Civil Engineering Reference
In-Depth Information
EXAMPLE 12.4
Design for load transfer from a 14-in.
14-in. column to a 13-ft-0-in.
13-ft-0-in. footing with a
P
u
of 800 k,
3000 psi in the footing and 5000 psi in the column, and
f
y
60,000 psi. The col-
umn has eight #8 bars.
c
f
SOLUTION
Bearing force at base of column
P
u
800 k
(allowable bearing force in concrete)
(strength of dowels)
Design bearing strength in concrete at base of column
(0.65)(0.85)(5.0)(14
14)
541.5 k
800 k
no good
Design bearing strength on footing concrete
(0.65)(0.85)(3.0)(14
14)(Use 2)
649.7 k
800 k
no good
Therefore, the dowels must be designed for excess load.
Excess load
800
541.5
258.5 k
A
s
of dowels
258.5
60
4.31 in.
2
or (0.005)(14)(14)
0.98 in.
2
Use six #8 bars (4.71 in.
2
)
Development Length of Dowels into Column
d
(0.02)(1.00)(60,000)
5000
16.97
d
(0.0003)(60,000)(1.00)
18.00
b
d
8
c
Development Length of Dowels into Footing (Different from Column Values because
f
Val-
ues Are Different)
d
(0.02)(1.00)(60,000)
3000
21.91
k
d
(0.0003)(1.00)(60,000)
18.00
Use six #8 dowels extending 18 in. up into the column and 22 in. down into the footing.
12.9
RECTANGULAR ISOLATED FOOTINGS
As previously mentioned, isolated footings may be rectangular in plan if the column has a
very pronounced rectangular shape or if the space available for the footing forces the de-
signer into using a rectangular shape. Should a square footing be feasible, it is normally to
be desired over a rectangular one because it will require less material and will be simpler
to construct.
The design procedure is almost identical with the one used for square footings. After
the required area is calculated and the lateral dimensions are selected, the depths required
for one-way and two-way shear are determined by the usual methods. One-way shear will
very often control the depths for rectangular footings, whereas two-way shear normally
controls the depths of square footings.
