Title page for ETD etd-08242006-133037


Type of Document Master's Thesis
Author Hinkle, Stephen Dock
URN etd-08242006-133037
Title Investigation of Time-Dependent Deflection in Long Span, High Strength, Prestressed Concrete Bridge Beams
Degree Master of Science
Department Civil Engineering
Advisory Committee
Advisor Name Title
Roberts-Wollmann, Carin L. Committee Chair
Cousins, Thomas E. Committee Member
Weyers, Richard E. Committee Member
Keywords
  • shrinkage
  • prestress losses
  • creep
  • camber
  • time-dependent deflection
Date of Defense 2006-08-10
Availability unrestricted
Abstract
Accurate camber prediction in prestressed concrete bridge beams is important to all parties involved in bridge design and construction. Many current prestress loss prediction methods, necessary for proper camber calculation, were developed many years ago and are predicated on assumptions that may no longer be valid as higher strength concrete, wider beam spacing, and longer span lengths become more commonplace. This throws into question which models are appropriate for use in camber calculation by the bridge engineers and contractors of today.

Twenty-seven high-strength concrete modified 79 in. Bulb Tee beams with a design compressive strength of 9,000 psi were periodically measured to determine camber growth. Most available models for concrete creep and shrinkage were used to calculate creep and shrinkage strain. The modulus of elasticity equation of each model was used to predict modulus of elasticity of the studied mix. The Shams and Kahn compressive strength and modulus of elasticity equations were modified in order to approximate measured modulus of elasticity. The creep, shrinkage, and modulus of elasticity equations were used as inputs to an incremental time step method. The time-dependent change in beam curvature calculated by the time step method was used to calculate theoretical camber using the Moment-Area method. Predicted camber, using inputs from each considered model, was then compared with measured camber to determine the most accurate camber prediction models. Season of casting was also examined to determine what, if any, affect ambient temperature has on camber growth.

For the studied beams, the Shams and Kahn Model for creep, shrinkage, and modulus of elasticity, used as inputs for an incremental time step analysis, were found to most accurately predict camber values. Lower concrete compressive strength was observed for test cylinders from beams cast in summer versus beams cast in winter. Differences in beam deflection based on season of casting showed mixed results.

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