posted 2 months ago
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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