Flat Mat foundation design in Etabs Bangla

Under normal conditions, square and rectangular footings such as those described in
Chapters 3 and 4 are economical for supporting columns and walls. However, under certain
circumstances, it may be desirable to construct a footing that supports a line of two or more
columns. These footings are referred to as combined footings. When more than one line of
columns is supported by a concrete slab, it is called a mat foundation. Combined footings
can be classified generally under the following categories:
a. Rectangular combined footing
b. Trapezoidal combined footing
c. Strap footing
Mat foundations are generally used with soil that has a low bearing capacity.
The mat foundation, which is sometimes referred to as a raft foundation, is a combined
footing that may cover the entire area under a structure supporting several columns and
walls. Mat foundations are sometimes preferred for soils that have low load-bearing capacities, but that will have to support high column or wall loads. Under some conditions,
spread footings would have to cover more than half the building area, and mat foundations
might be more economical. Several types of mat foundations are used currently. Some of
the common ones are shown schematically in Figure 6.4 and include the following:
1. Flat plate (Figure 6.4a). The mat is of uniform thickness.
2. Flat plate thickened under columns (Figure 6.4b).
3. Beams and slab (Figure 6.4c). The beams run both ways, and the columns are
located at the intersection of the beams.
4. Flat plates with pedestals (Figure 6.4d).
5. Slab with basement walls as a part of the mat (Figure 6.4e). The walls act as stiffeners
for the mat.
Approximate Flexible Method
In the conventional rigid method of design, the mat is assumed to be infinitely rigid. Also,
the soil pressure is distributed in a straight line, and the centroid of the soil pressure is coincident with the line of action of the resultant column loads. (See Figure 6.11a.) In the
approximate flexible method of design, the soil is assumed to be equivalent to an infinite
number of elastic springs, as shown in Figure 6.11b. This assumption is sometimes
referred to as the Winkler foundation. The elastic constant of these assumed springs is
referred to as the coefficient of subgrade reaction, k.
To understand the fundamental concepts behind flexible foundation design, consider a beam of width having infinite length, as shown in Figure 6.11c. The beam is
subjected to a single concentrated load Q. From the fundamentals of mechanics of
materials,

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