A set of Neoprene isolation bearings remaining from a 1981 shake table test series conducted at the Earthquake Simulator Laboratory at the Earthquake Engineering Research Center (now the Pacific Earthquake Engineering Research Center) were retested in 2012. The new test series was conducted at the NEES site at the Richmond Field Station, University of California, Berkeley. The retest program had the dual purpose of determining the changes in the isolation properties, namely, the horizontal stiffness and equivalent viscous damping, and to study the behavior of the isolators at dynamic inputs that constitute beyond design-basis earthquake levels.
Two types of isolators were considered in the 1981 test series: the first made with a stiff compound (50 durometer), and the second made with a much softer compound (40 durometer). The 2012 study investigated a total of six isolators, three of each compound. The experimental testing revealed that a significant increase in horizontal stiffness and an associated decrease in equivalent viscous damping had occurred. The increase was larger—or occurred in a shorter time span—than results from other studies on the natural or accelerated aging of Neoprene isolators. The changes in properties are believed to be amplified by the annular design of the isolators considered, which increases the ratio of exposed surface area to the volume of Neoprene. Although the increase in horizontal stiffness in this study may be partially attributed to the lower vertical force applied to the bearings, the increase is primarily attributed to aging effects. In spite of the considerable changes in the horizontal stiffness and damping characteristics of the bearings, the isolation system provided adequate protection for the structure throughout a large number of earthquakes; many of which constituted beyond design-basis earthquake levels that generated extreme isolator displacements.
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