Foundations up North: A Survey 3

[WARose]

I'm someone who has done the vast majority of his design work in the southeast (USA) and the rest in places like Phoenix. The few times I've done foundations up North the frost depth/penetration forced me to put the foundation pretty deep (especially compared to southeast). In other cases however, the geotech let me put them at much shallower depths.

So the purpose of this thread is kind of a survey for those of you who practice in the northern states (where the frost depth is high): How often do you wind up having to put them deep? And on average: how deep?

I ask because lately I had one come back where my MTO going in was off.....and working backwards, that's where the difference was.

Thanks.

 

[WARose]

Having been born and raised in the north, I'm curious going the other way. How deep do you go down south? Just the thickness of the footing?

Depends where we are talking. (Obviously "the south" can include places like Tennessee and Virginia where it gets deeper than the 12" min. that (IIRC) the IBC requires.)

I live in the upstate of South Carolina, and pretty much all the geotechnical reports I've read for this area go with 12" (if they mention it at all). For all practical purposes, in residential that means basically the depth of footing + some soil to cover the footing. (For my parent's house for example, it's a 8" wall footing + about 6" of soil (on the exterior side).) In commercial and industrial, it will mean the depth of footing + whatever slab/floor is needed. Sometimes you wind up needing a bit more depth for hold down purposes (in a uplift situation).

 

[peal724]

Frost depth is around 24" where I am in the Southeast. At exterior footings, I do top of footing at 24" below FFE, which puts bottom of ftg around 36" below FFE but allows for some step down and sloping exterior grade.
For Florida projects, I do top of footing at 16" below FFE, unless I need more for wind uplift resistance.

 

[hawkaz]

In the Southwest, bottom of footing is generally 18" below grade