Experimental Study of the Seismic Behavior of Hinged-Rubber Bearing

Although seismic base isolation has been implemented for several decades for global protection of structures from the effects of earthquake shaking, it has not been routinely adopted for local seismic protection of sets or piece of equipment and components in essential facilities. In this study, effectiveness of a type of seismic isolation system called hinged-rubber bearing (HBB) for seismic protection of equipment is investigated. The HBB consist of rubber bearing mounted on a hinge joint. The bottom hinge joint comprised a rolling ball that can rotate in all directions. The main objectives are: (1) to introduce an isolation system and contribute new knowledge on the bi-directional behavior of hinged-rubber bearing isolated systems under earthquake ground and floor motions; (2) to study the potential uses of HBB as passive protective system for equipment. To achieve the overreaching objectives, physical earthquake simulation tests were performed on an isolation platform that consisted of a rigid steel frame supported by four HBB isolators.  Concrete blocks were used as artificial mass to simulate the self-weight of a relatively heavy equipment. The isolation platform was subjected to recorded ground and floor accelerations of multistory buildings under historic earthquake events. Percentage reduction of the simulated equipment acceleration (i.e. ratio between maximum acceleration at top of the mass to maximum earthquake-simulator acceleration) was served as a performance index to evaluate the efficacy of the isolation system. Experimental test results evidenced that in most cases the isolation platform was effective in reducing acceleration demand above the isolation plane. Accordingly, acceleration ratios that ranged from 26.89% to 96.65% were obtained. In some cases, due to the influence of building dynamic properties on the response of the isolation system, 114.06% to 123.08% acceleration amplifications were observed. Clearly, when recorded ground acceleration of a 13-story commercial building under the 1987 Whitter earthquake at Sherman Oaks was applied, the acceleration ratio was 38.76%. In another test sequence in which the eighth and roof floor acceleration records of the same building under the same earthquake event were used, the percentage reductions were 50.17% and 51.73% respectively. These implies that the seismic performance of HBB is affected by floor level. Recorded base shear-displacement hysteresis loops of the isolators depicted that friction developed between the rotating ball of the hinge connection and it’s casing had contribution to the overall energy dissipation capacity of the device. In general, the test results demonstrated the feasibility of Hinged-Rubber bearings in attenuating seismic effects on relatively-heavy equipment such as cooling towers under earthquake ground shaking.