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Structural Concrete - Hassoun

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13.4 Design Considerations 449<br />

Figure 13.9<br />

Soil pressure distribution in cohesionless soil (sand).<br />

Figure 13.10<br />

Soil pressure distribution in cohesive soil (clay).<br />

The allowable bearing soil pressure, q a , is usually determined from soil tests. The allowable<br />

values vary with the type of soil, from extremely high in rocky beds to low in silty soils. For<br />

example, q a for sedimentary rock is 30 ksf, for compacted gravel is 8 ksf, for well-graded compacted<br />

sand is 6 ksf, and for silty-gravel soils is 3 ksf.<br />

Referring to Fig. 13.8, when the load P is applied, the part of the footing below the column<br />

tends to settle downward. The footing will tend to take a uniform curved shape, causing an<br />

upward pressure on the projected parts of the footing. Each part acts as a cantilever and must<br />

be designed for both bending moments and shearing forces. The design of footings is explained<br />

in detail later.<br />

13.4 DESIGN CONSIDERATIONS<br />

Footings must be designed to carry the column loads and transmit them to the soil safely. The design<br />

procedure must take the following strength requirements into consideration:<br />

1. The area of the footing based on the allowable bearing soil capacity.<br />

2. One-way shear.<br />

3. Two-way shear, or punching shear.

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