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.

 

[KootK]

Canada. 4' for heated structures in lot of places. Serious buildings usually have their foundations go down to frost depth. Less serious stuff may utilize other techniques such as shallow frost protected foundations for or soil replacement with non-susceptible material. Nice to have a geotech on board for making these kind of decisions.

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.

 

[structSU10]

In NY and frost depth is 4' typically. We generally set everything down for frost along the perimeter of the building, interior foundations are brought up to just below the floor.

 

[WinelandV]

Iowa

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.

 

[JAE]

Iowa, Nebraska, South Dakota, Illinois...etc. - 3'-6" below grade.

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.

Check out Eng-Tips Forum's Policies here:
faq731-376

 

[MrHershey]

Chicago area. 42" is codified in Chicago Building Code and is the typical value I've seen for work in surrounding areas as well. We've broached the subject of maybe doing some extra insulation/site work per ASCE 32 to not have to do that but costs have worked out so far to just bury them.

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).

 

[WARose]

Thanks for the feedback so far. Do some of these numbers include residential foundations? (Seems like that could get expensive for a house.)

 

[structSU10]

basements.

 

[mtu1972]

Like many above 4' below grade in most of WI and MI. That may then be 4'-6" +/- below first floor. Can't remember what we used in Northern MN. It's much colder there.

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

 

[kipfoot]

Local convention and geotechs will give the particulars, but this barely legible map gives the HUD ideas about frost depths in the US: HUD Appendix H

 

[DETstru]

WARose: yes, residential as well.
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.

 

[KootK]

Do some of these numbers include residential foundations? (Seems like that could get expensive for a house.)

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.

 

[Once20036]

I've always been told that for frost heave to occur, you need three things:
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.

 

[darthsoilsguy2]

Vermont is 5 feet. Up to 2016, it use to be 4'-6" in my region of VT but the state office made it one number for the whole state in their adoption of the IBC. Same goes for residential. It's the reason Yankees all have basements.

 

[jayrod12]

Where I am, Longitudinal centre of Canada about an hour from the US border we default to 6-8 ft of frost. Spread-bore piers and piers on pads are common for end-bearing commercial projects with no basements. However the majority of commercial projects just go straight to friction or driven piles.

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.

 

[VEBill]

Nova Scotia Canada, minimum of 4-foot deep footings is pretty standard. And once you go down 4-feet, it's almost standard to make a full (8-foot) basement of poured concrete - maybe 6-feet of it underground and maybe 2-feet above (YMMV).

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.

 

[ajk1]

Toronto practice is 1.2 m (4 foot) depth to underside of perimeter footings, for residential, commercial and industrial, irrespective of soil type, unless insulated foundation is used (it is rare to use insulated foundation).

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.

 

[JLNJ]

If I don't have a recommendation from a geotech or a jurisdiction which has a code requirement, I use the attached frost map. Despite being about 75 years old it seems to match up pretty well with local requirements.

 

[atrizzy]

I've done spread footings at 2.5m deep in the past, but this was for industrial structures.
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.

 

[John2025]

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? I guess that's obvious, but it seems sooo shallow to me.