A combined footing is a long footing supporting two or more columns in (typically two) one row. A combined footing is a rectangular or trapezoidal shaped footing.
The function of a footing foundation is to transmit the load form the structure to the underlying soil. The choice of suitable type of footing depends on the depth at which the bearing strata lies, the soil condition and the type of superstructure.
Summary of combined footing design is shown in the following steps.
1. Select a trial footing depth.
2. Establish the required base area of the footing: Uniform soil pressure is achieved by making the resultant of the service column loads coincide with the centroid of the footing base.
3. Evaluate the net factored soil pressure.
4. Check footing thickness for punching shear.5. Draw S.F.D and B.M.D for footing:
5. Draw S.F.D and B.M.D for footing:
The shear force and bending moment diagrams for the footing are to be drawn, considering the footing as an inverted beam subjected to distributed load. Column loads may be considered as concentrated loads to simplify the analysis.
6. Check footing thickness for beam shear.
Beam shear capacity is checked against critical factored shear evaluated from S.F.D. Depth of footing may be increased to satisfy beam shear requirement.
7. Compute the area of flexural reinforcement:
Flexural reinforcement required in the longitudinal direction is evaluated. Furthermore, the areas of flexural reinforcement under columns in the transverse direction are computed. In computing this reinforcement, it will be assumed that each column load is uniformly distributed over a band centered on the column and having a width extending a distance d/2 from its faces. This area of reinforcement is to be uniformly distributed across the band width (C2+d), and the rest of the footing is to be provided with shrinkage and temperature reinforcement.
8. Check bearing strength of column and footing concrete.
9. Check for anchorage of the reinforcement.
10. Prepare neat design drawings showing footing dimensions and provided reinforcement.
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