Structural Concrete - Hassoun

13.2 Types of Footings 445
Vertical loads are usually applied at the centroid of the footing. If the resultant of the applied
loads does not coincide with the centroid of the bearing area, a bending moment develops. In this
case, the pressure on one side of the footing will be greater than the pressure on the other side.
If the bearing soil capacity is different under different footings—for example, if the footings
of a building are partly on soil and partly on rock—a differential settlement will occur. It is usual
in such cases to provide a joint between the two parts to separate them, allowing for independent
settlement.
The depth of the footing below the ground level is an important factor in the design of footings.
This depth should be determined from soil tests, which should provide reliable information on
safe bearing capacity at different layers below ground level. Soil test reports specify the allowable
bearing capacity to be used in the design. In cold areas where freezing occurs, frost action may cause
heaving or subsidence. It is necessary to place footings below freezing depth to avoid movements.
13.2 TYPES OF FOOTINGS
Different types of footings may be used to support building columns or walls. The most common
types are as follows:
1. Wall footings are used to support structural walls that carry loads from other floors or to
support nonstructural walls. They have a limited width and a continuous length under the
wall (Fig. 13.1). Wall footings may have one thickness, be stepped, or have a sloped top.
2. Isolated, orsingle, footings are used to support single columns (Fig. 13.2). They may be
square, rectangular, or circular. Again, the footing may be of uniform thickness, stepped,
or have a sloped top. This is one of the most economical types of footings, and it is used
when columns are spaced at relatively long distances. The most commonly used are square
or rectangular footings with uniform thickness.
3. Combined footings (Fig. 13.3) usually support two columns or three columns even if not in
a row. The shape of the footing in the plan may be rectangular or trapezoidal, depending on
column loads. Combined footings are used when two columns are so close that single footings
cannot be used or when one column is located at or near a property line.
4. Cantilever,orstrap, footings (Fig. 13.4) consist of two single footings connected with a beam
or a strap and support two single columns. They are used when one footing supports an eccentric
column and the nearest adjacent footing lies at quite a distance from it. This type replaces
a combined footing and is sometimes more economical.
5. Continuous footings (Fig. 13.5) support a row of three or more columns. They have limited
width and continue under all columns.
6. Raft, ormat, foundations (Fig. 13.6) consist of one footing, usually placed under the entire
building area, and support the columns of the building. They are used when
a. The soil-bearing capacity is low.
b. Column loads are heavy.
c. Single footings cannot be used.
d. Piles are not used.
e. Differential settlement must be reduced through the entire footing system.
7. Pile caps (Fig. 13.7) are thick slabs used to tie a group of piles together and to support and
transmit column loads to the piles.