Browsing by Subject "Girders"

Now showing 1 - 5 of 5
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    BR27568 – Experimental Shear Capacity Comparison Between Repaired and Unrepaired Girder Ends
    (Minnesota Department of Transportation, 2018-02) Shield, Carol; Bergson, Paul
    Over time, the southbound exterior girder ends on each side of Pier 4 and Pier 26 of Bridge 27568 suffered significant corrosion damage that exposed transverse reinforcement, prestressing strands in the exterior side of the bottom flange and the sole plate anchorages. The girder ends were repaired in 2013 by encasing supplementary steel reinforcement in shotcrete over a 4 ft. length of the girder. The two repaired girders and two companion girders, removed when the bridge was replaced in 2017, were brought to the University of Minnesota and tested to failure in shear to determine the effectiveness of the repair. The laboratory testing showed that the repair was able to return the girders with significant corrosion damage to the strength of the companion girders, indicating that the repair was effective.
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    Debonded Strands in Prestressed Concrete Bridge Girders
    (Minnesota Department of Transportation., 2019-07) Osman, Mahad; French, Catherine
    There are three potential options to reduce end stresses in prestressed concrete bridge girders: drape strands, debond strands, or a combination of the two. In the draping option, a portion of the strands are raised from harp points within the girder to reduce the strand eccentricity at the girder ends. Large vertical reactions are required at the hold down points within the girder to resist the uplift of the draped strands. In addition, end cracking that follows the draped strand pattern is often observed, particularly in deeper sections. In the debonding option, a portion of the strands are debonded toward the girder ends to reduce the resultant prestress force. Concerns with debonding are its potential to reduce shear strength and to cause corrosion issues if moisture and deicing chemicals make their way into the girder ends along the debonded path. Due to potential corrosion concerns, MnDOT has prohibited strand debonding. However, as a means to eliminate some of the end cracking observed during fabrication with draped strands, this study was conducted to explore the use of debonded strands and to develop design recommendations. To this end, an extensive literature review was conducted regarding debonded strand research, and state Departments of Transportation with similar climates and fabricators were queried to learn from their experiences. Design recommendations and potential material specifications to protect debonded strands from corrosion are presented in this report.
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    Effect of Temperature on Prestressed Concrete Bridge Girder Strand Stress During Fabrication
    (Center for Transportation Studies, University of Minnesota, 2015-12) Swenson, Tanner W.; French, Catherine E. W.
    The Minnesota Department of Transportation has reported erection cambers of many prestressed concrete bridge girders that were much lower than anticipated. A previous University of Minnesota study (O’Neill and French, MnDOT 2012-16) attributed the discrepancies to inaccurate estimates of the concrete strength and stiffness at release and strand force loss due to temperature during fabrication. The objective of this study was to further investigate the effects of temperature on strand force and camber during precast, prestressed girder fabrication and to make recommendations for the design and fabrication processes to reduce the potential loss of prestress due to temperature effects during fabrication and to improve the release camber estimation. A thermal effects analysis was developed based on four key steps in the girder fabrication process: tensioning, concrete-steel bond, release, and normalization. The study included fabricating six short prestressed concrete segments released at early ages to determine the time/temperature associated with bonding the prestressing strand to the concrete. To investigate the non-recoverable prestress losses during girder fabrication, four sets of girders (MN54 and 82MW) were instrumented with thermocouples, strain gages, and in some cases load cells, that were monitored during the fabrication process to separate the thermal and mechanical strain components. Effects investigated included casting during a cold season, casting during a warm season, casting with the free length of strand covered, and casting with different bed occupancy during any season. A recommended procedure for adjusting strand force during tensioning was proposed to account for non-recoverable strand force changes due to temperature changes between tensioning and bond.
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    Reusability and Impact Damage Repair of Twenty-Year-Old AASHTO Type III Girders
    (1992-01) Olson, Steven A.; French, Catherine E.; Leon, Roberto T.
    Prestressed concrete has been used as a bridge construction method in the United States since 1949. Presently, there are thousands of pretensioned prestressed concrete bridges in service in North America. Each year, approximately 200 girders are damaged as a result of impact damage (primarily overheight vehicles striking a bridge from below). This thesis describes the results of a four girder test series which evaluated impact damage and repairs. The girders used for the study were fabricated in 1967 and placed in service. They were removed from service in 1984 as a result of a road realignment project. The objectives of the research project were: 1) to determine the effective prestress in the strands after 20 years, 2) to determine the influence of impact damage on girder performance, 3) to evaluate the performance of two impact damage repair schemes under static, fatigue, and ultimate loadings, and 4) to develop a model to estimate the strand stress ranges in damaged girders.
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    Validation of Prestressed Concrete I-Beam Deflection and Camber Estimates
    (Minnesota Department of Transportation, 2012-06) O'Neill, Cullen; French, Catherine
    The camber at the time of bridge erection of prestressed concrete bridge girders predicted by the Minnesota Department of Transportation (MnDOT) was observed to often overestimate the measured cambers of girders erected at bridge sites in Minnesota, which, in some cases, was causing significant problems related to the formation of the bridge deck profile, the composite behavior of the girders and bridge deck, delays in construction and increased costs. Extensive historical data was collected from two precasting plants and MN counties and it was found that, on average, the measured cambers at release and erection were only 74% and 83.5%, respectively, of the design values. Through data collection, analysis, and material testing, it was found that the primary causes of the low camber at release were concrete release strengths that exceeded the design values, the use of an equation for concrete elastic modulus that greatly under-predicted the measured values, and thermal prestress losses not accounted for in design. Fourteen girders were instrumented and their camber measured and the program PBEAM was used to evaluate the influence of various time-dependent effects (i.e., solar radiation, relative humidity, concrete creep and shrinkage, length of cure and bunking/storage conditions) on long-term camber. Once investigated, these effects were included in long-term camber predictions that were used to create sets of both time-dependent and singlevalue camber multipliers. The use of these multipliers, along with modifications made to the elastic release camber calculations, greatly reduced the observed discrepancy between measured and design release and erection cambers.