| Abstract | High Damping Bridgestone Isolation Bearings are used in several recently completed base isolated buildings in Japan. These isolators carry the vertical load, provide low horizontal stiffness and generate significant damping, thus obviating the need for additional mechanical energy dissipating mechanisms. These bearings are produced in several sizes up to very large diameters and are guaranteed by the manufacturer to be able to sustain shear strains of around 400%. Four high damping bearings were obtained from Bridgestone and tested at the Earthquake Engineering Research Center to verify their dynamic properties and their failure mechanisms. The test program included fully reversed horizontal shear tests at 0.5 Hz. These tests involved five cycles of horizontal shear from a low of 5% shear strain to a maximum of 350% shear strain, each series carried out at four different levels of vertical pressure, namely, zero pressure, 500 psi, 1000 psi, and 1500 psi. Each bearing was subjected to five cycles of vertical loading centered at three levels of vertical pressure. Each bearing was subsequently tested under monotonic loading to failure under four levels of vertical pressure. It was found that the failure strain is very high and is somewhat sensitive to pressure ranging from 475% at the highest pressure, 10.34 MPa (1500 psi), to 560% at zero pressure. At both zero and 3.45 MPa (500 psi) pressure the displacement at failure exceeds the diameter of the bearing. The most significant aspect of the bearing failure besides the high levels of strain is the fact that the bearing stiffness increases by a factor of 6 when the strain is greater than 250%. The breaking force at failure is at least 6 times tha force at 200% strain. It follows that if a structureis designed to be just at yield when the isolators are at 200% the structure will have reached coolapse before the isolators fail. This means that the weakest element in the overall structural system will not be the........ |
