. Four noninvasive methods for estimating the core eccentricity of the spheres are evaluated (X-ray computed tomography, camera-based motion tracking, tilt tests, and average period estimation), of which all except X-ray computed tomography use the system’s equation of motion to estimate the core eccentricity. All methods agreed well with the results of the computed tomography, identifying the core eccentricity, with mean errors below 15%. This finding shows that these methods are good candidates for developing an easy, fast, and reliable prequalification procedure for the spheres.