Behaviour and Modelling of Lead-Rubber Bearings Subjected To Tensile Actions

Abstract eng:
The general backbone curves for Lead Rubber Bearings (LRB) uses the tension stiffness as equal to the compression stiffness (up to 2G force). This leads to impractical unit loads for the manufacturers bidding for the supply to satisfy very high “ghost” tension forces (generally in MN), together with misleading design loads for the design of the structures and its connections to the isolation plane. Current backbone curves were determined using small units (eg. 180-400mm Dia), stiff rubber (eg G100) and often comparatively large lead cores (eg 60mm). These sizes are much smaller that units that we place under buildings: (600-1400mm dia), with softer rubber (G40-G60), and significantly smaller plug/bearing size ratio. By including a tension test as part of our production protocol, we have collected a broad data set, allowing us to better determining tension stiffness of LRB units. The tension testing has been undertaken on full-scale LRB of various sizes, heights and lead core diameters. An obvious trend in these data highlights that the units are significantly softer in tension that the current backbone curves utilized in modelling by consultants. Presented results show that LRB tension stiffness is non-linear in nature, however can be closely approximated to a bi-linear behavior with a defined lead yielding (Q dt ), and a post yield rubber stiffness (K vt ). The initial stiffness, attributed to the lead core (Q dt ), can be approximated from the core diameter, and the post yield tension stiffness of the rubber (K vt ) can be related back to the compression stiffness (K vc ) of the unit. There is also a noted scale effect that occurs with smaller diameter stiffer units converging on K vc approx. equal to K vt , which highlights issues with small scale testing and extrapolation to real size units. Results in the data collected show the effective axial tension stiffness (K vt,eff ) of LRBs varies according to the size of the unit and has a ratio of K vt,eff /K vc from 1/10 (small diameter) to 1/80 (large diameter) . The axial tension testing also revealed the LRBs can sustain significant elongation while remaining below the currently accepted cavitation limits (between 2G - 3G). Testing for the unit results elongation will be in the order of 20-30mm before the lower bound cavitation forces are reached (10-12% tension strains in unit rubber height), however the tensile stiffness from the test results is much softer than the recommended backbone curves of K vc up to 2G loads. From the data presented in this paper from approximately 190 full scale tests, there is minimal degradation in the post yield rubber stiffness upon repeat cycles, when working below 3G forces. The maximum MCE uplift displacements from the computer model should be set as the lower bound limits for the elongation of the units tested. This paper provides a simplified method of modelling tension stiffness in LRB units to more accurately predict analysis tension forces in units. Actual tension stiffness should be verified by prototype, and production testing

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