This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
Example. Assume that the columns on one side of a building are to be supported by a continuous footing; that the columns are 22 inches square, spaced 12 feet on center; and that they support a load of 195,000 pounds each. If the soil will safely support 6,000 pounds per square foot, the area required for a footing will be 195,000 ÷ 6,000 = 32.5 square feet. Since the columns are spaced 12 feet apart, the width of footing will be 32.5 ÷ 12 = 2.71 feet, or 2 feet 9 inches. To find the depth and amount of reinforcement necessary for this footing, it is designed as a simple inverted beam supported at both ends (the columns), and loaded with an upward pressure of 6,000 pounds per square foot on a beam 2 feet 9 inches wide. In computing the moment of this beam, the continuous-beam principle may be utilized on all except the end spans, and thus reduce the moment and therefore the required dimensions of the beam. Many engineers ignore this principle, since it merely increases the factor of safety to do so.