Seismic response of selective pallet racks with XY-FP isolators at the base subjected to far-field and near-field ground motions

Abstract

Past earthquakes have shown that selective pallet racks are susceptible to to pallet shedding and collapse along the cross-aisle direction. Seismic base isolation, a well-established and proven technology in buildings and bridges, is not widely adopted for steel pallet racks. The primary challenge lies in the wide range of pallet weights and unpredictable stacking patterns, making it difficult to design a base isolation system suitable for all load conditions. This is where XYFP isolators offer a significant advantage. During seismic action, these isolators generate a fundamental natural period independent of the weight placed on them. Additionally, these isolators feature a built-in uplift-restraining mechanism to prevent rack overturning during seismic action. The paper investigates the seismic behaviour of a selective pallet rack equipped with XY-FP isolators at their base. The parametric study explores numerous combinations of the isolator’s friction coefficient and radius of curvature, the pallet weight on the rack, and the diaphragm’s rigidity. Each baseisolated model is subjected to three sets of ground motions: far-field, near-field with PGV/PGA < 0.2, and near-field with PGV/PGA > 0.2. A nonlinear finite element analysis that accounts for the nonlinearity of the isolator and the geometric nonlinearity is employed. This research proposes ideal combinations of radius of curvature and friction coefficients for XY-FP isolators. By optimizing these parameters, peak shelf-level acceleration and isolator displacement can be kept within safe limits, effectively preventing pallet sliding and mitigating earthquake-induced damage. For far-field ground motions, most base-isolated configurations with friction coefficients of 0.11 and 0.14 at high sliding velocities (μmax) perform adequately. In near-field conditions with PGV/PGA < 0.2, only two configurations -μmax of 0.14 with natural time periods of 2.5s or 3.0s - meet the performance criteria. However, for near-field ground motions with PGV/ PGA > 0.2, there is no configuration that effectively constrains critical response parameters within reasonable bounds.