Comparison of Methods for Determining the Mechanical Parameters of Rubber Bearings

Based on existing specifications, the mechanical parameters of rubber bearings are primarily derived from the third hysteresis curve of its compression-shear test. However, when subjected to near-fault velocity pulse ground motion, the isolated structure will experience a greater concentration of energy input over a shorter period of ground motion, whereas the rubber bearing of the isolation layer typically only experiences one or two cycles of large horizontal deformation, which may lead to a significant departure from the desired outcomes when applying the mechanical parameters obtained from the third circle for the building’s seismic responses. In this paper, high-speed compression-shear apparatus is used to conduct a series of compression-shear tests at varying frequencies (0.20 Hz, 0.25 Hz, 0.30 Hz), and shear strains (50%, 100%, 250%) on rubber bearings. The mechanical properties corresponding to the first and third hysteresis curves of the laminated natural rubber bearings (LNR700) and lead rubber bearings (LRB700) are extracted and compared. In the third circle, an increase in the amplitude of shear strain results in a corresponding increase in the rates of change of the characteristic strength, horizontal equivalent stiffness, and equivalent damping ratio of the isolation rubber bearing. Under 250% shear strain, the characteristic strength, horizontal equivalent stiffness, equivalent damping ratio of LRB700 decrease by 34%, 15%, and 33%, respectively, compared to the first circle, while the horizontal post-yield stiffness of LRB700 and the horizontal equivalent stiffness of LNR700 remain essentially unchanged. Therefore, in the design of building isolation, the risk of increasing seismic response of the superstructure due to the increase of bearing characteristic strength and horizontal equivalent stiffness caused by instantaneous pulse ground motion must be taken into account.