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Thread: Cabin foundation

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    Default Cabin foundation

    Looking for some help with my cabin foundation. We plan on building a 24x32 cabin and have drilled 8 pilings for it. The pilings are 3 1/2" and range from 30'-40' in depth. After talking to a couple people they dont think that is enough support. Any advice?? Any input is appreciated

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    Member mit's Avatar
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    I built a 16X20 16X24 2 story on permafrost on post that where close to what you have there.
    Tim

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    hmm, well seeing as how this cabin is twice the length doesn't look promising.

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    Do you already have the holes drilled?The 8' span from the post to the outside of the cabin should be of concern. How high are the pilings out of the ground? I would think that you will need cross bracing welded on the pilings. If there was some way to set off some type of explosive in the bottem of the pillings to expand the base to give it some type of footing suport,otherwise I think that the pilings will gradualy sink with such a small footprint of 3.5".

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    yes the piles are already drilled in. The 8' gap is also my concern. The piles are about 2'(feet) out of the ground. I would think that since the piles go 30' to 40'(feet)deep into the muskeg and were drilled in with 3000 psi that they should have a strudy base. Another concern is that if i did weld some sort of offset im not sure if the 3 1/2" galvanized pipe would be strong enough. I was thinking of maybe getting some kind of extra wide beams to sit on top of the piles (something like 2' wide by 32' long) then atleast there would only be 7' without support. Any thoughts?

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    Add 2 more posts on each row and back off the over hang on the ends or add more posts there too. I used 12" TJI's they support a long span.
    Tim

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    I'm trying to get away from adding more pilings. I'm already in for $6,000 for the ones i put in. Im worried if i get him to come back for a couple more ill crack that 10k mark just for piles.

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    Your pilings are 3 1/2"? What are they made of? If you don't want more just go with a smaller footprint and go up.
    Tim

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    I hate to be the bearer of bad news, but the concerns expressed by the people you talked to are correct. That huge 8' cantilever on each side is the biggest concern. It would be cheaper to add more pilings than to come up with the engineering (even if it could be done) to build what you want on that current setup. You really need a third row of pilings and even that is only going to get you a realistic 20 x 20 cabin. Right now, your current pilings will support a 12 x 20 if there is any snow load at all.

    Regardless, the maximum load bearing cantilever on a BCI is 2'. As are other wood I joists.
    Bunny Boots and Bearcats: Utility Sled Mayhem

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    Quote Originally Posted by AKDoug View Post
    Regardless, the maximum load bearing cantilever on a BCI is 2'. As are other wood I joists.
    That depends on the size of the beam, 12" where good for 4 feet.
    Tim

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    Every single piece of documentation I have says that the maximum load bearing cant on a 12" BCI or TJI is 2'. 11 7/8 BCI's or TJI's are good for a 4' cantilever in a non-loadbearing applicaiton. In fact, in most cases you can't even go 24" with eave end snowloads of Alaskan levels. The big issue is when you add snowload on the eave ends and transfer that load to the ends of cantilevered floor joists. On a truss design roof the snowload of a 16x20 cabin runs right at 16,000# on the eave walls with a 100#/sqft snowload. If you have a load bearing ridge it gets a lot better at 4000# along the eave end walls, but that still only allows a 24" cantilever per BCI and TJI data.
    Bunny Boots and Bearcats: Utility Sled Mayhem

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    ``
    Quote Originally Posted by AKDoug View Post
    Every single piece of documentation I have says that the maximum load bearing cant on a 12" BCI or TJI is 2'. 11 7/8 BCI's or TJI's are good for a 4' cantilever in a non-loadbearing applicaiton. In fact, in most cases you can't even go 24" with eave end snowloads of Alaskan levels. The big issue is when you add snowload on the eave ends and transfer that load to the ends of cantilevered floor joists. On a truss design roof the snowload of a 16x20 cabin runs right at 16,000# on the eave walls with a 100#/sqft snowload. If you have a load bearing ridge it gets a lot better at 4000# along the eave end walls, but that still only allows a 24" cantilever per BCI and TJI data.
    I not going to dig out my paperwork if I could even find it..... in order to get the 4' you had to put in plywood on each side of the beam. That is how I did it, but I also added 3 4X4 posts on the sill joist on the end. Its been woking for the last 10 years.....
    Tim

