Experimental study of variable-frequency rocking bearings for near-fault seismic isolation

Near-fault earthquakes, which usually have a long-period pulse-like component, may result in excessive isolator displacement for a conventional isolation system, and consequently lead to an oversized isolator design or increase the risk of an isolation pounding effect. To alleviate these problems, seismic isolation using variable-frequency rocking bearings (VFRBs), which have variable mechanical property, are investigated experimentally in this study. A general VFRB has an axially symmetric rocking surface with variable curvature. By properly selecting the geometry of the rocking surface, the VFRB isolation frequency, which is independent of the super-structural mass, becomes variable and can be designed to meet the required specifications. To experimentally verify the VFRB isolation theory, a full-scale steel frame isolated by several prototype bearings is tested in this work by using a shaking table. To fabricate these bearings, a six-order polynomial function is proposed to define their rocking surface, so the bearings possess a relatively higher initial stiffness followed by softening mechanical behavior. The experimental results, which have very good agreement with the simulated ones, demonstrate that the prototype bearings exhibit the desired variable hysteretic property. The test results also show that the VFRB system with the proposed mechanical property is able to effectively suppress the excessive isolator displacement induced by a near-fault earthquake, while retaining a reasonably good isolation efficiency for the superstructure.