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Structural Concrete - Hassoun

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470 Chapter 13 Footings<br />

Solution<br />

1. Determine the footing that has the largest ratio of live load to dead load. In this example, footing<br />

3 ratio of 1.43 is higher than the other ratios.<br />

2. Calculate the usual load for all footings. The usual load is the dead load and the portion of live<br />

load that most commonly occurs on the structure. In this example,<br />

Usual load = DL + 0.2(LL)<br />

The values of the usual loads are shown in the following table.<br />

3. Determine the area of the footing that has the highest ratio of LL/DL:<br />

DL + LL 140 + 200<br />

Area of footing → 3 =<br />

= = 56.7ft 2<br />

allowable soil pressure 6<br />

The usual soil pressure under footing 3 is<br />

Usual load<br />

Area of footing = 180 = 3.18 ksf<br />

56.7<br />

4. Calculate the area required for each footing by dividing its usual load by the soil pressure of<br />

footing 3. The areas are tabulated in the following table. For footing 1, for example, the required<br />

area is 150/3.18 = 47.2 ft 2 .<br />

5. Calculate the maximum soil pressure under each footing:<br />

q max = D + L ≤ 6ksf (allowable soil pressure)<br />

area<br />

By Footing Number<br />

Description 1 2 3 4 5<br />

Live load<br />

Dead load<br />

1.25 1.22 1.43 0.90 1.14<br />

Usual load = DL + 0.2 (LL) (kips) 150 224 180 224 258<br />

usual load<br />

Area required =<br />

3.18 ksf (ft2 ) 47.2 70.4 56.7 70.4 81.1<br />

Max. soil pressure = D + L (ksf) 5.72 5.68 6.00 5.11 5.55<br />

area<br />

Example 13.6<br />

Design a plain concrete footing to support a 16-in.-thick concrete wall. The loads on the wall consist of<br />

a 16-K/ft dead load (including the self-weight of wall) and a 10-K/ft live load. The base of the footing<br />

is 4 ft below final grade. Use f c ′ = 3 ksi and an allowable soil pressure of 5 ksf.<br />

Solution<br />

1. Calculate the effective soil pressure. Assume a total depth of footing of 28 in.:<br />

Weight of footing = 28 × 145 = 338 psf<br />

12<br />

The weight of the soil, assuming that soil weighs 100 pcf, is (4 − 2.33) × 100 = 167 psf. Effective<br />

soil pressure is 5000 − 338 − 167 = 4495 psf.

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