United States Department of Transportation
i
Improving the design of u-beams for Indiana.
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2014-06-01
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[PDF-46.71 MB]
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Abstract:The use of prestressed U‐beams, developed as an economical and aesthetic alternative to traditional I‐beams, is becoming increasingly
common. While U‐beams are seeing increased use, questions have arisen in the design of bridges utilizing this girder type. Specific issues
include live load distribution, bridge deck behavior, and debonding limits. First, since the U‐beam is a new girder type, limited guidance
exists for the live load distributions to be used in design. Second, due to the increased stiffness and strength of this girder, fewer girder
lines are required resulting in increased deck spans. Finally, current AASHTO debonding limits severely restrict the efficiency and
economy of the section. The objective of this research program is to develop design strategies to improve the efficiency and optimize
the design of the Indiana modified U‐beam with a focus on the concerns related to the design of U‐beams. In particular, this research
program will evaluate the live load distribution appropriate for the design of U‐beams, assess the behavior and design of the bridge deck
when supported by U‐beams, and evaluate both the shear strength and shear design of the composite U‐beam system. It is important
that the strength strength of pretensioned concrete beams with debonded strand be fully evaluated. Five phases of research were
conducted. The first phase consisted of evaluation of a U‐beam bridge constructed in Indianapolis, Indiana to assess the live load
distribution factors and flexural deck behavior of this bridge. The second phase evaluated the effectiveness of debonding sheathing to
ensure that proper debonding is being achieved. The third phase evaluated the influence of the percentage of debonded strands on
shear strength using rectangular and I‐shaped cross sections to investigate flexure‐shear and web‐shear strengths, respectively. While
this phase concentrated on the shear strength resisted by the concrete, the influence of transverse reinforcement was also evaluated.
The fourth phase evaluated the influence of varying strength concretes, which are commonly used in composite sections, on shear
strength. Finally, the fifth phase evaluated the shear strength of a 50% debonded U‐beam both with and without transverse
reinforcement. From the results, an improved understanding of the influence of debonding on shear strength is obtained.
Recommendations are provided to improve the analysis, design, and construction of U‐beams as well as the design and construction of
girders utilizing debonded strand in general.
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