Determination of Service Stresses in Pretensioned Beams

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Determination of Service Stresses in Pretensioned Beams

Author: Hamilton, H R O'Neill, Christina | Size: 7.73 MB | Format: PDF | Quality: Original preprint | Publisher: University of Florida, Gainesville | Year: 2009 | pages: 112

This report presents research on the evaluation of service flexural stresses and cracking moment in prestressed concrete members and on the minimum reinforcement requirements that are currently controlled by the flexural cracking moment. In prestressed concrete girders, the cracking moment changes when prestressing steel quantities are adjusted. If bonded prestressing steel is considered to contribute to the minimum reinforcement, then a single minimum reinforcement quantity is not possible. Furthermore, as bonded prestressing steel quantities are increased to satisfy the minimum reinforcement, the minimum steel requirement increases proportionately. A parametric study was conducted on hollow core, Florida bulb tee, and segmental box girders to evaluate the current minimum steel provisions. New minimum reinforcement provisions were then derived based on recommendations by Leonhardt 1964. These reinforcement provisions were then compared to the existing American Concrete Institute (ACI) 318 and American Association of State Highway and Transportation Officials (AASHTO) provisions using the sections from the parametric study. Ten precast, pretensioned pile cut offs from an FDOT construction project were salvaged and tested to determine cracking moment and to evaluate cracking behavior. Half of the piles were loaded monotonically to cracking and half were loaded cyclically to cracking. Cyclic loading was used in conjunction with AE monitoring and strain gage data to determine the initiation of microcracking. Structural cracking was determined using visual identification combined with interpolation from the load deflection plot. Six precast, pretensioned I-girders were also constructed and tested cyclically to determine cracking moment and evaluate cracking behavior. As both the piles and girders were loaded, microcracking and structural cracking were found to occur at lower stresses than would be calculated from the measured modulus of rupture and precompression. The stress range between the initiation of microcracking and the formation of a structural crack was found to increase with the prestress level. The current AASHTO provisions limiting tensile stress in harsh environments appear to be adequate in light of the girder test results.

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