A combined footing generally withstands the load of two or more adjoining columns. This kind of footing is offered under the following circumstances:
1) When the columns are very close to each other as a consequence their footing overlap.
2) When the bearing capacity of soil is fewer, necessitating more region under singular footing.
3) When the end column is near a property line so that its footing cannot be spread in that direction.
A combine footing may be of different in plan such as rectangular or trapezoidal. The goal is to obtain identical pressure distribution under the footing. For this the CG of footing are should coincide with the CG of the combine loads of the two columns. There are basically four sorts of combine footing ordinarily used. They are as follows:
1) Combine rectangular footing
2) Combine trapezoidal footing
3) Strap beam footing
4) Raft footing.
To get better understanding not only it is mandatory to download pdf and but this numerical is of combine trapezoidal footing which is must watch also.