Many of the bridges in the United States are being used beyond their initial design intentions, classified as structurally deficient, and are in need of rehabilitation or replacement. A portion of these bridges suffer from specific bridge vulnerabilities that have been categorized as fracture prone. The safe operating life of fracture prone details is governed by the stress range experienced by the detail. Providing alternate load paths through a supplemental apparatus attached to the bridge structure can relieve high stress ranges, and the limited safe bridge service life due to these vulnerabilities may be safely extended. As part of the apparatus, the utility of a mechanical amplifier, the scissor jack, is carefully investigated; the amplifier allows for a very localized application and much smaller stiffness and damping device demands. The mathematical relationships for the apparatus, in particular the magnification factor for displacement and force, are formulated analytically and verified through numerical modeling. The effects of the mechanical amplifier are investigated on a simple beam numerical model as well as through more comprehensive parameter studies on numerical bridge models of an in-service fracture critical bridge. The parameter studies reveal that longer apparatuses and larger cross-sectional member area improve performance. A relatively small passive stiffness and damping device provides adequate safe life extension when employing the mechanical amplifier and vastly outperforms an apparatus without the amplifier. The apparatus parameters are optimized through a series of simulations, and small amounts of device damping with no stiffness perform the best. Much larger damping and stiffness coefficients are necessary to achieve similar performance without the mechanical amplifier. Safe life extension of over 100 percent can be achieved with apparatus member cross-sectional area of 25 percent of the bridge girder area. For implementation on a general bridge, a long and slender mechanically amplified RM apparatus is recommended for safe life extension. For a passive system, a RM device with a small damping coefficient and no stiffness should be employed. The cross-sectional area of the RM apparatus members will need to be sufficiently large to provide adequate safe life extension and will have to be evaluated on a case-by-case basis. A simple bridge model should be used to gauge initial member size. Frequency response analyses of the modified bridge structures show response amplification at some loading frequencies. Analyses also found different optimal device characteristics for decreasing the magnitude of the maximum or minimum moment range experienced at the vulnerability. These findings lend support to the hypothesis that semi-active control strategies allowing for changes in device characteristics may ultimately be more beneficial and should be further investigated.
University of Minnesota Ph.D. dissertation. December 2013. Major: Civil Engineering. Advisor: Steven J. Wojtkiewicz. 1 computer file (PDF); xv, 158 pages.
Innovative approach for Lifetime extension of an aging inventory of vulnerable bridges.
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