| Principal Investigator and Institution
Michel Bruneau, University at Buffalo
Objective
A large number of steel truss bridges were constructed throughout the
United States when earthquake-resistant design was not a major
consideration. Many of these bridges are located in zones of moderate to
high seismicity, and are therefore likely to be subjected to a major
earthquake over their remaining service life. Recent structural analyses
of some of these bridges indicates that they may suffer significant damage
and a risk of collapse. Contributing significantly to this undesirable
behavior are the latticed built-up members typically used in these bridge;
they possess a high potential for buckling under compression and have
little effective ductility.
The primary objective of this task is to investigate the seismic
behavior of these structural elements, in order to quantify the range of
reliable energy dissipation for these members. This knowledge will help
establish the threshold below which seismic retrofit is not necessary.
However, other components in these bridges are also vulnerable, and may
require retrofitting. For example, the diagonal braces in these piers are
elements that may be the first to fail during an earthquake, and are thus
targeted for retrofit. Retrofit strategies focusing on strengthening are
not advocated, as this will typically require proportional strengthening
of the entire truss pier, along with the anchors and foundations.
Therefore, the objective here is to develop seismic retrofit strategies
that can effectively enhance structural ductility of the braces while
controlling the maximum forces transmitted to the adjacent structural
elements, in a capacity design approach.
Approach
The first year of this two-year task is an exploratory research program
that will investigate and compare the merits of various innovative
retrofit strategies. The initial intent is to develop schemes that would
make it possible to require retrofit to only the lower braced-panel of a
steel truss pier, as this would provide a most economical solution.
However, solutions applicable to all panels along the height of the pier
may also be considered. The following are some of the retrofit strategies
that will be considered under this task:
Ductile steel energy dissipation inserted at the brace intersection
point - This approach relies on the introduction of members specially
detailed to dissipate energy while controlling the maximum force
transmitted to the existing structure. These members could be inserted at
the brace attachment points, preferably where the braces intersect in the
middle of the panel. Under large lateral loads, these structural
"fuses" will sacrificially yield and dissipate energy; the laced
members should remain undamaged as they are effectively capacity
protected. Immediately following an earthquake, any damaged fuses can be
replaced and the bridge can be quickly restored to full service.
Unbonded brace encasement - This is an approach that can be used
in those instances where the strength or details of the existing braces
are such that capacity protection of the braces is not viable. Encasement
of the existing brace (e.g., wrapping it in a special membrane so that it
is unbonded from the encasing material) makes it possible to have a
bracing member capable of axially yielding both in tension and
compression. This would require special details to ensure yielding of the
member prior to fracture at the member connections.
Replacement of first-tier panel brace members - In some
instances, the best retrofit solution may require entirely replacing the
bracing members in the lower panel by new specially designed and detailed
members that are able to dissipate energy in an optimal manner, providing
the desirable capacity protection to the rest of the structure. This would
be possible only for structures in which this local concentration of
energy dissipation is advisable.
Products
Seismic analysis and retrofit guidelines for steel truss bridge piers,
in the format that is compatible with the design and retrofit manual being
developed under Task C1-1.
Technical Challenges
The primary challenge of this task is to develop practical seismic
retrofit strategies based on capacity design principles and that are can
be easily adopted by the various transportation agencies. The ideal
retrofit strategy should be economical, simple to accomplish, and durable. |