Definition and Types of Combined Footing
When there is a need to accommodate two or at times even more columns by an individual footing it is known combined footing. It can also be defined as the footing which is provided when two or three columns are located close to one another. Reinforced concrete is used to make combined footing. In plan a combined footing is usually rectangle or trapezoidal.
The idea is to get equal amount of pressure distribution under the footing. Therefore, it is critical for C.G of the footing area to coincide with the C.G of combined loads of the two-column.
When Do We Use Combined Footing?
The following are the situations when combined footing is used:
- When the footing overlaps as a result of two columns which are close by.
- There is a requirement for more area under individual footing when the load bearing capacity of the soil is low.
- It is also used when the column is close to the property line. The footing cannot be extended to that direction and the center of gravity of the columns will not coincide with the center of gravity of the footing.
What Are The Different Types Of Combined Footing?
1. Rectangular combined footing
2. Trapezoidal combined footing
3. Strap footing
4. Raft footing
Rectangular Combined Footing
This footing is used when two columns carry equal load.
Trapezoidal Combined Footing
This type of footing becomes essential when the heavily loaded column is near the property line or even when there are certain restrictions on the overall length of the footing. Also, when one column carries a heavier load than the other.
This kind of footing is used when the external column is very close to the property line. A strap footing is a combination of spread footings of two columns which is connected by a strap beam.
When the beam is connected to two spread footing it does not transfer any load to the soil. The transfer the load of the heavily loaded column to the inner column takes place as a result of the strap beam.
Raft footing is a type of combined footing which expands all throughout the entire area beneath a structure and can provide supports to all walls and columns.