Task C3-3: Seismic Retrofit of Steel Truss Piers
Subject Area: Special Bridges - Substructures and
|Principal Investigator and Institution
Michel Bruneau, University at Buffalo
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.
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.
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.
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.
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