A footing when used for two columns or more than two columns is called combined footing. Combined footing is mainly two types.
Rectangular Combined Footing.
Trapezoidal Combined Footing.
This type of footing is provided under following situations:
1. When the columns are located extremely close to each other and their individual footings are overlapping.
2. In case of soil having low bearing capacity and a large area is required under the individual footing.
3. When the column end is situated near the property line and it is not possible to extend the footing area on the side of the property line.
The main purpose of using combined footing is to distribute uniform pressure under the footing. To accomplish this target, the center of gravity of the footing area should be equal to the center of gravity of the two columns.
If the outside columns close to the property line, conveys heavier load, it is necessary to provide trapezoidal footing to maintain the center of gravity of the footing in line with the center of gravity of the two column loads. Otherwise only a rectangular footing may be provided. Following extra points should be kept in view to provide combined footings.
1. This type of footing is considered as an inverted floor, where footing load is carried by the columns and loaded from underneath by uniform earth reaction.
2. The area enclosed by combine footing should be equivalent to or more than that acquired by dividing the total combined load into the columns by safe bearing capacity of the soil.
3. The shape of the combined footings should be selected in such a manner that it concurs the center of gravity with the center of gravity of soil reaction.