Properties of Slender Elastomeric Isolation Bearings During Shake Table Studies of a Large-Scale Model Bridge Deck

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Title: Properties of Slender Elastomeric Isolation Bearings During Shake Table Studies of a Large-Scale Model Bridge Deck

Author(s): Ian G. Buckle and James M. Kelly

Publication: Special Publication

Volume: 94

Issue:

Appears on pages(s): 247-270

Keywords: axial loads; bridge bearings; buckling; dynamic tests; earthquake-resistant structures; isolation; models; rubber; stability; shear properties; static tests; Construction

Date: 9/1/1986

Abstract:
Elastomeric bearings are now being used as isolation bearings for the protection of bridges and buildings from earthquakes. In most cases, the bearings are of the same general form as those used in conventional bridge design for the relief of thermal and creep movements. However, isolation bearings are thicker than standard bearings and are expected to perform to much higher levels of shear strain than usually encountered under nonseismic conditions. Furthermore, some isolation bearings are provided with lead cores to dampen seismic vibrations and to reduce the shear displacement demand. These modifications were studied, inter alia, during a recent project on the shake table at the University of California at Berkeley. Here the seismic performance of a number of natural rubber bearings, with and without lead cylinders, was investigated. The bearings had been designed to isolate a single span of a large-scale model bridge deck that was mounted on the table in variously skewed configurations. To satisfy the dynamic similitude laws for the model, the bearings (8« in.ý in plan by 7 7/8] in. high) were unusually slender by current code definitions and outside the range of height/width ratios commonly accepted for stability in the U.S. The paper describes the design of the bearings, the preliminary static tests undertaken to establish fundamental stiffness and buckling loads, and their dynamic performance. Of particular interest is the confirmation of the theoretical buckling loads using a Southwell Plot procedure, the reduction in lateral stiffness due to vertical load, and the confirmation of limiting shear displacements defining the onset of overturning instability.