| 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. |