Civil Engineering Reference
In-Depth Information
As in reinforced concrete members, the amount of steel in prestressed sections is lim-
ited to ensure tensile failures. The limitation rarely presents a problem except in members
with very small amounts of prestressing or in members that have not only prestress
strands, but also some regular reinforcing bars.
Example 19.3 illustrates the calculations involved in determining the permissible ulti-
mate capacity of a rectangular prestressed beam. Some important comments about the solu-
tion and about ultimate-moment calculations in general are made at the end of the example.
EXAMPLE 19.3
Determine the permissible ultimate moment capacity of the prestressed bonded beam of Figure 19.9
if
f
py
c
240,000 psi,
f
pu
is 275,000 psi, and
f
is 5000 psi.
SOLUTION
Approximate Value of
f
ps
from ACI Code
p
A
ps
1.40
(12)(21.5)
0.00543
bd
f
py
f
pu
240,000
275,000
0.873
p
0.40, as given immediately after the presentation of ACI Equation 18-3 earlier in this
section.
f
ps
estimated stress in prestressed reinforcement at nominal strength. Note that
1
0.80 for
5000 psi concrete and
d
, the distance from the extreme compression fiber of the beam to the cen-
troid of any nonprestressed tension reinforcement is 0 since there is no such reinforcement in this
beam.
1
p
p
f
pu
d
f
ps
f
pu
1
d
p
(
)
(ACI Equation 18-3)
c
f
1
0.40
275
5
275
0
233.9 ksi
0.80
0.00543
Figure 19.9
