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Building Design and Construction Handbook - Merritt - Ventech!

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9.110 SECTION NINE<br />

Footing Thickness. The minimum thickness allowed by the ACI 318 <strong>Building</strong><br />

Code for footing is 8 in for plain concrete footings on soil, 6 in above the bottom<br />

reinforcement for reinforced-concrete footings on soil, <strong>and</strong> 12 in above the bottom<br />

reinforcement for reinforced-concrete footings on piles. Plain-concrete pile-cap<br />

footings are not permitted.<br />

Concrete Cover. The minimum concrete cover required by the ACI 318 <strong>Building</strong><br />

Code for reinforcement cast against <strong>and</strong> permanently exposed to earth is 3 in.<br />

9.75 SPREAD FOOTINGS FOR WALLS<br />

The critical sections for shear <strong>and</strong> moment for spread footings supporting concrete<br />

or masonry walls are shown in Fig. 9.44a <strong>and</strong> b. Under the soil pressure, the<br />

projection of footing on either side of a wall acts as a one-way cantilever slab.<br />

Unreinforced Footings. Requirements for design of unreinforced concrete footings<br />

are included in the ACI 318 <strong>Building</strong> Code. For plain-concrete spread footings,<br />

the maximum permissible flexural tension stress, psi, in the concrete is limited to<br />

5��ƒ�, where � � strength-reduction factor � 0.65 <strong>and</strong> ƒ��specified<br />

concrete<br />

c c<br />

compressive strength, psi. For constant-depth, concentrically loaded footings with<br />

uniform factored soil pressure q s <strong>and</strong> neglecting the weight of the projection of the<br />

footing, the thickness h, in, can be calculated from<br />

qs h � 0.08X � (9.90)<br />

�ƒ�c<br />

where X � projection of footing, in <strong>and</strong> qs � net factored soil pressure, psf.<br />

Shear is not critical for plain-concrete wall footings. The maximum tensile<br />

stress, �5�ƒ�, due to flexure controls the thickness.<br />

c<br />

Reinforced Footings. Because it is usually not economical or practical to provide<br />

shear reinforcement in reinforced-concrete spread footings for walls, V u is usually<br />

limited to the maximum value that can be carried by the concrete, �V �<br />

c<br />

�2�ƒ�b d.<br />

c w<br />

Area of flexural reinforcement can be determined from A s � M u/�ƒ yjd as in-<br />

FIGURE 9.44 Critical sections for factored shear <strong>and</strong><br />

moment in wall footings.

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