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Cabin foundation

 
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I am building a timber frame cabin this summer. The frame/beams are already cut. It will be 16'x12' and was made to be built on skids. I don't want to put the skids directly on the ground, so I am looking into different foundation options for this structure. I live in the interior of British Columbia, Canada, where the frost line is approximately 4 feet deep. I have thought about bigfoot footings with sonotube piers. If I go this route I will need to dig down about 5 feet, and then have the tubes protruding a good amout so that there is space underneath the cabin for insulation and plumbing, etc. I have read that code is to have the sonotube diameter be 3x as much as the size of the beams. Can I get away with having them smaller? Maybe 2x the diameter to save on cement? I have also thought about using rammed earth or cement filled tires for the footings, as well as ground screws or Cement footings and cinder blocks. Does anyone have any experience with foundations for a timberf rame structure of this size on skids? Any experience/advice is much appreciated. I am still in the brainstorming stage, and just trying to figure out what makes the most sense. Since it's on skids it can be lifted and moved in the future. I also plan on doing an addition to double the size in the future.

TIA
 
pollinator
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I have experience with skidable structures, but not with a four foot frost line.

My favorite solution is the big recycled concrete blocks. 2x2x4 foot and weigh a ton each.  But I only have to sink them a foot down.

The right answer depends a lot on your subsoil.

No rocks and you can find someone local ish to do it, screw piers should be cheapest. But they are new and overpriced in many areas.

 
pollinator
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ARE YOU TAlking about beams or posts?
There is a difference.
From my general reading the 3 x seems the standard, with a bell bottom.
I suggest installing the floor insulation and plumbing during construction, from above to make it easier.
Surely for the depth of snow a few feet clearance is needed anyway.
Its not a good idea to crimp on foundations anyway!

 
steward
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I am not familiar with:   bigfoot footings with sonotube piers.  So I asked ....

https://www.bigfootsystems.com/index.htm

sounds like a lot of work for a skiddable cabin.  Why?

 
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Location: 55 deg. N. Central B.C. Zone 3a S. Nevada. Hot and dry zone
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Might be wise to put more emphasis on bearing than frost line. A timber frame built on piers will simply flex or tilt a bit if the ground heaves. No drywall, no worries, maybe a sticking door.
We built a 700 s.f. sauna/storage building out here almost 30 years ago. My dad used 10 in sonotube <3ft. in the ground, with a 12-18 crawlspace under the building. All flexible wod finishes inside. The only visible issue is the pier nearest the lake is subsiding as the shore recedes/softens. Built too close to the lake.
One way to think of it is bearing surface of your foundation. We live +-55 deg North in B.C. Our soil is silty/sandy glacier gravel/rock. I would use a figure of 3-4000 lb./s.f. as the amount of pressure undisturbed/compacted soil up here can withstand.
I can't see your structure, but 8x12 skids and 6x8 purlins with 2x8 joists is a pretty solid floor.
Estimate total weight of structure and you can begin to figure number of piers needed and size. As long as the pier provides full bearing the width of the skid/beam, quality reinforced concrete pier can easily tolerate the weight required, even cantilevered 2 ft. out of the soil.

If access to delivered concrete is the issue, you don't specify, then a 5 c.ft. PTO driven mixers are not that expensive, you'll need a 25-30 h.p. tractor to mix/move/dump it and access to mixed aggregate. Likewise the little 2 c.ft. electrics. The advantage of sonotube is you can mix and pour in whatever full tube amount you desire. Walls/footings/foundations in general like to be poured continuous, no dry joints, without keying and reinforcement.
If costs are the issue, then there are other ways to go about this, and you have some reading to do. Some have been explored on this forum, look about.
To be honest, 12x16 is not that big, complex, or difficult. Don't cheap out on a foundation, but don't lose sleep over it either.
If you are not positive your location is it's forever home, then I would not hesitate to level the house on wood cribbing, while you think about the future. Even out here, as wet as it can be, sacrificial cribbing will last for years. While you are thinking of insulating your floor, I would absolutely figure to sheet it tight in with plywood once utilities are done. Packrats will destroy anything they can hide in or access.
For me, a basement would be the answer, cheap but harder to build space, great food/dry goods storage.
Caveat, if you have building codes, (we don't) then you will do as instructed, and your friendly comrade direktor/public safety officer will want engineering for foundation and wastewater.
 
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