High-strain tests on lead-rubber bearings for earthquake loadings

Abstract

Dynamic tests on two lead-rubber bearings, 280 x 230 x 113 mm, are described in which a dynamic actuator was used to apply shear displacements of ±10 to ±140 mm at frequencies of 0.1 to 3 Hz, giving shear strains in the rubber of up to ±200%. The weight of the structure on the bearing was applied by a vertical jacking system and ranged from 35 to 455 kN. Sequences of cycles were performed to simulate earthquake conditions and force-displacement hysteresis loops were obtained. It was concluded that at strains approaching 200% in the rubber, the force-displacement hysteresis loop could be represented by the linear elastic slope of the rubber component plus a rectangle appropriate to the plastic shear of the lead, with some rounding of the corners of the hysteresis loop. There was a decrease in the area of the hysteresis loop of 20% after six cycles at 1 Hz with a maximum shear strain of 125% in the rubber; but recovery followed within minutes. In one bearing an attempt was made to locally confine the lead by screwing external plates to the first internal plate within the bearing at the top and the bottom, but no difference in performance between the two bearings was noted. The scope of the tests indicated that, with peak strains in excess of 100% in the rubber, the bearings would continue to perform satisfactorily for a sequence of very large earthquakes.