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Confused by cement slab foundations

workalone | Posted in Construction Techniques on December 23, 2003 07:39am

    I’m building in central PA, frost line is at 36″. My understanding is that perimeter footings must go below the frost line. But now, I want to have a cement 4″ slab on ground, so I end up with a slab connected to a 36″ perimeter footing. Piffin pointed out that this is dangerous as uneven settling of perimeter and/or the ground under the slab might cause stresses and cracks. He sounds right.  So, I’m totally confused on how to build my foundation. Can it be that slab foundations don’t need a perimeter? If so, won’t the slab heave in the winter, and cause plumbing leaks in the pipes that go from slab into the ground??

  The building that I’m putting up is a steel folded plate structure(looks like an accordian). It’s an airplane hangar-type building with vertical sides, which I plan to live in. It requires a monolithic rebar-ed cement footing under both walls about 16″ by 16″ by 30 feet, which is the length of the building. The two end walls are open. The two steel side walls bolt to the footing to keep the building from spreading apart or flying away in a wind. In looking at the plans, I assumed that because my frost line is at 36″, that the 16″x16″x30′ cement footing has to rest on a perimeter below frost. But now I am totally confused.

Help!!

BTW does anybody know of a good book specifically about cement slabs construction?

PaulBen

 

 

  

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Replies

  1. BobKovacs | Dec 23, 2003 07:58pm | #1

    Paul-

    A technicality, but the proper term is "concrete slab" not "cement slab".  Portland cement is one component of concrete.  This was drilled into my head in college- "you 'place concrete'- you don't 'pour cement' ".  Sorry, but it was drilled into me, and now it's a pet peeve...lol.

    Now onto your question.  Yes, the perimeter footings need to go below frost line, regardless of what your pre-engineered building's plans say.  Depending on how you detail the foundation, the slab is usually placed "inside" the perimeter walls, and therefore in conditioned space.  You then use rigid insulation around the perimeter to keep the cold from affecting the slab (combined with the heat from the building).

    Not to put you down, but it sounds to me like you don't have nearly enough knowledge to be attempting this project on your own, and even the knowledgable folks here at BT won't be enough help, as we're not there working on the project. 

    Who designed the foundation?  Is there an architect or engineer involved?  Someone's gotta design this thing to keep the building from falling down.  The type of structure you described relies heavily on the connection between the structure and the foundation to keep it from rolling over- this isn't a DIY 6'x8' shed you're building.

    Bob

    1. workalone | Dec 23, 2003 08:59pm | #4

      Not to put you down, but it sounds to me like you don't have nearly enough knowledge to be attempting this project on your own, and even the knowledgable folks here at BT won't be enough help, as we're not there working on the project. 

      Who designed the foundation?  Is there an architect or engineer involved?  Someone's gotta design this thing to keep the building from falling down. 

      Bob Kovacs

      No offense Bob, as I do sound uninformed in my post. But in fact it's only in the area of cement... oops...concrete foundations that I'm inexperienced. The foundation details on the drawing are ambiguous as I read them.

      I've designed and built (completely alone) two airplane hangers from scratch - 12' high, 40' clear span, pole foundations (each dug by hand...). They are standing perfectly after 15 years. I also built a rustic 12x26 cabin that's 20 years old. The cabin has a 3'x6' skylight made of just 1/16" thick acrylic plastic that works perfectly because I have a 4x8 plywood sliding cover on wheels that protects it when not in use.

      The steel building came on a truck - a sigle pallet containing nested steel plates and two 60lb buckets containing over 3,000 bolts - 4,500 pounds in all. It came with signed drawings by an engineer licensed in PA. The plates bolt together into a 30x30 foot building, clear span, 16' center peak and vertical sidewalls 10' high.

      The interesting thing is that the building <must> be erected quickly because of the danger of collapse if a high wind should develop during construction - the raised arches will act like a giant sail. So I need to get it up in a day. The steel plates are bolted together on the ground to form a 2'x30' arch,  then tilted up and bolted to the previous completed arches. The manual advises a crew of 6 people to raise each section. I've studied the procedure and conclude that I could do it alone with a winch and some other stuff, as the crew doesn't do very much except guide the arch into position. An arch weighs just 500 lbs, so it's man-handable. All I need other people for is to place those damn 3,000 bolts.

      I'd love to post some photos. Anybody know how to do it here?

      Paul Ben

  2. KwanChoi | Dec 23, 2003 08:48pm | #2

    I am an average carpenter, so I don't have answers for you, but maybe I can help with the wording of your question.  I am assuming that the builders of this structure figured that 16x16 footing will carry the load.  This footing is based on the footing resting on the ground level, it sounds.  They did not give you a scenario where footing will be 3 feet under ground.  My guess is that you need footing 3 feet underground and maybe 4 feet concrete wall above the footing, extra foot to get the structure above the ground level.  In this case, the footing needs re-engineered to account for the extra weight of the wall.  Opposite walls may have to be somehow tied together to prevent spreading out as well.  I think you need to consult an engineer or contact the builder of this structure.  They may already have this information.  Can anyone else suggest a use of insulated shallow footing?  I've heard of this footing for a while but can it be used in this area?

