Effects of Vertical Load, Strain Rate and Cycling on the Response of Lead-Rubber Seismic Isolators

Abstract

Lead-rubber isolators represent a valid and economic solution for the seismic isolation of bridge structures and modern manufacturing techniques make available large devices. Velocity effects on small to medium-scale isolators have been discussed by several authors (e.g., Clark et al., 1997 Clark, P. W. , Aiken, I. D. and Kelly, J. M. 1997. Experimental studies of the ultimate behavior of seismically isolated structures, Berkeley, CA: Earthquake Engineering Research Center, University of California. Report No. UCB/EERC-97/18 [Google Scholar] ; Thompson et al., 2000 Thomson, A. C. , Whittaker, A. S. , Fenves, G. L. and Mahin, S. A. 2000. “Property modification factors for elastomeric seismic isolation bearings”. In Proceedings of the 12th World Conference on Earthquake Engineering, New Zealand: Auckand. January [Google Scholar] ) as well as included in reports of experimental programs (e.g., CERF, 1999 CERF. Civil Engineering Research Foundation, Summary of evaluation findings for the testing of seismic isolation and energy dissipation devices. CERF Report No. 40404. 1999. [Google Scholar] ). Only recently, however, the behavior of large devices was validated under full-scale displacements, loads, and velocities. In this article, results obtained from an experimental investigation on the effects of axial load and strain rate on the performance of a full-scale lead-core elastomeric bearing for bridge applications, are reported. The bearing response was analyzed with particular attention to the variation of critical performance characteristics in order to produce a set of information that could be implemented in a physically motivated numerical model. The results, in line with additional tests performed on similar full-scale bearings at the Caltrans SRMD Testing Facility at the University of California San Diego, indicate a moderate effect of the applied vertical load but a significant effect of the strain rate and cycling on all the significant response parameters. This information should be taken into account by designers, particularly when high component of velocities are associated with the expected seismic motion. A simplified numerical model is proposed for the assessment of lead-rubber bearing performance.