United States Department of Transportation
i
Load Rating of Existing Continuous Stringers on Louisiana’s Bridges
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2021-10-01
[PDF-7.08 MB]
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Edition:Final Report: 06/2018 – 4/2021
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Abstract:Some of Louisiana’s bridges built in the 1950s and 1960s used two-girder or truss systems, in which floor beams are carried by main members and continuous (spliced) stringers are supported by the floor beams. The main members are either two edge (fascia) girders or trusses. Stringer bottom flanges are in compression at the negative moment region, which could result in lateral torsional buckling. When the continuous stringers are load-rated using AASHTOWare Bridge Rating analysis software, Cb is calculated in accordance with the AASHTO LRFD Bridge Design Specifications, which does not properly account for the bracing effect of non-composite deck and therefore underestimates the flexural strength. As a result, the rating may become low enough to require restrictive load posting or even closure. This issue affects bridges that are key parts of Louisiana’s highway system. The current load rating would cause expensive (and possibly unnecessary) bridge rehabilitation or replacement with significant disruption for the traveling public. This project reassesses the methodology behind load rating the stringers, with efforts focusing on more realistic values for Cb. The project objective was to evaluate the capacity of the aforementioned bridges built with continuous stringers and to develop a new approach for rating those stringers by more accurately representing moment gradients using Cb.
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