FHWA Resource Center
STRUCTURES TEAM
Slide
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.
Slide 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.
Slide 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.
Slide 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.
Slide 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.
Slide 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.
TRAINING
