Civil Engineering Reference
In-Depth Information
Figure 7.20
At point C we reach the cutoff point of the bars, and from C to D (a distance equal to
d
) the
design moment capacity will increase from 185.3 ft-k to 359.7 ft-k. (In Figure 7.21(a) the bars seem
to be shown in two layers. They are actually on one level, but the author has shown them this way
so that the reader can get a better picture of how many bars there are at any point along the beam.)
At no point along the span may the design strength of the beam be less than the actual bend-
ing moment caused by the loads. We can then see that point C is located where the actual bending
M
u
= (44.8)(8.25) - (2.8)(8.25)
(
)
8.25
2
= 274.3 ft-k
Cutoff points
4 bars
2 bars
2 bars
(a)
Design moment capacity
of beam
359.7 ft-k
359.7 ft-k
D
358.4 ft-k
B
C
274.3 ft-k
274.3 ft-k
185.3 ft-k
185.3 ft-k
185.3 ft-k
185.3 ft-k
Moment diagram
due to beam loads
A
d
= 35
"
'
15
"
'
d
= 35
"
'
d
= 35
"
'
15
"
'
d
= 35
"
'
15 6
"
'
22 4
'
"
(length of cutoff bars)
(b)
Figure 7.21
