Property modification factors for seismic isolation bearings

The overall objective of this task was to develop an improved understanding of the expected performance and behavior of sliding isolationsystems;evaluate their functionality, reliability, and longevity; and assist in the development of improved codes and specifications for their use. This report deals with the problem of establishing upper and lower bound values of properties of seismic isolation bearing for use in the analysis and design of seismicallyisolated bridges. These bounding values of properties are determined by using system property modification factors. The concepts and the values presented represent the basis on which bounding analysis is described in the new 1999 "AASHTO Guide Specifications for Seismic Isolation Design"

Experimental study of bridge elastomeric and other isolation and energy dissipation systems with emphasis on uplift prevention and high velocity near/source seismic excitation

This report focussed on three parts: (a) the design and construction of an isolator testing machine, (b) the testing and modeling of prestressed isolators, and (c) an experimental study of bridge elastomeric isolation systems with an emphasis given to near source high velocity seismic excitation. An isolator testing machine, that overcomes intrinsic difficulties encountered in such testing, was designed and constructed. The machine is capable of testing a variety of isolators under controlled conditions of axial load, lateral displacement and rotation. The prestressing of isolators for preventing uplift or tension has been experimentally investigated for the purpose of demonstrating its effects on the behavior of isolators and for evaluating the validity and accuracy of theoretical predictions of the behavior of prestressed isolators. Flat sliding bearings, spherical FPS bearings and elastomeric bearings were tested prestressed within the developed testing machine under imposed combined horizontal displacement and variable axial load. A theory of prestressed isolators is presented and evaluated on the basis of the experimental results. Earthquake simulator tests were performed on a quarter scale bridge model representing a two span bridge. A total of four isolated configurations were studied. These consisted of a low damping elastomeric isolation system without and with linear and nonlinear viscous dampers, and a high damping elastomeric isolation system. In addition, three non-isolated configurations, without and with dampers, were investigated. The testing of these systems had multiple objectives such as: (a) Observation of the behavior of high damping elastomeric isolation systems under conditions of changing bearing properties due to the phenomena of scragging and related recovery. (b) Comparative study of the effectiveness of elastomeric systems and elastomeric systems enhanced with viscous dampers. (c) Observation of the behavior of elastomeric systems enhanced with linear and nonlinear viscous dampers in near-field source high velocity seismic excitation. (d) Study of the behavior of non/isolated bridges with supplemental energy dissipation systems. (e) Demonstration of the significance of enhanced damping in isolation systems for the reduction of displacement demand and the effective redistribution of the force transmitted to the substructure. (f) Investigation of the validity and degree of accuracy of currently available analytical tools for the prediction of the dynamic response of bridges equipped with seismic isolation and energy dissipation systems