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PERFORMANCE ASSESSMENT AND DESIGN OF LEAD RUBBER SEISMIC ISOLA TORS USING A BILINEAR SPECTRUM METHOD By WEIXIAO SUN, B.ENG.SOC A Thesis Submitted to the School of Graduate Studies in Partial Fulfillment of the Requirements for the Degree Master of Applied Science McMaster University © Copyright by Weixiao Sun, April 2011
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DESCRIPTIVE NOTE MASTER OF APPLIED SCIENCE (Civil Engineering) McMaster University Hamilton, Ontario TITLE: Performance Assessment and Design of Lead Rubber Seismic Isolators Using A Bilinear Spectrum Method AUTHOR: Weixiao Sun, B.Eng.Soc. (University of Ottawa) SUPERVISOR: Dr. John C. Wilson NUMBER OF PAGES: ix, 105,A2,B3 ii
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ABSTRACT Seismic isolation has been widely adopted for structural protection. This technique, which introduces a flexible layer between the structure and the support, isolates the structure from earthquake ground motions by lengthening the structural period. The lead rubber bearing (LRB) is one of the most commonly used seismic isolators. The sizes of the rubber bearing and the lead core determine its stiffness and damping characteristics. The parameters, which characterize the seismic performance of a LRB, are the elastic stiffness (k 1 ), post-elastic stiffness (k 2 ), yield strength (Fy) and the total weight (w) of the isolated structure. In this study, an assessment of the nonlinear performance of LRB isolators is carried out using a series of spectra, which are referred to as bilinear spectra, as they are based on the bilinear behaviour of LRBs. The LRB parameters are non- dimensionalized using post-to-pre elastic stiffness ratio (n=k2/kl) and yield strength to weight ratio (r=Fy/w) to construct the bilinear spectra. Feasible ranges of nand r have been considered according to design code recommendations. The spectra are constructed from statistical analyses of LRB responses due to sets of real earthquake ground motions. These spectra plot the displacement and the shear force response of isolated structures for various combinations of nand r, vs. the elastic period. The results of the study show that displacement decreases as the lead content increases, as expected. However, the corresponding shear forces fluctuate over different isolated periods. An increase in the rubber bearing size increases only the shear response, but has iii
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negligible influence on the displacement. It is also found that earthquakes with a lower ratio of PGAlPGV tend to result in higher displacement and shear force responses of the LRB compared to ground motions with higher PGA/PGV ratios. A new chart-based method (referred to as the Chart Method) is developed by using a regression-based bilinear spectrum for estimating the LRB isolator displacement and shear force responses. The design capability of the Chart Method is compared to a more conventional method for designing LRBs, by solving several examples. The study concludes that the Chart Method has improved accuracy and versatility and can be used to evaluate the design suitability of commonly available LRB sizes.
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