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dk09
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# Posted: 19 May 2016 06:18pm
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Hi folks, I'm new to the forum and currently in the planning phase of our small cabin. What are your thoughts on a foundation for a 10x16 cabin (1 story w/small sleeping loft)?
A few things to get started: We're building in northern IA -- footing depth to get below the frost line is 42". Soil is silt/loam. Can't get a concrete truck or other heavy machinery to the site. And finally, we are novices in the truest sense of the word 
Thoughts after reading a lot about foundations:
1. Skids. Is 10x16 too big for skids? I'm worried about frost heave -- for those who have built on skids in cold environments, how much movement have you experienced?
2. Pylex screw piles.
URL
Their website advertises these as foundation for sheds, and there are a few threads out there about people using them for cabins, but I'm skeptical. They seem too good to be true. Anyone have experience with these or thoughts on why they would/wouldn't work?
3. Deck blocks. Any advantage to this over skids?
4. Sonotubes. Kind of a last resort, but keeping it on the list. What's the best way to dig the holes for these? I'm assuming most just use a powered auger of some sort -- anyone dig them by hand?
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bldginsp
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# Posted: 19 May 2016 07:56pm - Edited by: bldginsp
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Welcome dk09. Here's what I think-
-Skids definitely not enough for 10x16, in an area with a lot of frost heave.
-Don't see any reason the screw piles wouldn't work, if you use enough of them. They aren't cheap. There are some that screw by hand like the Pylex, but the larger ones require hydraulic equipment and that's pricey to rent or hire.
-If by deck blocks you mean precast piers put on the surface of the soil then no, no difference between that and skids.
-There are post hole boring attachments for tractors with different diameter augers, hydraulic augers that mount on backhoes or skid steers, and hand held machines as well. The latter probably won't handle the diameter hole you need. You could hire a tractor operator, or rent a small tractor or skid steer with the attachment.
-Another alternative is a mat slab, which is a 1 foot thick slab with lots of rebar that sits on top of the ground and floats on the heaving soil below. But you need a concrete truck for that.
I think you are a good candidate for the screw piles. Each one holds about 4,000 lbs so it won't take that many for a 10x16. And, if your silt/loam soil is not very rocky they will install easily. I'd use a lot of them, more than the minimum for weight calcs, just to provide even load distribution.
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Wilbour
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# Posted: 19 May 2016 08:15pm
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Quoting: bldginsp -Skids definitely not enough for 10x16, in an area with a lot of frost heave.
Well it is do-able but I have to jack up my 10 x 16 each spring and fiddle with the footings. Not something I look forward to. Perhaps I will invest in the Pylex screws down the road
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Don_P
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# Posted: 19 May 2016 09:24pm
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Post frame is another option. Auger 6 holes 42" deep and pour 8" of concrete. Call a local ag barn builder and get six 6x6 posts that are tall enough to run from that footing to the top of the sidewalls. The footing depth prevents frost heave, the continuous posts braced by the walls resist wind. I don't believe the screw in piles are bracing, you're getting uplift resistance but little horizontal stiffness. We tend to just look at holding the building up. Gravity is only one force of nature, Wind is a horizontal force pushing sideways on the building. For instance if the total height of the 16' side of the house, to top of roof, is about 12' then that side presents a 200 square foot sail to the wind. A heavy gust can push sideways on that with about 20 pounds per square foot, that's a 4,000 pound horizontal load. On a relatively level site it is easier to keep things low, the shorter those lever arms are with 4kips pushing on them the better.
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rockies
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# Posted: 19 May 2016 11:10pm - Edited by: rockies
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I plan on using screw piles from Techno Metal Post. They come in a variety of sizes and there is a frost sleeve available.
http://www.technometalpost.com/en-CA/brochures/
If you look at their "Cottage" brochure they state that the posts have a load bearing capacity up to 50625 lbs. I also like that their posts are engineer certified. Screw piles can be pricey but they install in a couple of hours and you can build on them immediately. You weld a plate to the top and attach your beams to that. No digging, formwork, sonotubes, gravel, rebar, hand mixing concrete, back filling, waiting for curing, etc, and if you ever want to move the cabin they can be unscrewed and taken with you.
There is also a good feature on a house built on a very steep slope.
http://www.technometalpost.com/en-CA/etude/detached-single-family-home-on-a-sloping-l ot/
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Don_P
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# Posted: 20 May 2016 12:15am
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Rockies, I believe you said you've worked with these before. What is their lateral resistance? Who is responsible for giving them the design load info for gravity and lateral?
