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

Response history analysis of structures with seismic isolation and energy dissipation systems : Verification examples for Program SAP2000

SAP2000 is a recently released commercial structural analysis program with capabilities for dynamic analysis of structures with seismic isolation and energy dissipation systems. This report presents five verification examples in which results obtained by SAP2000 are compared to experimental results and to results obtained by programs 3D-BASIS and ANSYS. Three of the examples involve seismically isolated structures, of which one was tested on the shake table under conditions resulting in bearing uplift. The other two examples involve structures with linear and nonlinear fluid viscous energy dissipation devices, which were also tested on the shake table. In general, SAP2000 produced results in excellent agreement with other analysis programs and in good agreement with experimental results, except for the case of the structure tested with nonlinear viscous damping devices. In this case, SAP2000 underpredicted the displacement response of the structure

Response modification factors for seismically isolated bridges

The 1997 AASHTO "Guide Specifications for Seismic Isolation Design" specify response modification factors for the substructures of isolated bridges that are lower than those specfied for the substructures of non-isolated bridges. this report presents the rationale behind these specifications and presents reseach results that lead to the establishment of appropriate response modification factors for isolated bridges. The reseach concertrated on dynamic analysis of simple models of seismic-isolated and non-isolated bridges for a range of isolation system and substructure behaviors and for seismic excitation characterized by AASHTO ground motion spectra for a range of soil conditions and acceleration coefficients. the study investigated the displacement ductility demand in the substructure of these bridges and established the appropriate value of the ductility-based portion of the response modification factors.The study concludes that response modification factors should be lower in the substructures of isolated bridges than in the substructures of non-isolated bridges because : A) elastic or nearly elastic substructure behavior is required for proper behavior of the isolation system, and B) isolated bridges exhibit more sensivity in the substructure ineslastic response due to variability the seismic input (AU)

Seismic protective systems for bridges in the United States : State - of- the- art and State - of - practice

The use of seismic isolation and passive energy dissipation hardware for bridge applications has gained widespread acceptance in all seismic regions in the United States. Much research and the development of guidelines has accompanied themarket upswing in isolation-related construction. This review paper presents information on the state of the art and state of practice of seismic protective systems for bridges in the United States. Current and recently completed research and studies on system and component behavior are summarized, and the impact of this work on the newly revived AASHTO Guide Specification for Seismic Isolation of Bridges is identified.(AU)

Sensitivity of response of isolated bridges to modeling and design parameters : A case study

The concept of base isolations in bridges implies that structural components should experience limited inelastic behavior associated only with minor damage. Most isolation systems are designed to eliminate altogether inelastic behavior. The estimation uncertainties during the design process of the future ground motions, associated with the uncertainties involved in the constructed practice, leave the possibility that the entire system will ecperience inelastic behavior with permanent deformations and damage.(AU)

Property modification factors and response modification factors for seismically isolated highway bridges

The ballot version of the 1997 AASHTO Guide Specifications for Seismic Isolation Design (March 1997) [1] contains a number of modifications and improvements over its predecessor, the 1991 AASHTO Guide Specification[2]. Of these, two major changes are (a) the introduction of isolator Property Modification factors, and (b) the specification of substructure Response Modification Factors that are lower than those of non-isolated bridges. This paper presents the rationale behind these modifications and some reseach results that led to led to the establishment of the aforementioned factors. (AU)