Civil Engineering Reference
In-Depth Information
High-strength steels are necessary to produce and keep satisfactory prestress forces in
members. The strains that occur in these steels during stressing are much greater than
those that can be obtained with ordinary reinforcing steels. As a result, when the concrete
elastically shortens in compression and also shortens due to creep and shrinkage, the
losses in strain in the steel (and thus stress) represent a smaller percentage of the total
stress. Another reason for using high-strength steels is that a large prestress force can be
developed in a small area.
Early work with prestressed concrete using ordinary-strength bars to induce the
prestressing forces in the concrete resulted in failure because the low stresses that could
be put into the bars were completely lost due to the concrete's shrinkage and creep.
Should a prestress of 20,000 psi be put into such rods, the resulting strains would be
equal to 20,000/(29
10
6
)
0.00069. This value is less than the long-term creep and
shrinkage strain normally occurring in concrete, roughly 0.0008, which would com-
pletely relieve the stress in the steel. Should a high-strength steel be stressed to about
150,000 psi and have the same creep and shrinkage, the stress reduction will be of the
order of (0.0008)(29
10
6
)
23,200 psi, leaving 150,000
23,200
126,800 psi in
the steel (a loss of only 15.47% of the steel stress).
2
Three forms of prestressing steel are used: single wires, wire strands, and bars. The
greater the diameter of the wires, the smaller become their strengths and bond to the con-
crete. As a result, wires are manufactured with diameters from 0.192 in. up to a maximum of
Prestressing girders for bridge in Butler, Pennsylvania. (Courtesy of Portland Cement
Association.)
2
Winter, G., and Nilson, A. H., 1991,
Design of Concrete Structures
, 11th ed. (New York: McGraw-Hill),
pp. 759-760.
