Civil Engineering Reference
In-Depth Information
EXAMPLE 12.9
c
A 14-in.
14-in. column is supported by a 13-ft-0-in.
13-ft-0-in. footing (
3000 psi and
f
y
60,000 psi for both). For vertical compression force transfer, six #6 dowels (2.65 in.
2
) were selected
extending 18 in. up into the column and 18 in. down into the footing. Design for a horizontal fac-
tored force
V
u
of 65 k acting at the base of the column. Assume that the footing concrete has not
been intentionally roughened before the column concrete is placed. Thus
0.6
(0.6)(1.0) for
normal-weight concrete
0.6. (See ACI Code 11.7.4.)
f
SOLUTION
Determine Minimum Shear Friction Reinforcement Required by ACI Section 11.7.4
V
n
V
u
65
0.75
86.7 k
V
n
A
v
f
f
y
(ACI Equation 11-25)
86.7
(60)(0.6)
2.41 in.
2
2.65 in.
2
A
vf
OK
The six #6 dowels (2.65 in.
2
) present may also be used as shear friction reinforcing. If their
area had not been sufficient, it could have been increased and/or the value of
could be increased
significantly by intentionally roughening the concrete, as described in Section 11.7.4.3 of the Code.
Check Tensile Development Lengths of These Dowels
As described in ACI Sections 12.2.3 and 11.7.8, shear friction reinforcing acts in tension and thus
must have tensile anchorage on both sides of the shear plane. It must also engage the main reinforc-
ing in the footing to prevent cracks from occurring between the shear reinforcing and the body of
the concrete.
Assuming
c
K
tr
d
b
2.5
d
(1.0)(1.0)(0.8)(1.0)
2.5
60,000
3000
3
40
2.41
2.65
d
b
23.91 diameters
d
(23.91)(0.75)
17.93 in.
Say 18 in.
Compute Maximum Shear Transfer Strength Permitted by the Code (11.7.5)
c
A
c
, but not
(800
A
c
)
V
u
0.2
f
(0.75)(0.2)(3)(14
14)
88.2 k
65 k
OK
but not
(0.75)(800)(14
14)
117,600 lb
117.6 k
12.14
PLAIN CONCRETE FOOTINGS
Occasionally, plain concrete footings are used to support light loads if the supporting soil
is of good quality. Very often the widths and thicknesses of such footings are determined
by rules of thumb such as “the depth of a plain footing must be equal to no less than the
projection beyond the edges of the wall.” In this section, however, a plain concrete foot-
ing is designed in accordance with the requirements of the ACI.
Chapter 22 of the ACI Code is devoted to the design of structural plain concrete.
Structural plain concrete is defined as concrete that is completely unreinforced or that
