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FOUNDATIONS 



FORMULAS FOR THE DESIGN OF FOOTINGS 



The theoretical design of a spread reinforced-concrete 

 footing consists first in determining the total load on the 

 column to be supported by the footing, and then finding the 

 required area of the bottom reinforced-concrete footing or 

 layer of concrete by dividing the load by the assumed 

 allowable bearing value of the soil. The next thing to find 

 is the area of the base of the column where it bears on the 

 upper tier, or layer, of concrete, because the size of this por- 

 tion of the footing is fixed by the size of the column base. 

 After the areas of the bottom and top tiers, or layers, of con- 



FIG. 3 



crete have been ascertained, the projection of the bottom tier 

 beyond the upper is known, and the bending-moment stress 

 on the lower tier of concrete can be calculated. Sufficient 

 steel rods may then be introduced, and the footing made of a 

 depth that will resist this bending moment. 



The usual formulas for bending moments and the resist- 

 ance of reinforced-concrete rectangular sections may be 

 applied in determining the strength of reinforced-concrete 

 spread footings. However, in office practice, it is desirable 

 to use direct formulas for finding the area of steel 

 reinforcement required and for determining the unit 

 compression created in the upper portion of the concrete 

 footing. 