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dk09
Member
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# Posted: 20 May 2016 10:23am - Edited by: dk09
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Thanks everyone, several good ideas here.
Quoting: Don_P We tend to just look at holding the building up. Gravity is only one force of nature
Yeah, good point -- I wasn't thinking about this. I'm assuming other methods, like skids, would run into the horizontal load issues as well, correct? How do people counteract that? Mobile home anchors?
At this point I'm leaning towards the Pylex screws for ease of getting started with the rest of the build. I went ahead and bought one just to try out this weekend before I invest in any more.
Looks like they have a product to add horizontal stabilization to the screws: http://www.pylex.com/en/page_produit.asp?sku=10575
Or maybe, better yet, I could use one of their commercial screws on each corner (if I can find a place that sells them). http://www.pylex.com/en/page_produit.asp?sku=10730
Those supposedly support "4,781 lb. of pull-off force and 1,641 lb. of side thrust"
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Don_P
Member
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# Posted: 20 May 2016 08:47pm
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I looked up the ESR, engineering services report, on the techno posts. It's a good idea to look for an ESR on any unfamiliar product to see if it has approvals and how it is meant to be used.;
http://www.technopieux.com/wp-content/uploads/2014/10/ESR-3418.pdf
Fig 4B is the listed method of laterally supporting the posts.
The whole document is worth reading carefully. eg in the footnotes, most of the numbers given are for very stiff clay.
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Gary O
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# Posted: 21 May 2016 08:44pm
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Quoting: bldginsp -Skids definitely not enough for 10x16, in an area with a lot of frost heave.
I tend to shy from arguing a structural point with anyone of professional experience, especially Don_P of whom I admire, and, well, have come to love...there...I said it.
Howevah
Since I'm building my 12x16 shop on skids, and the previous cabins were built the same way (first one over ten yrs ago), I feel I should offer my experience.
Our soil is pumice.
We get little rain here, but we do get some.
Have no idea of the frost line here, but have experienced -42°F and 5.5 ft of snow accumulation.
Little movement if any to the structures.
I do use pavers to lay the skids on and tie the skids in every six feet (cross members).
My skids are PT 4x4s.
The ties assist in final adjustments (sledge hammer to the corners) in getting everything square.
Guess my point is, nothing noticeable in frost heave induced movement over the ten years.
I do build my own doors and leave ample room in the jambs, but everthing is flush when closing.
and, welcome
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Don_P
Member
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# Posted: 21 May 2016 10:05pm
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Cabbie, he's found the jug again 
Actually if you're not going to put in a foundation, I prefer skids to piers... how far are you gonna fall off the ground? Frost is a problem but nobody gets hurt. A handful of mobile home screws and it isn't going to roll. At 10x16 if you want to get up out of the ground a bit a crib of 6x6's would create a low permanent wood foundation. That said, every time you jack a building it loosens things. not a great plan.
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Gary O
Member
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# Posted: 21 May 2016 10:35pm
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Quoting: Don_P Cabbie, he's found the jug again
I love bldginsp too
.....aaand 12 yr old single malt
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bldginsp
Member
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# Posted: 21 May 2016 11:42pm
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I'm touched.
My friends think I'm touched in the head.
Particularly when I drink dry red wine.
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Gary O
Member
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# Posted: 21 May 2016 11:55pm
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Quoting: bldginsp I'm touched.....Particularly when I drink dry red wine.
And well you should be.
Wine
Sheeesh
Scotch, my friend, is the foundation of all worthy discussions
(see? not off topic..heh heh)
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bldginsp
Member
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# Posted: 22 May 2016 12:09am
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(hic) I'm serfectly pober, Occifer. No, I never learned the alphabet, and I can't walk straight down that line because it's all squiggly to begin with. Don't worry about that bottle in the back seat, its empty. (hic)
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Anna
Moderator
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# Posted: 22 May 2016 04:10pm
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getting off topic gentlemen..
there is a topic on Fav Cabin Drinks
an now back to Foundations..
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bldginsp
Member
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# Posted: 22 May 2016 04:22pm
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You are perhaps making a gross assumption, Anna, referring to us as gentlemen.