  3. UncleDunc | Dec 23, 2003 08:50pm | #3

    As an alternative to putting the footings below the frost depth, you can bring the frost depth up to meet your footings. Google for "frost protected shallow foundation".

  4. VaTom | Dec 23, 2003 09:38pm | #5

    In looking at the plans, I assumed that because my frost line is at 36", that the 16"x16"x30' cement footing has to rest on a perimeter below frost. But now I am totally confused.

    We deal with half that depth for frost line.  One corner of our place was so close to the surface that I was looking at dropping the rest of the building down and buying a large quantity of concrete that was otherwise unneeded.  As my plan called for insulating all around the perimeter, out 20' for PAHS (passive annual heat storage) anyway, I got an appointment with the bldg. dept. variance guy.  He looked at my plans, asked if I was really going to insulate the foundation on the outside of the building and not on the inside, rolled his eyes when I affirmed, and told me to consider the bottom of my exterior slab as grade (on top of the insulation and stone).  This effectively raised all my footers nearly 12", saving me a large amount of concrete.  And, no, he didn't want to know why I was insulating outside the building.

    This is the essence of shallow footings.  They'll work very well in your application and save you considerable money.  My slab is poured inside my concrete walls, independent of them. 

    The other choice that I used with a post&beam building was to have a frost-depth footing only under the posts, poured monolithically with the slab, 2'x2' IIRC.  Looking at it, it looks like a simple slab on grade.  Works fine and the inspector was happy.

    Ambitious project.  Good luck. 

    PAHS Designer/Builder- Bury it!

    1. brownbagg | Dec 23, 2003 09:43pm | #6

      pour the footing below frost grade, either block up or form wall to elevation. pour slab on top of that. if you block up, fill block solid

      The best employee you can have but you wouldn't want him as a neighbor " He the shifty type"

      1. RalphWicklund | Dec 23, 2003 10:56pm | #8

        Can't speak to the insulation part but I would consider grading the site flat, putting in perimeter form boards as if you were pouring a 4" patio and then get the old shovel out and dig. If you have to use a mini excavator then dig first and then form it. Soil poisoning, 6-mil barrier, put the steel in, lay out the 6x6 wire and pour the whole thing a once. Monolithic pour, it's called.

        Somewhere in there, either inside or out, would be the insulation you want. Some one from the northern states can help there.

  5. Piffin | Dec 23, 2003 10:50pm | #7

    PB,

    Here is a rendition of how you might satisfy both you local codes guys and the building designers.

    You remove all organic material from the ground and then compact inch minus gravel to a depth of 18" to provide drained, hard base for the weight of the building to ride.

    The edge of the slab is thickened, in this case to provide a perimeter footing of 16" x 16". Use rebar to make it all intergral.

    Then you bury foam 2" thick four feet out from the edge of the slab. This keeps the building heat and heat from the earth contained to prevent the soils from freezing. Don't forget the perimeter drainage too. You could also place the foam under the slab center to keep warmth in the building.

    IMO, the footing size stipulated for this building is a one-size-fits-all for Anytown, USA. It serves as a grade beam, footing, and anchor against the wind. I typically do shallow monoslab foundations with a 12" perimeter for a one story when the drainage is good.

    If your building dept will not approve shallow foundations, then just form the perimeter deeper. The idea of the shalow system is dependent on having the same well drained and well compacted mineral soils under the whole slab and in maintaining good drainage, and in keeping it from freezing. The gravale will not hold water to freeze. The drain lines lead it away, and the foam controls temperatures.

    No frozen water means no frost heaves or building movement.

    BTW, Get yourself some help. Got any idea what a five hundred pound arch will do to your body? I've seen these things and can't recommend trying this wtith less than three able bodies plus some tooling.

    .

    Excellence is its own reward!

    1. workalone | Dec 23, 2003 11:24pm | #9

      I swear, this group has the smartest and most helpful people in the world. It's amazing how much I've learned from just a few posts. I can't thank you all enough. What's doubly amazing is the breath of your knowledge; you guys have some serious experience under your belts.

      It looks like the "frost-protected shallow foundation" is the answer to my needs, and I never even knew that it existed until today. Once again, Thank you  Thank you  Thank you  Thank you  Thank you  Thank you  Thank you  Thank you  Thank you  Thank you  Thank you  Thank you  Thank you  Thank you  Thank you  Thank you  Thank you  Thank you  Thank you  Thank you  Thank you  Thank you  Thank you  Thank you  Thank you  Thank you  Thank you  Thank you  Thank you  Thank you  Thank you  Thank you  Thank you  Thank you  Thank you  Thank you  Thank you  Thank you  Thank you  Thank you  Thank you  Thank you  Thank you  Thank you  Thank you  Thank you  Thank you  Thank you.

      PaulBen

      1. toast953 | Dec 25, 2003 09:23am | #10

        I swear, this group has the smartest and most helpful people in the world. It's amazing how much I've learned from just a few posts  Paul Ben, that's why I check in most nights, reading mostly, learning, always learning. There are lots of posters who possess so much knowledge, and share so willingly. Believe it or not, I  think often of this group/board, while I'am working. I've been learning A LOT on the "business end", of this business. So to all the poster's here, I thank you, and have a safe Holiday,, Jim J

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