Civil Engineering Reference
In-Depth Information
EXAMPLE 5.9
c
Design a rectangular beam for
M
D
325 ft-k and
M
L
400 ft-k if
4000 psi and
f
y
60,000
psi. The maximum permissible beam dimensions are shown in Figure 5.16.
f
SOLUTION
M
u
(1.2)(325)
(1.6)(400)
1030 ft- k
Assuming
0.90
M
n
M
u
1030
0.90
1144.4 ft- k
Assuming maximum possible tensile steel with no compression steel and computing beams
nominal moment strength
max
from Appendix Table A.7
0.0181
A
s
1
(0.0181)(15)(28)
7.60 in.
2
M
u
bd
2
from Appendix Table A.13
For
0.0181
912.0
M
u
1
(912.0)(0.9)(15)(28)
2
9,652,608 in.- lb
804.4 ft- k
M
n
1
804.4
0.90
893.8 ft- k
M
n
2
M
n
M
n
1
1144.4
893.8 ft- k
250.6 ft- k
Checking to See Whether Compression Steel Has Yielded
(7.60)(60)
(0.85)(4)(15)
8.94 in.
a
c
8.94
0.85
10.52 in.
10.52
3
10.52
s
(0.00300)
0.00214
0.00207
Therefore, compression steel has yielded.
3"
A'
31"
25"
A
s
3"
15"
Figure 5.16
