Civil Engineering Reference
In-Depth Information
Figure 12.14
The next step is to select the reinforcing in the long direction. These longitudinal bars
are spaced uniformly across the footing, but such is not the case for the short-span reinforc-
ing. With reference to Figure 12.14, it can be seen that the support provided by the footing
to the column will be concentrated near the middle of the footing, and thus the moment in
the short direction will be concentrated somewhat in the same area near the column.
As a result of this concentration effect, it seems only logical to concentrate a large
proportion of the short-span reinforcing in this area. The Code (15.4.4.2) states that a cer-
tain minimum percentage of the total short-span reinforcing should be placed in a band
width equal to the length of the shorter direction of the footing. The amount of reinforcing
in this band is to be determined with the following expression, in which
is the ratio of
the length of the long side to the width of the short side of the footing:
Reinforcing in band width
Total reinforcing in short direction
2
(ACI Equation 15-1)
1
The remaining reinforcing in the short direction should be uniformly spaced over the
ends of the footing but should at least meet the shrinkage and temperature requirements of
the ACI Code (7.12).
Example 12.5 presents the partial design of a rectangular footing in which the depths
for one- and two-way shears are determined and the reinforcement selected.
EXAMPLE 12.5
Design a rectangular footing for an 18-in. square interior column with a dead load of 185 k and a
live load of 150 k. Make the length of the long side equal to twice the width of the short side,
f
y
60,000 psi,
c
f
4000 psi, and
q
a
4000 psf. Assume the base of the footing is 5 ft 0 in. below grade.
SOLUTION
Assume 24-in. Footing (
d
19.5 in.)
24
12
36
12
q
e
4000
(150)
(100)
3400 psf
185
150
3.4
A
required
98.5 ft
2
Use 7
0
14
0
98.0 ft
2
q
u
(1.2)(185)
(1.6)(150)
98.0
4.71 ksf
