Civil Engineering Reference
In-Depth Information
If the computed development length of dowels is greater than the distance available
from the top of the footing down to the top of the tensile steel, three possible solutions are
available. One or more of the following alternatives may be selected:
1.
A larger number of smaller dowels may be used. The smaller diameters will result
in smaller development lengths.
2.
A deeper footing may be used.
3.
A cap or pedestal may be constructed on top of the footing to provide the extra de-
velopment length needed.
Should bending moments or uplift forces have to be transferred to a footing such that
the dowels would be in tension, the development lengths must satisfy the requirements for
tension bars. For tension development length into the footing, a hook at the bottom of the
dowel may be considered effective.
If there is moment or uplift, it will be necessary for the designer to conform to the
splice requirements of Section 12.17 of the Code in determining the distance the dowels
must be extended up into the wall or column.
Examples 12.3 and 12.4 provide brief examples of column-to-footing load transfer
calculations for vertical forces only. Consideration is given to lateral forces and moments
in Sections 12.12 and 12.13 of this chapter.
EXAMPLE 12.3
Design for load transfer from a 16-in.
16-in. column to a 9-ft-0-in.
9-ft-0-in. footing if
D
200 k,
L
160 k,
c
f
4000 psi for both footing and column, and
f
y
60,000 psi.
SOLUTION
Bearing force at base of column
(1.2)(200)
(1.6)(160)
496 k
Allowable bearing force in concrete at base of column
c
A
1
)
(0.65)(0.85)(4.0)(16
16)
(0.85
f
566 k
496 k
Allowable bearing force in footing
co
ncrete
A
2
A
1
(0.65)(0.85)(4.0)(16
16)
9
1.33
1.33
9
c
A
1
)
(0.85
f
(0.65)(0.85)(4.0)(16
16)(Use 2)
1131.5 k
496 k
OK
Minimum
A
s
for dowels
(0.005)(16
16)
1.28 in.
2
Use 4 #6 bars (1.77 in.
2
)
Development Lengths of Dowels (ACI 12.3)
0.02
d
b
f
y
f
(0.02)(0.75)(60,000)
4000
d
14.23
k
c
d
0.0003
d
b
f
y
(0.0003)(0.75)(60,000)
13.50
d
8.00
14
2
Use four #6 dowels extending in. up into column and down into footing and set on top of
reinforcing mat, as shown in Figure 12.13.
