Civil Engineering Reference
In-Depth Information
The critical section for bending is taken at the face of a reinforced concrete column or
halfway between the middle and edge of a masonry wall or at a distance halfway from the
edge of the base plate and the face of the column if structural steel columns are used
(Code 15.4.2).
The determination of footing depths by the procedure described here will often re-
quire several cycles of a trial-and-error procedure. There are, however, many tables and
handbooks available with which footing depths can be accurately estimated. One of these
is the previously mentioned
CRSI Design Handbook
. In addition, there are many rules of
thumb used by designers for making initial thickness estimates, such as 20% of the foot-
ing width or the column diameter plus 3 in. Computer programs such as SABLE32 easily
handle this problem.
The reinforcing steel area calculated for
f
ootings will often be appreciably less than
the minimum values and specified for flexural members in
ACI Section 10.5.1. In Section 10.5.4, however, the Code states that in slabs of uniform
thickness the minimum area and maximum spacing of reinforcing bars in the direction of
bending need only be equal to those required for shrinkage and temperature reinforce-
ment. The maximum spacing of this reinforcement may not exceed the lesser of three
times the footing thickness, or 18 in. Many designers feel that the combination of high
shears and low
(
200
b
w
d
/
f
y
)
(
3
f
c
b
w
d
/
f
y
)
values that often occurs in footings is not a good situation. Because of
this, they specify steel areas at least as large as the flexural minimums of ACI Section
10.5.1. This is the practice we also follow herein.
Example 12.2 illustrates the design of an isolated column footing.
EXAMPLE 12.2
Design a square column footing for a 16-in. square tied interior column that supports a dead load
D
200 k and a live load
L
160 k. The column is reinforced with eight #8 bars, the base of
the footing is 5 ft below grade, the soil weight is 100 lb/ft
3
,
f
y
60,000 psi,
c
f
3000 psi, and
q
a
5000 psf.
SOLUTION
After Two Previous Trials Assume 24-in. Footing (
d
19.5 in. estimated to c.g. of top layer of
flexural steel)
24
12
36
12
q
e
5000
(150)
(100)
4400 psf
200
160
4.400
A
required
81.82 ft
2
Use 9 ft 0 in.
9 ft 0 in. footing
81.0 ft
2
Bearing Pressure for Strength Design
q
u
(1.2)(200)
(1.6)(160)
81.0
6.12 ksf
