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Thread: Foundation/span question

  1. #1

    Default Foundation/span question

    I've been doing some engineering getting ready to haul in materials for a cabin and I'm stuck on one point here.

    For a 14' to 16' span (this will be my front/rear cabin walls) will I need a pad for center support? I'm using 12" logs and 2'x2'x6" concrete pads. The cabin will be either 12x14 or 14x16 so the question is whether I need 4 pads, one at each corner, or 8, with pads at the corners and in the center supporting the bottom logs?

    Any thoughts?

  2. #2
    Member itchibon69's Avatar
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    I would go with the middle supports . The 12" logs should be able to span that but its always nice to be on the safe side. Not sure about all your other dimensions as in 1 story or loft ??? So dont know your load exactly.

  3. #3

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    You will want do like bobblehead said about three supports over time you would probely get a sag in the middle . You should also think about putting some supports in the middle of the building to take the bounce out of the floor. Also you will want to cross brace your support post, that is assuming that you are going to put it on post as that will be much easier with any thing that might need to be done later under it.

  4. #4

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    Thanks for the input. To answer your question, it will be one story, no loft. It's essentialy a trapper cabin design, 12x14 maybe 14x16.

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    I'd go 6 around the perimeter (4 corners and 1 in the middle of each 16' stretch), but I always overbuild.

    And adjustable saddles on every support so you can adjust each's height, over time. Costs just a couple bucks more up front.

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    Member Hunt'N'Photos's Avatar
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    I am doing basically the same thing right now. Just bought the wood last night to haul in this weekend. I am building a 12x16 gen shed/workshop up at the cabin. I am using 6 sonotubes for the pilings, 4x8x16' beams and 2x10 x12 floor joists. I am going to cantilever the beams in 6" from the sides to give me an 11' span between beams and the tubes will span 7 feet. I would consider that a minimum for my project. Cant wait to start diggin the holes this spring!
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    Premium Member bmunsell's Avatar
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    Center beams never work out very well. Problem is frost heave. Center post will tend to move differently than outside posts, maybe because of more heat or less moisture or who knows what, plus it is way harder to reset/relevel the center. Cabins that don't have full foundations always move around and require releveling. What has worked best for me is to build a cabin like you would a shed on skids, with the long length first layer set in from the outside like mentioned above, but I go up to 18" in from the outside to reduce the center span to around 9'. You can do the same for the support pads, come in from the end of the log/beam 12" or so and then put one in the middle for a total of 6 support pads. Then the actual floor joists run perpindicular to the skids/long length first layer. If that makes sense.


  8. #8

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    That's an interesting point about a center beam under the front and rear walls.

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    Member Stanly's Avatar
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    Something to consider for those choosing 2x10/12 for floor joists. Check out the engineered I-beams, they're much lighter, much stronger and always straignt. You can use them for your joists and rafters. I used to use them when I was building homes. The price dif was not excessive a couple of years ago. Just go to the contractor section of Lowes or Home Depot and they can give you all the specs...
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    Member Dirtofak's Avatar
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    Quote Originally Posted by Stanly View Post
    Something to consider for those choosing 2x10/12 for floor joists. Check out the engineered I-beams, they're much lighter, much stronger and always straignt. You can use them for your joists and rafters. I used to use them when I was building homes. The price dif was not excessive a couple of years ago. Just go to the contractor section of Lowes or Home Depot and they can give you all the specs...
    And with JCIs, you can drop to 12' on center if you need to to get your floor load up to par. Then it is very rigid even on a 20' wide cabin.

    Do NOT cantilever a log much at all, a couple inches if at all. All the wieght is on the outside walls. 2X12s bending at the cantilever is not a good thing.
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    Member Dirtofak's Avatar
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    Quote Originally Posted by bobblehead View Post
    That's an interesting point about a center beam under the front and rear walls.
    My cabin has no beam under the middle. It has a beam completely around the base. and one under the front wall. There are 3 risers in the middle. Front, 8 foot back under the front wall and under the back wall. It floats on the tundra and moves about 1/2" per season. 3 cranks on the front, 1/2 crank less as I go back. Takes less than an hour. I am going to shoot for middle ground this spring and just live with 1/4 inch of unlevel.
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  12. #12

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    Good idea about the JCI floor joists, I have worked with them before with good results.

