AN EXPERIMENTAL STUDY ON PRACTICAL USE OF A LEAD RUBBER BEARING WITH FAIL-SAFE MECHANISM

The authors have been developing a new seismic isolation bearing, which consists of a lead rubber bearing (LRB) and a slider bearing as a fail-safe (FS) mechanism in series. The bearing called “FSLRB” behaves as a general LRB which is just friction connected to the building structure at either of upper or lower end, when subjected to design level earthquakes or smaller, while the sliding behavior keeps LRB away from giving excessive hardening and shear break in severe earthquakes beyond expectations. Hence, FSLRB has much higher seismic safety compared to general isolation bearings including conventional LRBs. In this paper, dynamic and quasi-static horizontal loading tests under constant or varying compressive loadings for several kinds of scaled and full-scale FSLRB specimens are conducted, to evaluate the effect of bi-directional excitations on additional torsion in the LRB part and changes in friction properties in the slider part. Ultimate performance limit of a full-scale bearing is also investigated under extremely high surface pressures beyond expectations. Dynamic loading tests of two types of □-200mm scaled specimens, each has different second shape factor in the LRB part, make clear that there is no apparent difference in friction properties between uni-directional and bi-directional excitations, and no damage to the LRB part due to the torsional behavior in bi-directional excitation, as far as the bearing is applied under a reasonable surface pressure around 10 N/mm2. Bi-directional dynamic loading tests of a □-900mm full-scale specimen show that there is no significant scale-effect between the full-scale FSLRB and the □-200mm scaled one on both the torsional behavior of the LRB part and friction properties of the slider part. Uni-directional dynamic loading tests of □-600mm full-scale specimens indicate that dynamic and static friction coefficients have quite low velocity dependence in reasonable sliding speed expected in the practical use. Finally, uni-directional quasi-static loading tests of □-600mm specimens show that the bearing should be applied under surface pressure of 20 N/mm2 or lower, to give a well-ordered tri-linear hysteresis curve without causing any unexpected incidents such as partial uplift of the slider part and buckling or shear break of the LRB part. Furthermore, surface pressure dependence of dynamic and static friction coefficients are inductively estimated applying all the data obtained in the series of the research, in