Civil Engineering Reference
In-Depth Information
19.2
Compute the stresses in the top and bottom of the beam shown at the ends and centerline immediately after the ca-
bles are cut. Assume straight cables. Initial prestress in 170 ksi.
1 k/ft (not including beam weight)
22"
27"
0.9 in.
2
27'
5"
14"
shown in the accompanying illustration support for a 60-ft
simple span in addition to its own weight? Assume 20%
prestress loss.
c
19.3
The beam shown has a 30-ft simple span;
5000 psi,
f
pu
250,000 psi, and the initial prestress is
160,000 psi and
f
py
0.85
f
pu
.
f
(a)
Calculate the concrete stresses in the top and bottom
of the beam at midspan immediately after the tendons are
cut. (
Ans. f
top
0.183 ksi,
f
bott
1.041 ksi)
(b)
Recalculate the stresses at midspan after assumed
prestress losses in the tendons of 20%. (
Ans.
f
top
0.076 ksi,
f
bott
0.762 ksi)
(c)
What maximum service live load can this beam
support in addition to its own weight if
a
llowable stresses
of
c
c
0.45
f
in compression and
6
f
in tension are
permitted? (
Ans.
1.182 k/ft.)
19.5
Compute the cracking moment and the permissible
ultimate moment capacity of the beam of Problem 19.3 if
5000 psi and
f
py
200,000 psi. (
Ans. M
cr
144.74 ft-k,
M
n
300 ft-k)
c
f
21"
24"
0.9 in.
2
19.6
Compute the stresses in the top and bottom at the
L
of the beam of Problem 19.1 if it is picked up at the
L
.
Assume 100% impact. Concrete weighs 150 lb-ft
3
.
3"
14"
19.7
Determine the stresses at the one-third points of the
beam of Problem 19.6 if the beam is picked up at those
points. (
Ans. f
top
1.009 ksi,
f
bott
2.151 ksi)
19.4
Using the same allowable stresses permitted in
Problem 19.3, what total uniform load can the beam
