Civil Engineering Reference
In-Depth Information
have an embedment length beyond the point of inflection no less than the effective depth
of the member, 12 bar diameters, or one-sixteenth of the clear span of the member,
whichever is greatest.
Example 14.4, which follows, presents a detailed consideration of the lengths re-
quired by the Code for a set of straight negative-moment bars at an interior support of a
continuous beam.
EXAMPLE 14.4
At the first interior support of the continuous beam shown in Figure 14.35, six #8 straight uncoated
top bars (4.71 in.
2
) are used to resist a moment
M
u
of 390 ft-k for which the calculated
A
s
required is
4.24 in.
2
If
f
y
60 ksi,
3 ksi, and normal sand-gravel concrete is used, determine the length of
the bars as required by the ACI Code. The dimensions of the beam are
b
w
14 in. and
d
24 in.
Assume bar cover
2
d
b
and clear spacing
3
d
b
.
c
f
Note:
The six negative-tension bars are actually placed in one layer in the top of the beam, but
they are shown for illustration purposes in Figures 14.35 and 14.36 as though they are arranged in
three layers of two bars each. In the solution that follows, the “bottom” two bars are cut off first, at
the point where the moment plus the required development length is furnished; the “middle” two are
cut when the moment plus the needed development length permits; and the “top” two are cut off at
the required distance beyond the P.I.
SOLUTION
1.
Required development length
c
side cover
(2)(1.0)
1.0
2
2.5
3
2
1.0
2
c
one- half of c. to c. spacing of bars
(1.0)
2.0
k
Figure 14.35
