Footings - Design of Reinforced Concrete - page 347

Placing concrete, Big Dig, Boston, MA. (Steve Dunwell/The Image Bank.)
2. An isolated or single-column footing [Figure 12.1(b)] is used to support the load
of a single column. These are the most commonly used footings, particularly
where the loads are relatively light and the columns are not closely spaced.
3. Combined footings are used to support two or more common loads [Figure
12.1(c)]. A combined footing might be economical where two or more heavily
loaded columns are so spaced that normally designed single-column footings
would run into each other. Single-column footings are usually square or rectangu-
lar and, when used for columns located right at property lines, would extend
across those lines. A footing for such a column combined with one for an interior
column can be designed to fit within the property lines.
4. A mat or raft or floating foundation [Figure 12.1(d)] is a continuous reinforced con-
crete slab over a large area used to support many columns and walls. This kind of
foundation is used where soil strength is low or where column loads are large but
where piles or caissons are not used. For such cases, isolated footings would be so
large that it is more economical to use a continuous raft or mat under the entire area.
The cost of the formwork for a mat footing is far less than is the cost of the forms
for a large number of isolated footings. If individual footings are designed for each
column and if their combined area is greater than half of the area contained within
the perimeter of the building, it is usually more economical to use one large footing
or mat. The raft or mat foundation is particularly useful in reducing differential set-
tlements between columns—the reduction being 50% or more. For these types of
footings the excavations are often rather deep. The goal is to remove an amount of