Structural Concrete - Hassoun

13.5 Plain Concrete Footings 469
14''
No. 11 bars
30''
No. 8
dowel bars
2'–0''
10'
Figure 13.21
Example 13.4 Dowels details.
Lap length of no. 8 bars governs. No. 8 dowel bars should extend not less than 30 in. into
the column.
b. For development into the footing, no. 8 dowels must extend full development length:
[ ] [
]
0.02fy
l dc =
λ √ 0.02 (60,000)
d b =
f c
′ 1.0 √ (1.0) =19 in.
4000
l dc,min =(0.0003f y )d b = 0.0003 × 60,000 × 1.0 = 18 in.
The length may be reduced to account for excess reinforcement according to ACI Code, Section
12.3.3:
A s (required)
A s (provided) = 2.74
3.16 = 0.86
l dc = 0.86 × 19 = 16.47 in.
Then
Available length for dowel development = 24 − 3 − 2(0.875)−1.0
= 18.25 in. >16.47 required (safe)(Fig.13.21)
Example 13.5
Determine the footing areas required for equal settlement (balanced footing design) if the usual live
load is 20% for all footings. The footings are subjected to dead loads and live loads as indicated in the
following table. The allowable net soil pressure is 6 ksf.
Footing Number
1 2 3 4 5
Dead load 120 K 180 K 140 K 190 K 210 K
Live load 150 K 220 K 200 K 170 K 240 K