Foundation Design based on soil test reports and column loads

Part 5

Mat Foundations

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Combined footings and Mat (Raft) Foundations

Combined footing: footing that supports a line of two or more columns Mat or Raft foundation: A ground concrete slab that supports more than one line of columns

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Combined footings can be classified generally under the following categories: a. Rectangular combined footing b. Trapezoidal combined footing c. Strap footing

Combined footings

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1. Flat plate (a). The mat is of uniform thickness. 2. Flat plate thickened under columns (b). 3. Beams and slab (c). The beams run both ways,

and the columns are located at the intersections of the beams.

4. Flat plates with pedestals (d). 5. Slab with basement walls as a part of the mat (e).

Mat Foundations

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Bearing Capacity of Mat Foundations

The gross ultimate bearing capacity of a mat foundation

For mats on sands, since the factor of safety against bearing capacity failure is very large, the designs are governed by the settlement considerations

Net allowable bearing capacity of a mat foundation based on settlement considerations