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Structural Concrete - Hassoun

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480 Chapter 13 Footings<br />

Figure 13.29 Footing subjected to P and biaxial moment. If e x<br />

< L/6 and e y<br />

< B/6,<br />

footing will be subjected to upward soil pressure on all bottom surface (nonuniform<br />

pressure).<br />

The soil pressure at corner 1 is<br />

At corner 2,<br />

At corner 3,<br />

At corner 4,<br />

q max = P A + M xc y<br />

I x<br />

q 2 = P A − M xc y<br />

I x<br />

q 3 = P A − M xc y<br />

I x<br />

q 4 = P A + M xc y<br />

I x<br />

+ M yc x<br />

I y<br />

+ M yc x<br />

I y<br />

− M yc x<br />

I y<br />

− M yc x<br />

I y<br />

Again, note that the allowable soil pressure must not be exceeded and the soil cannot take any<br />

tension; that is, q ≥ 0.<br />

Example 13.8<br />

A12× 24−in. column of an unsymmetrical shed shown in Fig. 13.30a is subjected to an axial load<br />

P D = 220 K and a moment M d = 180 K ⋅ ft due to dead load and an axial load P L = 165 K and a moment<br />

M L = 140 K ⋅ ft due to live load. The base of the footing is 5 ft below final grade, and the allowable soil<br />

bearing pressure is 5 ksf. Design the footing using f c ′ = 4ksiandf y = 60 ksi.<br />

Solution<br />

The footing is subjected to an axial load and a moment:<br />

P = 220 + 165 = 385 K<br />

The eccentricity is<br />

e = M P<br />

M = 180 + 140 = 320 K ⋅ ft<br />

=<br />

320 × 12<br />

385<br />

= 9.97 in. say, 10 in.

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