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Concrete slabs and concrete posts
I am about to build a 50 x 60 pole barn. I live in Alberta and typically have frost to 6-8 feet in winter. I have been advised to dig my post holes to 8 feet, set 2 feet of concrete in the bottom for a day to harden and then set my posts on the concrete and fill to the surface. These piles will then be uneffected by the heave and drop of the frost in the winter. Should I be using Soma tubes and what size auger holes? I have 6 X 8 posts. Does this all make sense? I will then be adding a heated concrete floor at a later time. How do I tie the concrete to the pre-set posts? Do I need footing around the edges that go to 8 feet? will be very expensive if this is required.
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Concrete slabs and concrete posts
You might want to forego the post-type building and just erect a standard wood-framed or even an open-truss or clearspan steel building. This is better for many reasons. Cost is about the same; easier and quicker to make the footer; no issues about post alignment or rotting at or below ground; insulating or isolating the slab is easier; insulating a stud wall is easier as is installing any type of wall board.
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Concrete slabs and concrete posts
I'd suggest finding out the applicable building codes and prevailing practice in your area. (That brand is "Sonotube," named for the maker, Sonoco.)
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Concrete slabs and concrete posts
jbrannan, the Sonotubes are critical, they are the forms for the concrete poured for the footings of the posts, you should also IMHO (and if you can manage it) use the special cone-shaped pieces that go at the bottom too.
The reason the Sonotubes are so important is that they form the concrete into smooth cylinders, not just what ever shape the inside of the hole may have been. This is required because as ground freezes it also expands slightly, if the soil freezes around a rough shape it can grip it tight enough to raise it up, over repeated years this can lift the building enough to do real damage.
By making a smooth piling there is little for the earth to grab onto and it slips up along the piling instead of lifting it. Likewise, the cone at the bottom will act as an anchor to help prevent the lifting action.
Around here we use an 'isolator' (usually plastic or foam) around the post so that when the floor is poured it doesn't bond to the posts, that way if one (floor or posts) moves, it doesn't affect the other.
Best of luck.
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Concrete slabs and concrete posts
Another reason to use a "Sonotube" is hole collapse. I've augered many a hole--up to 24" diameter and 13' deep. ANY gound water allowed to gather in the hole will in short order collapse a hole. It can take a days--or the conditions are right--minutes.
And the deeper you go the harder it is to make the hole straight or plumb---unless you're siing a rig that drills perfectly straight. Aside from that, you'll have to over-auger the hole size plenty to be sure you can just drop the tube in and not have to pound on it which will destroy it.
There are different types of generic Sonotubes too. Some are waxed inside or out. Some are made to be easily removed (for above-ground use). Because they're made of paper and will rot if in moist conditions, you might want to consider wrpapping them with thick-mil polyethylene sheeting for reasons Murf mentioned about frost heaving.
Also, the coated tubes are made to resist water in a hole if they have to sit there unfilled. If not they will come apart.
You didn't specify what you would be backfilling with--earth, stone, pea gravel, or concrete. Around here the debate rages about pouring concrete around the poles which leads to rot because the posts shrink which gives water a place to stay. I think I'd consider applying tar to the ends and sides of the posts then wrapping them with poly sheeting--the tar isolates them from moisture and the poly allows for slippery anti-heave surface.
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Concrete slabs and concrete posts
Thanks for the responses. I'll be sticking with the wooden barn and using the waxed Sonotubes. Any comments about the hole sizes for 6X8 posts?
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Concrete slabs and concrete posts
Depends what's down there. In terms of downward pressure, a concrete post can bear an enormous amount of weight (a bit more if rebar-reinforced in the center) so strength isn't the concern, it's sinking.
Drill a test-hole or two. Poke at the bottom. If it feels like rock, you can drill a skinny hole. If it gives any, drill a fatter one, and lay down up to a foot of cement first -- or use the conical tubes that someone above referred to.
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