Civil Engineering Reference
In-Depth Information
0.276 in. (about in.). In posttensioning work, large numbers of wires are grouped in paral-
lel into tendons. Strands that are made by twisting wires together are used for most preten-
sioned work. They are of the seven-wire type, where a center wire is tightly surrounded by
twisting the other six wires helically around it. Strands are manufactured with diameters
from to in. sometimes large-size, high-strength, heat-treated alloy steel bars are used for
posttensioned sections. they are available with diameters running from to in.
High-strength prestressing steels do not have distinct yield points (see Figure 19.10)
as do the structural carbon reinforcing steels. The practice of considering yield points,
however, is so firmly embedded in the average designer's mind that high-strength steels
are probably given an arbitrary yield point anyway. The yield stress for wires and strands
is usually assumed to be the stress that causes a total elongation of 1% to occur in the
steel. For high-strength bars the yield stress is assumed to occur when a 0.2% permanent
strain occurs.
9
32
1
4
1
2
3
4
1
8
19.5
STRESS CALCULATIONS
For a consideration of stresses in a prestressed rectangular beam, reference is made to
Figure 19.5. For this example the prestress tendons are assumed to be straight, although it
will later be shown that a curved shape is more practical for most beams. The tendons are
assumed to be located an eccentric distance
e
below the centroidal axis of the beam. As a
result, the beam is subjected to a combination of direct compression and a moment due to
Figure 19.5
