Spacing between Column Footings 1

[learning2geotech]

How far apart should column footings be apart from each other (edge to edge)?

If I use footing pressure of 0 at a depth of 2B, does that mean the footings should be 4B from each other (edge to edge) assuming a pressure distribution of 1:1?

When does it make sense to build a raft foundation connecting two or more column footings?

 

[r13]

If combined footing is undesirable, the minimum distance will be controlled by the compaction equipment. Other than basement, raft foundation makes sense is when 1) the force/settlement is excessively large for spread footing that makes raft foundation more attractive/economical, and 2) pile foundation is undesirable.

 

[mfrad]

Using a 1.5V:1.0H slope would have an average pressure of about 7.5% at 2B depth. 1:1 slope about 4% avg pressure at 2B. Since these are approximations at footing center lines the footing edge stress at B depth following a Boussinesque curved profile will have somewhat lower average pressure so the adjacent footings could be closer together. If you sketch the 1.5:1.0 with the Boussinesque curve you can judge for yourself how close the footings could be apart.

Regarding your next question, most engineers would consider a mat when the footing areas exceed 50% of the structure's footprint. This is an economic decision regarding footing forms, access, etc project by project. Don't forget that some footings carrying larger/smaller column loads are resized larger for balanced settlement to avoid differential settlements.

 

[emmgjld]

I gave mfrad the star because of the 2nd paragraph.
In my experience, a large number of engineers designing foundations have little understanding of the concept of balanced settlement (or minor to moderate heaving) computations. In my practice, this is notable in the design of lightly loaded (residential) foundations.