Civil Engineering Reference
In-Depth Information
A plain concrete footing will obviously require considerably more concrete than will
a reinforced one. On the other hand, the cost of purchasing reinforcing and placing it will
be eliminated. Furthermore, the use of plain concrete footings will enable us to save con-
struction time in that we don't have to order the reinforcing and place it before the con-
crete can be poured. Therefore, plain concrete footings may be economical on more
occasions than one might realize.
Even though plain footings are designed in accordance with the ACI requirements,
they should at the very least be reinforced in the longitudinal direction to keep tempera-
ture and shrinkage cracks within reason and to enable the footing to bridge over soft spots
in the underlying soil. Nevertheless, Example 12.10 presents the design of a plain con-
crete footing in accordance with the ACI Code.
EXAMPLE 12.10
Design a plain concrete footing for a 12-in. reinforced concrete wall that supports a dead load of 12 k/ft,
including the wall weight, and a 6-k/ft live load. The base of the footing is to be 5 ft below the final
grade,
c
f
3000 psi, and
q
a
4000 psf.
SOLUTION
Assume 24-in. Footing (see Figure 12.25)
24
12
36
12
q
e
4000
(145)
(100)
3410 psf
18
3.41
5.28 ft
Width required
Say 5
6
Bearing Pressure for Strength Design
q
u
(1.2)(12)
(1.6)(6)
5.5
4.36 ksf
Checking Bending Strength, Neglecting Bottom 2 in. of Footing
M
u
for 12 in. width of footing
2.25
2
(4.36)(2.25)
11.04 ft- k
(12)(22)
2
6
bd
2
6
S
968 in.
3
c
S
(0.55)(5)(
3000)(968)
M
n
5
f
145,804 in.- lb
12.15 ft- k
11.04 ft- k
OK
12
"
wall
q
u
= 4.36 ksf
1
'
0
"
5
'
6
"
2
'
3
"
2
'
3
"
Figure 12.25
