Pole barn and frost heave

[Jared]

Let's say I built a pole barn, and then poured a slab. Since all the weight is on the poles that would be set into the ground, would the fact that there was no footings under the pad affect it at all?
Jared

[glenn-k]

I forget where you are at but in a bad area moisture could go under the edge of the slab, freeze and break it.  In mild areas a small footing - we call it a shovel footing or thickened edge is dug around the perimeter.  You could still insulate to prevent the problems but may be getting away from the reason you went to the poles.

[glenn-k]

The poles being below the frost line shouldnt have a problem as I see it.

[Jared]

I knew the poles wouldn't, but I wasn't sure about the pad. I'm in NE Oklahoma and we don't get any really hard winters, but we do get snow about every winter and some have been pretty dang cold. But nothing like the northern states. I just wouldn't want my sheetrock to crack every winter.
Jared

[Amanda_931]

Since you started with "lets say."

You could build a pole building--with the floor joists attached to the poles the 18 inches or whatever your area says for termites.

Or a rubble trench or insulated shallow foundation of some kind inside the poles for the roof.  That's not simpler, though.

Or that slab with thickened edges.

[Jared]

The slab with thickened edges is what I was looking for. Typicall, people around here dig down 18" for footings. So, with the poles set and cemented in at 18", and me wanting to pour a slab, how "thick" would I be looking for at the edges? 6" in the ground? Further?
Jared

[Amanda_931]

This is the book on vertical log cabins.  I wouldn't call it a plan, and my copy is on a different computer, durn it.  I think that he give instructions on how to do the floating slab, presumably suitable for permafrost--although the heavily forested parts of Alaska--look at the size of the logs in that photo--may be the same winter chill zone as you are.

http://www.alaskacabin.net/

[glenn-k]

Poles are usually set deeper at least on pole barns because they do the bracing too.  At only 18" you will have to do all the normal bracing -good anyway- along with temporary bracing until things get fastened together.because the poles will hinge and fall over.  With a power auger 4' or so is usually pretty easy.

Here there is no freeze problem and the shovel footing goes 6 inches below the slab on shop buildings.  I don't know if that would work there or if your freeze level is deeper than that -- seems John or someone mentioned that you can insulate horizontally also if you don't want to dig that deep.

[Amanda_931]

That's about what I remember for the "Alaska foundation."  Slab plus another six inches.  

[John Raabe]

Just because a slab is not load bearing doesn't mean it will act any differently if the ground under it freezes and expands.

If you have wet expansive soils under the slab then this soil can freeze down to whatever the frost depth is and lift the slab. If the poles are below this then the roof and wall structure will not move... but the floor will!  

The main question is can you provide drainage/and/or underslab material that will not hold water (crushed stone, for instance) so that freezing does not expand the soil.

For freeze protection you can either insulate the foundation (rather pointless in a building that won't be heated) or insure that the combination of water and expansive clay can be eliminated under the footings and slab supports. This will break the necessary conditions needed for frost heave.

Building in the Alaska permafrost is more complex and has different rules.

[Amanda_931]

John said:



and of course he's right.  Somewhere recently I've seen pictures of sinkholes formed when the permafrost starts melting.  Nothing much would survive that.