When individual footing has to accommodate two or in exceptional case more than Two columns is known as the combined footing. or when two or three columns are located very near to each other, we provide a single substructure for them and the footing is known as a combined footing.
Combined footings are also usually made of reinforced concrete. A combined footing may be rectangle or trapezoidal in plan. The object is to get uniform pressure distribution under the footing. For this, the C.G of footing area should coincide with the C.G. of combined loads of the two-column.
When the combined footing is used?
A combined footing is used in the following conditions:
- When two columns are very close enough to each other such that their footing overlap.
- When the load-bearing capacity of the soil is low, the requirement of more area under the individual footing.
- When the column is close to the property line, So that footing cannot be spread in that direction and centre of gravity of column will not coincide with the centre of gravity of footing.
Types of Combined Footing
- Rectangular Combined Footing
- Trapezoidal Combined Footing
- Strap footing
- Raft Footing
1. Rectangular Combined Footing
This footing is used when two columns carry equal loads.
2. Trapezoidal Combined Footing
Combined footing for two columns become essential when the heavily loaded column is near the property line or when there is some restriction on the total length of the footing. or when one column carries heavy load compared to others.
3. Strap footing
This footing is useful when the external column is very near to property line. A strap footing is a combination of spread footings of two columns connected by a strap beam. Beam connecting two spread footing does not transfer any load to the soil. the function of the strap beam to transfer a load of the heavily loaded column to the inner column.
4. Raft Footing
Raft footing is a type of combined footing that covers the entire area beneath a structure and supports all the walls and columns.