Footings supporting columns are usually made considerably larger than the columns to keep pressure and settlement within reasonable limits. Generally, each column is also placed over the centroid of its footing to obtain uniform pressure distribution under concentric loading. In plan, the footings are usually square, but they can be made rectangular to satisfy space restrictions or to support rectangular columns or pedestals.
Under soil pressure, the projection on each side of a column acts as a cantilever slab in two perpendicular directions.
The effective depth of footing d is the distance from the extreme compression surface of the footing to the centroid of the tension reinforcement.
Bending Stresses. Critical sections for moment are at the faces of square and rectangular concrete columns or pedestals (Fig. 9.45a). For round and regular polygon columns or pedestals, the face may be taken as the side of a square having an area equal to the area enclosed within the perimeter of the column or pedestal. For structural steel columns with steel base plates, the critical section for moment may be taken halfway between the face of the column and the edges of the plate.
Flexural Reinforcement. In square spread footings, reinforcing steel should be uniformly spaced throughout, in perpendicular directions. In rectangular spread footings, the ACI 318 Building Code requires the reinforcement in the long direction to be uniformly spaced over the footing width. Also, reinforcement with an area 2/ ( ß +1) times the area of total reinforcement in the short direction should be uniformly spaced in a width that is centered on the column and equal to the short footing dimension, where ß is the ratio of the long to the short side of the footing. The remainder of the reinforcement in the short direction should be uniformly spaced in the outer portions of the footing. To maintain a uniform spacing for the bars in the short direction for simplified placing, the theoretical number of bars required for flexure must be increased by about 15%. The maximum increase occurs when ß is about 2.5.
The required area of flexural reinforcement can be determined as indicated in Art. 9.46. Maximum-size bars are usually selected to develop the yield strength by straight tension embedment without end hooks. The critical length is the shorter dimension Ld shown for wall footings in Figs. 9.44a and b.
