Frost Depth on Spread Footing

[BayPointArchitect]

As you all know, water freezes at 32 degrees. So my thinking is that frost heave occurs where there is moisture in the soil and it is freezing cold outside.
Here in the land of extreme temperatures, we sometimes get a structural engineer with a southern accent wrestling with the fact that we actually enforce something called "frost depth". It is most unfortunate after the foundation is poured and we then realize that the surrounding grade needs to remain low. Not good.

Looking at the attached detail, would you say that this is okay or should we add an inch of insulation anywhere between the grade level and bottom of the horizontal spread footing?

Thanks!

ICC Certified Plan Reviewer
NFPA Certified Fire Plan Examiner

 

[tmurray]

Assuming from your notes, the soils will be subject to ice lensing and assuming that the engineer did not design the foundation to resist frost heave, I would be looking for insulation. In my neck of the woods, 1" of foam is equivalent to 1' of frost cover.

 

[BayPointArchitect]

Part of the problem, in this case, is that the architect illustrated on his grading plan that the bottom of the foundation needed to step down on the "low-side" of the block in order to match the surrounding street and sidewalk grades. Maybe the structural engineer thought that we might raise the public streets to make the site "flat" because his foundation plan works great if the site was perfectly flat. After final grading was completed and seeing that the spread footing was clearly visible, it was decided that they would extend an 8" thick chunk of concrete down 42 inches and try to cover up the mistake with some dirt, grass, plantings - whatever it takes. I would have done the same thing but I would have added 2 inches of insulation to ensure that the horizontal ledge below the undisturbed spread footing remained above 32 degrees.

 

[tmurray]

The insulation would have been better and cheaper. Now I guess the questions is if the frost getting under the original foundation will heave the "new foundation". My feelings are that if it can heave the whole building, a little bit of extra concrete isn't going to stop it. Personally, I would ask for confirmation from the architect and engineer of record to confirm that the new foundation arrangement will not be susceptible to frost heave. So, a letter sealed and signed to put in the file clearly stating that the foundation is designed so that any movement from frost will not damage structural components or damage the environmental separator.

 

[Paul Sweet]

How about a Frost-protected shallow foundation (IRC R403.3)?

If this is under the IBC, 1809.5 mentions ASCE 32 (Design and Construction of Frost Protected Shallow Foundations)