Factors for improving the design and construction of U-Beam bridges: Live load distribution and prestressing strand debonding effectiveness

Matthew Adam Pavelchak, Purdue University

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 of the section. While it is important to evaluate these limits, questions have also arisen regarding the effectiveness of commercially available debonding products as many precasters have experienced problems with these materials. Determining the effectiveness of debonding is essential before determining the appropriateness of the AASHTO debonding limits and will be a focus of this study. Based on these research needs, the objective of this research study is to investigate the live load distribution and flexural deck behavior of a U-Beam bridge and to assess the effectiveness of common debond sheathing products. Two 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. This investigation was performed by instrumenting the bridge with strain gages and placing loaded dump trucks in different loading configurations on the bridge deck. Comparison of the measured live load distribution factors and flexural deck behavior with those calculated according the AASHTO provisions are provided. The second phase consisted of evaluation of several commercially available debond sheathing products. This investigation was performed by constructing a series of 27 concentrically prestressed, prismatic members. Strains were measured along the face of the specimens both before and after prestress transfer. Additionally, strains were measured at 14 and 28 days after transfer to detect any possible time dependant effects. The resulting strain distributions along the length of each specimen also allowed for the determination of transfer lengths for each specimen. Comparison of the effectiveness of the various debond sheathing products is provided. Recommendations are made about the live load distribution factors used in design and selection of debond sheathing used during construction.

Degree

M.S.C.E.

Advisors

Frosch, Purdue University.

Subject Area

Civil engineering

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