Dk09- while lateral resistance is a one of the more important things to calculate for in any building, I personally believe that for very small buildings such as your cabin and mine, the forces are relatively small and not very consequential. UNLESS you are in a unique situation, such as a broad open field in a very windy area, or on an exposed ridge. You can securely tie the beams to the screw posts, and securely tie the building to the beams, so the lateral failure danger here is the possibility of the posts bending at the ground from high wind. I just don't see that with a 10 x 16 building. IMO
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rockies
Member
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# Posted: 22 May 2016 08:04pm
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Another thing to consider in calculating for lateral resistance is "what is the cabin built from"? I plan on using SIPs for the floor and roof (and perhaps the walls). If the panel is made out of a 7 1/2 inch thick solid core of expanded foam sandwiched between layers of OSB then wouldn't the wind forces be transmitted through the entire panel uniformly?
When a wall, for example, is built with standard stick framing you usually have 2x4 or 2x6 studs covered in 1/2 inch or 5/8 inch OSB or plywood. Most of the resistance to the wind force is done just by the stud, isn't it?
It's sort of the same theory when calculating the roof framing required for snow loads. The roof may be framed with 2x12 rafters but in between the rafters is just the thickness of the sheathing. A full 12 1/4 inch thick SIP panel for the roof means that the support is present across the entire SIP panel, not just where a rafter lies.
So wind pushing against a SIP wall should have far more resistance built into it than a stud framed wall. As long as the panels are properly tied down to the foundation you should have a very sturdy cabin whatever the wind force is.
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Don_P
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# Posted: 22 May 2016 09:02pm - Edited by: Don_P
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When we talk about diaphragms, shear walls, braced walls, these are the sheathed parts of the frame that are transferring the lateral forces to the foundation and then to the ground. The sheathed box that contains the occupants is just one part of the structure, the entire structure needs to be braced to resist lateral, racking, forces. A building of vertical studs with horizontal plates and no bracing will lay over to the ground in a very light breeze, the sheathing is what prevents that, just as in a SIP. Unbraced posts under the structure are also subject to laying over just as the unbraced studwall is.
So, we have a rigid box built of sheathed walls some distance above the ground. Between the rigid box and the ground we can build a rigid box of masonry or pressure treated wood, or, we can stand it on several pencils, likely stuck a short distance into... what does dirt do when it rains, stuck a short distance into mud.
When the wind blows the rigid building is going to transmit all of that lateral load very effectively to the foundation, why, because as we have pointed out, it is very rigid. A pound of wind power against the wall results in a pound of horizontal force against the foundation at the bottom of the wall, nothing disappears in between. If that load is transferred to foundation walls through the floor diaphragm the load is resisted along the lengths of the foundation walls, if it is resisted by posts or individual elements the surface area of the post against that unknown mud is all you have.
A very common way people think about a pier failure is assuming that it has to bend over at the ground level. This isn't how they fail. The point of rotation is usually underground about 2/3 of the length of the post down. There isn't anything at ground level to form a "constraint" that would cause the failure bldginsp visualized. Unless you build it as shown in the Engineering Services Report... that concrete grade beam does what we are envisioning. Then the thing to check is the post diameter and effective length for bending in the manner described.
Professionals,
Go grab some lateral design CEU's over at apawood.org in their lateral design series of free courses. Once you understand more we can talk about this further.
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bldginsp
Member
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# Posted: 23 May 2016 08:55am - Edited by: bldginsp
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Regardless of the point of bending, I don't believe the issue of lateral strength with earth screws is as critical with a small building as it is with larger buildings.
Small buildings have forces working on them an order of magnitude smaller than full sized houses, just because they are that much lighter, and smaller area. Proportionately, the materials and strengths of connections are more robust in relation to the loads they carry. This is a generalization that wont apply where basic load paths are not constructed, or inadequately sized. Where they are correctly implemented, a small cabin is a Datsun B210 built with a Hummer chassis.
Earth screws hold a structure down as well as a spread footing, or close, certainly far better than piers (that are not deep). If one screw tilts in the earth from wind load, they all have to as the floor diaphragm pushes them. I can see this with four earth screws in a 10x16, in wet clay, in extreme wind. But not with a dozen or more.
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Don_P
Member
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# Posted: 23 May 2016 11:19am - Edited by: Don_P
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I simply read the ESR, effectively the approved directions. In the section for cottages and sheds, they seem to differ with you.
So as a homeowner, I'm sitting here with a building inspector and an architect telling me what I want to hear, and a set of code approvals telling me otherwise. An interesting quandary on several levels.
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