    I'm trying to maintain a low overal profile with the cabin; going for the traditional trapper cabin look while maintaining functionality and longevity (ex: not setting it directly on the ground). I'll have just enough room to crawl under it an work if necessary, but when the grass grows up in the summer it will look like it is set right on the ground, more or less. To achieve this I'm going to "hang" the floor. I plan to hew a flat surface on the inside of the bottom two logs that run lengthwise, fasten a 2x10 to that surface, and then hang the floor joists from there.

    These bottom logs will set on treated blocks, which will set on top of 2'x2'x6" concrete pads.

    I have three supporting a 16' span. Trying to determine if I need three for the 14' end walls or if two will suffice. Does the load from the ridge pole transfer all the way down through 7 or 8' of 12" logs enough to create a bow underneath the bottom?

    Fun stuff.

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    Member Dirtofak's Avatar
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    I was told by a guy that builds to think of the floor as a plain that is rigid. Build it thick and solid. Then when it is tied into the structure, if a corner drops, the floor will stay, the walls will stay, the roof won't crack. You either have to make everything move or nothing.

    I personally, would not hang the floor. Logs dance enough as it is. Worse when you don't have regulated humidity. shrink and swell. I would use 12" JCIs with a full rim board, braced every place they go over a beam. fill it with R34 or 38. Blue board on the bottom and OSB it. 3/4 or better floor, glued down. If you double or triple your front JCI you might be able to get away from the middle supports. Check span charts. Give the logs as solid of a surface as you can. Lay them carefully and tight. Know exactly how you are going to frame doors and windows. Have the roof sitting ready.
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    cabin foundation.jpg

    it should look something like this

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    Member AKDoug's Avatar
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    No, you shouldn't need a support there. I also wouldn't build it that way. It will be a nightmare to get the floor joists to hang off those logs and keep everything square and level. Bmunsell's drawing shows the best way to do it, but the leveling of the logs combined with the work to flatten the top of them would stear me away. I would use six pads like his drawing with 4x12 beams instead of the logs.
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    Member AKDoug's Avatar
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    I need to back off on my previous post. You don't need a support there unless you are going to need a post coming down to carry the load of your ridge beam. What type of roof are you going to build?

    I missed your post on the "look" you are trying to acheive. I still think it will be more of a pain than it's worth.
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    Premium Member bmunsell's Avatar
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    The other thing to think about with the trapper cabin look is that trapper cabins are only used in the winter when there is a lot of snow (we hope) on the ground, so they look close to the ground, but usually aren't. Its a tradeoff, build close to the ground and you get to spend all winter digging out your path and walking down into a slick hole or build higher and spend the summer going up the stairs.

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    Member AKDoug's Avatar
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    I have three supporting a 16' span. Trying to determine if I need three for the 14' end walls or if two will suffice. Does the load from the ridge pole transfer all the way down through 7 or 8' of 12" logs enough to create a bow underneath the bottom?
    Man, I need to read better. Yes, depending on where your door opening is, the load on the ridge pole support post may be enough to overstress the bottom log if it is unsupported.
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  19. #19

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    Quote Originally Posted by AKDoug View Post
    Man, I need to read better. Yes, depending on where your door opening is, the load on the ridge pole support post may be enough to overstress the bottom log if it is unsupported.
    The roof will be a shallow pitch, with horizontal logs all the way up to the ridge pole. It will not be a framed gable. I don't think I will need a vertical support post to the ridge pole do I? Not with the front wall bearing the load?

    I know that some of these designs might be more difficult than necessary, but I have the time, and I'm learning patience. Heck, stick frame would be a lot easier overall than scribe fit notches, but I've got a vision...

    Thanks for the all the discussion--good stuff.

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    Member AKDoug's Avatar
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    I think you would be fine with that span with no center post with a horizontal log wall supporting the ridge beam.
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