Foundations up North: A Survey
Foundations up North: A Survey
(OP)
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.
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.






RE: Foundations up North: A Survey
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.
RE: Foundations up North: A Survey
RE: Foundations up North: A Survey
42" minimum, and no one thinks twice about it. Many times we'll call out 48" so as to not worry about a sloped grading around the building/structure.
RE: Foundations up North: A Survey
This usually is indicated in the local code ordinances or at the very least your local geotechnical engineer would know what is appropriate for the locale.
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RE: Foundations up North: A Survey
Have also been doing/chasing a decent amount of work in the Carolinas recently. Frost line I've seen there has typically been 0" or sufficiently shallow that any normal footing thickness will exceed it.
If you're looking for various depths around the country just to get an idea, a good reference is actually UFC 3-301-01. They have frost penetration depths for various military installations and cities all over the US and worldwide. Flipping through the deepest one I see that doesn't indicate 'permafrost' is Valdez, Alaska at 132 inches (11 feet).
RE: Foundations up North: A Survey
RE: Foundations up North: A Survey
RE: Foundations up North: A Survey
Early in my career I saw a strip mall that had been designed by an Architect/Engineer from somewhere down south. They had an apron slab at the entrances that heaved and restricted the door swings.
gjc
RE: Foundations up North: A Survey
RE: Foundations up North: A Survey
Perimeter walls on houses will go down to frost. Usually they just excavate the trench and dump in concrete (with a bar or two literally tossed in). Typically no forming.
Commercial (and sometimes even residential) buildings also support slabs where exterior doors open to down to frost. That way the concrete just outside the door doesn't heave upwards and impinge on the door swing.
RE: Foundations up North: A Survey
Mine do. Just part of the cost of doing business in a northern climate I suppose. I flirted with some work in Alaska and that was really going to be something. Everything on piles that had to consider frost jacking etc. You sometimes need to be a bit careful with unformed foundation constructions in frost heave environments. Apparently, when the soil around your foundation moves, it can take your foundation with it if there's something to grab on to. A geotech talked me out of doing a corbel on a basement wall for this reason once.
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.
RE: Foundations up North: A Survey
1) Cold
2) Moisture
3) Frost susceptible soil.
I believe that the frost turn downs essentially eliminate the second item by preventing water from getting under the slab.
When I`m doing an exterior equipment pad approx. 1 in 10 geotech let me replace the soil below the equipment pad with gravel to address item 3. Its very rare though as I think they don't want the liability.
Without approval from the geotech, we pour everything (commercial, industrial) below frost depth.
RE: Foundations up North: A Survey
RE: Foundations up North: A Survey
In the residential market, basements are the norm with strip-footings, however a larger portion of residential is just defaulting to friction piles here as well.
RE: Foundations up North: A Survey
One needs to be careful about doors into the basement. The footing under the threshold of the door may have to go even deeper, to stay away from the frost line.
RE: Foundations up North: A Survey
Frost protection for interior spread footings in unheated parking structures is not specified by Code, so different engineers do different things, but something less than 1.2 m (4') is generally used (although my measurements of the air temperature in the bottom level of a 3 storey below grade garage a few years ago indicated that the it was still below freezing when it was below freezing outside). Get the soils engineer to make recommendation for this case.
RE: Foundations up North: A Survey
RE: Foundations up North: A Survey
Many geotechs will recommend that, if you're working way up north, you use piles, account for frost jacking, etc.
As was mentioned before, if you have any corbels within frost depth (such as a square pile cap on a round pile) make sure to use collapsible void form underneath the bits that are sticking out.
RE: Foundations up North: A Survey
RE: Foundations up North: A Survey
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).
RE: Foundations up North: A Survey
For Florida projects, I do top of footing at 16" below FFE, unless I need more for wind uplift resistance.
RE: Foundations up North: A Survey