Under what conditions combined footings are adopted ?

Combined footings are adopted under the following circumstances:- 

In this article, I discussed the condition in which we provide combined footing. 

1. When two or more columns/walls are located close to each other 
2. If the two columns are relatively heavily loaded or footing constructed are to be rested on soil with low safe bearing capacity.
3. When isolated footings for individual columns are touching or
   overlapping each other.
4. When the columns are located near the boundary lines.