Footings - Design of Reinforced Concrete - page 357

Figure 12.7 Shapes of isolated footings.
lar or octagonal. Rectangular footings are used where such shapes are dictated by the avail-
able space or where the cross sections of the columns are very pronounced rectangles.
Most footings consist of slabs of constant thickness, such as the one shown in Figure
12.7(a), but if calculated thicknesses are greater than 3 or 4 ft, it may be economical to use
stepped footings, as illustrated in Figure 12.7(b). The shears and moments in a footing are
obviously larger near the column, with the result that greater depths are required in that
area as compared to the outer parts of the footing. For very large footings, such as for
bridge piers, stepped footings can give appreciable savings in concrete quantities.
Occasionally, sloped footings [Figure 12.7(c)] are used instead of the stepped ones, but
labor costs can be a problem. Whether stepped or sloped, it is considered necessary to place
the concrete for the entire footing in a single pour to ensure the construction of a monolithic
structure, thus avoiding horizontal shearing weakness. If this procedure is not followed, it is
desirable to use keys or shear friction reinforcing between the parts to ensure monolithic ac-
tion. In addition, when sloped or stepped footings are used, it is necessary to check stresses
at more than one section in the footing. For example, steel area and development length re-
quirements should be checked at steps as well as at the faces of walls or columns.
Before a column footing can be designed, it is necessary to make a few comments re-
garding shears and moments. This is done in the paragraphs to follow, while a related sub-
ject, load transfer from columns to footings, is discussed in Section 12.8.
Shears
Two shear conditions must be considered in column footings, regardless of their shapes.
The first of these is one-way or beam shear, which is the same as that considered in wall
footings in the preceding section. For this discussion, reference is made to the footing of
Figure 12.8. The total shear ( V u 1 ) to be taken along section 1-1 equals the net soil pressure