Raft Foundation Minimum Thickness

[MN91]

Dear engineers,

I've been handed a plan for a raft foundation of thickness 30 cm to be executed. My question is, will a thickness of 30 cm have the rigid behavior a raft foundation is supposed to have?

Attached is a section of the foundation.

Regards

 

[hokie66]

Depends what you mean by "raft foundation". I wouldn't call that a raft. What do you mean "to be executed"? Are you building it?

 

[MN91]

The basement floor (raft) shown in the previous attachment has no intermediate supports for the upper slab, only the shown basement wall (upper slab designed for a live load of 4 tons/m2). The basic design was having a 40 cm footing along the basement wall (see attachment), then the decision was to go with a 30 cm raft along the entire area.

I know you hate centimeters but sadly these are the units we use here.

And yes, our company is responsible for both the design and execution of this basement.

 

[KootK]

Your intuition is correct: the mat footing will not be very rigid at all. That being said, a raft footing doesn't necessarily need to be rigid in order to do its job. You would simply need to perform an analysis that accounts for flexibility rather than assuming rigid behaviour.

Because the raft footing will be very flexible, the concrete in the middle of the raft won't be contributing much. The original design seems as though it would be more efficient unless you're trying to resist hydrostatic pressures etc.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[jayrod12]

If you're arguing about 10cm of concrete why not just thicken up the edge of the "raft" underneath the foundation walls to provide the 40cm determined from the original analysis.