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    Tim- There is no doubt it will work. Plenty of stuff works for a long time that isn't allowed in the books. I know, because I've done a few myself. However, there is NO way the foundation above will carry a 24x32 building in Alaska without some exotic engineering.
    Bunny Boots and Bearcats: Utility Sled Mayhem

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    Well it sounds like i will have to add at least one more row of piles. The bad thing about that is then the cabin will be off center with just 1 row. Realistically 2 rows of 4 would be best. The bad thing about that is we have done some land excavation since then and i know the piling drill will have troubles getting back in. We have looked into metal I beams but those are way out of budget. I wonder if i could put like 8 TJI's together with plywood on the sides like u say and get that gap down to 7' if that would work. I'm sure i am dreaming but its a thought. As for the snowload there shouldn't be much of one if any. We plan on putting in a high pitched A-frame.

    sigh...

    Im open to all exotic engineering ideas..(i like that term)

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    Railroad tie pads.
    Tim

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    I agree with mit.. It looks like your only course of action is some sort of pad system on the perimeter. There are several designs out there. Most use styrofoam insulation under the pad and some sort of adjustable post base.
    Bunny Boots and Bearcats: Utility Sled Mayhem

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    Kently21,

    What area is this cabin going in at?

  18. #18

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    Have you considered jetting your own holes for the pilings. If you are not in an area of gravel you can go it alone without the need to bring a driller out. I put in 15 pilings 3.5 well casing schedule 80 with a friend in 3 days. We went 14 feet deep and back filled with gravel...each hole took about 100 gallons of gravel. Since we have permafrost we put a 6" round plate on the bottom for a base and welded rebar and angle iron at right angles to the piling on the bottom 3 feet to anchor/hold from seasonal movement with the frost. No movement after 6 years. This is a house (24x40) not a cabin. I built my own jetting nozzle and used a high pressure honda pump.....of course you need a wataer source. We used a nearby lake. The best time we made on a hole was 60 minutes and that included back filling with gravel.............worst time was 5 hours.....hit old trees at the 7 foot level and just kept jetting away and broke parts away. pm me if you have specifics about it. Not talented enough to sketch and post online.
    Wherever you go ....there you are.

  19. #19

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    I just had a new foundation engineered for my house. 1500 sq feet, 22x40 with a couple pushouts. Engineer specified eight 6" piles, good for 100,000k lbs each. Span the piles with W14x22 I-Beams, at only $18/ft with two intermediate supports to limit defection in the beams. Entire thing was less than $10k.

    I think I-Beams will be your best bet, they will cantilever 8' no problem.

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    Speaking from experience, depending on the amount of moisture in the ground, and the amount of annual frost heave... you should either be totally above the frost line, or totally below it. I built my 16 x 24 on pier blocks and concrete block pads on the surface (topsoil scraped off). The cabin raises and lowers about 18" each year do to the high moisture content in the clay soil (it moves evenly, due to the rigid construction, and homogenous soil). 3 years later, I decide to build a 12 x 12 mud room, and this time bury 20' of drill stem for the foundation. Luckily, I had not secured the base of my mud room to the pipe yet, because in the winter, the timbers supporting the mud room were 18" from the top of the pipe. The cabin and mud room lifted since it was 100% above the frost line, but the 20' pilings stayed put. Therefore, I HIGHLY reccomend that if some of your foundation is buried below the frost line, all of it must be. That being said... I think your only options here are either to add more pilings, or to use steel (ibeams) for your foundation... lots of it.

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