MCEER HIGHWAY PROJECT
FHWA CONTRACT DTFH61-98-C-00094
Seismic Vulnerability of the Highway System

Task D1-4: Property Modification Factors for Seismic Isolation Devices

Subject Area: Seismic Response Modification Devices
Research Year 3

Principal Investigator and Institution

Andrew Whittaker, University at Buffalo

Objective

The addition of requirements for the explicit consideration of the effect of changes in isolator mechanical properties on the response of isolated bridges represents a significant positive change in the 1999 edition of the AASHTO Guide Specification for Seismic Isolation Design. However, the commentary to the Guide Specification provides no guidance on the effect of these changes on system response, nor are limits set on the values of the system property modification factors below which no bounding analysis is needed. In addition, the system reduction factors for non-critical bridges "…are based on engineering judgment." Research conducted under this task will establish a limit on the system reduction factors for which no bounding analysis is required for design, and system reduction factors for essential and other bridges.

Approach

Component property modification factors were introduced in the AASHTO Guide Specification to account for variations in the mechanical properties of individual seismic isolators over the service life of the bridge. Property modification factors for the response of isolation systems were also proposed, but the commentary to the Guide Specification notes that "…the reduction factors for essential and other bridges are based on engineering judgment." Recently completed experimental studies will provide improved values for the component factors for elastomeric bearings. One question not yet systematically addressed is the effect of variations in the properties of individual isolators on the response of isolated bridges.

The proposed work will study variations in material properties from a systems perspective, using a one-span isolated overcrossing as a test bed. The two primary objectives of this task are to identify threshold values of the system factors below which changes in isolator properties need not be considered for design, and to establish a technical basis for the reduction factors for essential and other bridges based on the likely statistical dependence of the individual component factors (e.g., wear, contamination, and corrosion) and the time taken for the factor under consideration to achieve the maximum value (e.g., for aging of an elastomeric bearing, does the stiffness increase uniformly over the life of the bridge or is much of the aging effect realized in the first five years after fabrication?).

This task will systematically address the effect of changes in individual isolator response characteristics on the displacements and associated forces in seismically isolated bridges. The objectives are to: (a) identify threshold values of the system factors below which changes in isolator properties need not be considered in design, and (b) establish a technical basis for the reduction factors for essential and other bridges based on the likely statistical dependence of the individual component factors (e.g., wear, contamination, and corrosion). Isolators will be characterized using Qd and Kd, and varied as described above. A one-span overcrossing will be considered. Construction geometries and details for the bridge will be coordinated with a State bridge department.

To achieve the first objective, advice on acceptable variations in system responses from the calculated values, for which no bounding analysis would be required, will be sought from the State agency. The five bins of earthquake histories developed under Task D1-3 will be used to establish the corresponding limits on the system property modification factors. To establish a technical basis for the system factors, the dependence of each component factor on the other (e.g., the factors for wear and contamination), and the time required to achieve the maximum value as a percentage of the life of the bridge, will be considered. The effect of these system adjustment factors on the response of the isolated bridge will be studied using the five bins of earthquake histories. Mean and mean-plus-one sigma percentage changes in the displacements and forces will be reported, and the effect of these changes on the response of the superstructure and substructure will be documented.

Products

  • Procedures to establish system property modification factors from component property modification factors.
  • Guidance on when system factors will substantially influence the response of a seismically isolated bridge as a function of ground motion type.

Technical Challenges

The primary challenge of this task is to ensure that the various system factors are systematically and thoroughly assessed, so as to ensure that the various interactions between factors are adequately identified and documented.

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