Design of Combined Footing Excel Sheet
A footing when used for two columns or more than two columns is called combined footing.
The combined footing is mainly two types:
Rectangular Combined Footing
Trapezoidal Combined Footing.
Combined footings are provided to support two or more column loads. These may be continuous with rectangular or trapezoidal.
A Combined footing is used to support the load of two or more adjacent columns.
To have a uniform bearing pressure and to minimize differential settlement between columns.
When Combined Footings provided
When the columns are nearer to each other so that their isolated footings overlaps.
when the bearing capacity of the soil is lower and required more area under the individual footing.
When one column is close to a property line or the Centre of gravity of column will not coincide with footing or dimensions of one side of footing are restricted to some lower value.
Combined footings become necessary under the following circumstances:
- when the isolated footings overlap.
- when the exterior column is close to the property line with the result symmetrical isolated footing cannot be provided.
This type of footing is provided under following situations:
When the columns are located extremely close to each other and their individual footings are overlapping.
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.
Bearing capacity of soil is so low.
Area of footing = Total Load/ Allowable Bearing Capacity of soil
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.
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.