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PCEF DESIGN PARAMETERS

PCEF DESIGN PARAMETERS.

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PCEF DESIGN PARAMETERS

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  1. PCEF DESIGN PARAMETERS • Strand pattern type - There are three basic categories of pretensioned strand patterns; draped, straight, and straight with unbonding. With proper attention to design and detailing, all three can provide an equivalent end-result. Recognizing this, it is the consensus of the committee that straight strand patterns (either with or without unbonding) are preferred and that draped designs can be substituted given that computations are provided which document the equivalency of the design.

  2. PCEF DESIGN PARAMETERS • It is the consensus of the committee that the maximum amount of unbonding used to reduce top tensile stresses (or bottom compressive stresses) at the end of the member should be limited to 25% of the total number of strands.

  3. PCEF DESIGN PARAMETERS • Many states either use, or allow the use of, 1/2" Special strands with an area = 0.167 in2. Some states are leaning toward the use of 0.6" strands spaced at 2" c/c. It is the consensus of the committee that either 1/2" Special or 0.6" strands spaced at 2" c/c should be used in the design of pretensioned bridge members.

  4. PCEF DESIGN PARAMETERS • Most states in the region design to a maximum allowable tensile stress in the precompressed tensile zone (i.e. At or very near midspan at the bottom of the girder) of 3* sqrt(f'c). A small number still design for zero allowable tensile stress. It is the consensus of the committee that 3*sqrt(f'c) tension is adequate to provide sufficient protection in severe environments and should be used throughout the region, unless the owner deems that a higher allowable tensile stress provides adequate protection in their area.

  5. PCEF DESIGN PARAMETERS • For I-beam, bulb-tee, and spread box beam bridges, many states use a single line of diaphragms at midspan to stabilize the girders during deck casting for spans up to 160 feet. Given the cost and safety implications of diaphragms, it is the consensus of the committee that a single line of diaphragms should be used for spans up to at least 160 feet.

  6. PCEF DESIGN PARAMETERS • Reinforcement projecting from the top of pretensioned bridge girders is not only costly, it is a safety (tripping) hazard for workers on the jobsite and is a logical pathway for corrosive materials to enter the girders. Therefore, it is the consensus of the committee that AASHTO horizontal shear reinforcement provisions should be used to reduce the amount of projecting reinforcement as much as possible.

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