Frost depth for turndown slab

[strlengr7]

I have virtually an unheated metal shack. It will sit on a slab on grade with a turndown. Is is necessary for the turndown to bear below the frost depth? In my mind, only unheated bearing foundations should be below the frost depth. Even though the shack will put load on the turndown, the load is minimum. Any thoughts?

 

[woodman88]

The IBC 2009 section 1809.5 gives an exception for meeting this depth for agricultural and minor storage buildings 600 sf or less with eaves 10 ft or less.

Garth Dreger PE
AZ Phoenix area

 

[BAretired]

In my area (Edmonton, Alberta) the frost penetration in an unheated area is about six to eight feet, depending on snow cover and natural insulation.

For sheds and unattached garages, it is common practice to use a slab on grade with a thickened edge all around the perimeter. The edge thickening is completely within the active zone of frost. If the underlying soil is frost susceptible, say a clay, heaving may be expected during the winter season. If heaving is uniform, it may not be noticed by the occupants, but if the moisture content is variable, the magnitude of heave can vary from one place to another. In buildings of this sort it is prudent to avoid rigid materials such as drywall because it is susceptible to cracking. The perimeter beam is usually reinforced to spread uneven settlement over a broader area.

For residential or commercial buildings, it is common practice to provide foundations below the frost depth in order to avoid the unsightly aspects of heaving.

BA

 

[CarlB]

Depends what "virtually unheated" means. If building is kept above freezing, you should be able to design the foundation using "frost protected shallow foundation" methods (FPSF) but maybe even more insulated depending on inside temperature. Generally 2 feet deep turndown/thickened edge, with 2" rigid insulation against that, then 2" x 4' horizontal insulation 2 feet down.

 

[DaveAtkins]

I am currently working on a project using the FPSF methods CarlB mentioned. Design depends on a number of factors, but for my case, construction will consist of a 10" slab over 14" of free draining gravel over 3" rigid insulation. Interestingly, the rigid insulation must extend several feet beyond the edge of the slab on all sides.

Dow makes below grade insulation in various compressive strengths for this application.

DaveAtkins

 

[dik]

BARet... Edmonchuk is almost as cold as Whipanegg... I don't know what your soils are, but we have, generally, a highly plastic clay for a depth of about 40'. Generally as long as silt is excluded, and good drainage is provided, then stiffened slabs do fairly well. Have used them in more northern locations. Also, I have several papers on the freezing of clays; for freezing to occur, there must be a phase change, and because of the impervious nature of some clays, you can have water (supercooled) in a liquid state at -40... I don't rely on this, but it may explain why they work as well as they do.


CarlB... I generally place the insulation closer to grade... about a foot down to take as much advantage of the geothermal heat and to provide some measure of protection.

DaveA... I spec the DOW, Type 4, extruded polystyrene almose exclusively... great product. DOW also has a bit of a tech blurb on the use of insulation for geotech info...

Dik

 

[BAretired]

dik,

The soil in the Edmonton area is predominantly clay. Geotechnical Engineers sometimes classify it as highly plastic, but I don't think it is as plastic as the clay in Winnipeg. I have heard that Saskatchewan is even worse.

Most engineers tend to avoid shallow foundations because of frost movement, but for a shed, perhaps a little frost heave is not such a big deal.

Retaining walls are also an issue. Some say the foundation should go below the frost line, but that makes it so expensive that most retaining walls are founded in the active zone. When that is done, the wall should never be tied rigidly to a building with deep foundations.

BA

 

[dik]

BARet... Can I call you Bart? <G>

Agree... and even with well founded retaining walls, I don't tie them into real structure.

Dik

 

[BAretired]

Dik,

You may call me anything you wish. Bart is okay, but my real name is Bruce...Bruce Alexander, sort of like James...James Bond, but I have not had the same success with those of the opposite gender...I wonder why? Any hints would be appreciated.

BA

 

[BAretired]

Let me re-phrase that last remark. I am completely happy in my present circumstances.

BA

 

[BAretired]

Or, perhaps, even if not completely happy, too bloody old to do anything about it.

BA

 

[DaveAtkins]

BAretired,

Maybe instead of saying "Bruce...Bruce Alexander," you should be saying...

"My initials are "BA," as in "B A Baracus!"

I thought that emoticon sort of looks like B A Baracus.

DaveAtkins

 

[BAretired]

B. A. (Bad Attitude) Baracus as played by Mr. T. I like it.

BA

 

[dik]

and what's wrong with Baccus?

Dik

 

[Splitrings]

If your structure can withstand both uniform and non uniform weave then providing a frost protected foundation isn't likely necessary.

Otherwise, I would start with ASCE 32-01 "Design and Construction of Frost Protected Shallow Foundations. Assuming you are in the US. The maps only cover the contiguous US plus Alaska.

 

[dhengr]

BA:
Hint.... Happy or not, you’re in the wrong chat room here, given what you claim is your deficiency or lack of luck.

While you seem to know an awful lot about structures, buckling lengths and unbraced lengths, and the like. You’ve been looking for all the wrong things in all the wrong places, it’s not unbraced your looking for, it’s embraced. I’m sure there is a 25 or 30 year old chick out there who could show you a thing or two about the kl/r of you cantilever.

 

[BAretired]

Thanks, dhengr. I wonder if Mrs. A would approve?

BA