Testing of a wind restraint for aseismic based isolation

Abstract

Base isolation has been proposed as an economical approach to aseismic design for many types of buildings and structural systems. In general, the base of a structure is isolated when a support system with extremely low horizontal stiffness limits the transmittal of horizontal shear from the ground to the structure. The displacement of such a structure under wind loading will, however, be excessive since the horizontal stiffness of the support system is so low. The horizontal deflection of a building with an isolated natural frequency of 0.5 Hz would, for example, be four inches for a wind load of one-tenth the weight of the building. While such a horizontal deflection would not be acceptable, a base isolation system could be equipped with a mechanical fuse which would be sufficiently strong to resist wind loading, but which would fracture during an earthquake, leaving the building free on the isolation system. In this report we describe an extensive series of shaking table tests of such a device. The wind restraint consisted of a notched shear pin. Several shear pins were tested in conjunction with a natural rubber isolation system placed beneath a three-storey 40,000 lb steel frame model structure. Pins with breaking forces ranging from 3% to 20% of the weight of the model were tested. The model structure was subjected to various peak accelerations of three earthquake inputs. The shear pins fractured rapidly and cleanly. The breaking force for each pin was reasonably predictable. Although higher mode accelerations were induced in the model structure when the pins broke, these disappeared rapidly. The design of a shear pin mechanical fuse system for a full-scale structure is discussed at the end of this report in view of the experimental results described.