#11  
Old 01-14-2013, 06:44 PM
Laborer
 
Join Date: Jan 2013
Location: New City, NY
Posts: 94
Default Re: foundation depth confusion

So Tscarborough, then you are saying that it doesn't matter if there are footings below the slab? Both will rise or crack.
This is how I think it should be done, like the guy I linked to above: http://www.fornobravo.com/forum/6/6/...rted-3701.html (New Oven started)

Silvfox, I don't think you could do this with tubes. You need continuous, but of course it's not a large footprint. The most fuss is in the digging, not the cost of the concrete for the footings.

Only difference from this gentleman, is that I have to foot down three feet. Ug. Less concrete than a slab though.

Edited the illustration to show the frost line.
Attached Thumbnails
foundation depth confusion-better-oven-footings-1.jpeg  

Last edited by ronwass; 01-14-2013 at 06:51 PM.
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  #12  
Old 01-14-2013, 07:07 PM
Tscarborough's Avatar
Il Pizzaiolo
 
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Default Re: foundation depth confusion

If you are going to build a floating slab, you do not need footings. Piers would also work fine, you do not need continuous footings.
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  #13  
Old 01-14-2013, 08:33 PM
Laborer
 
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Location: New City, NY
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Default Re: foundation depth confusion

I can only figure out one way how to use piers and still avoid frost movement. In the first two drawings below, you have to span the space between the piers, and that span, either a girder or a slab will be on the ground.

The third way, with a suspended girder seems much worse and uglier than my no-slab/continuous-footings idea in a post above. Might as well build a deck and put the oven on a stand on the deck.
Attached Thumbnails
foundation depth confusion-piers-girder.jpeg   foundation depth confusion-piers-slab.jpeg   foundation depth confusion-suspended-girder.jpeg  
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  #14  
Old 01-15-2013, 07:33 PM
Peasant
 
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Location: Minnesota
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Default Re: foundation depth confusion

If you use continuous or column footings under your stand you should still pour a slab on grade to support your stand. Rebar will tie the slab to the footings. Hope this helps clarify my ealier post.
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  #15  
Old 01-15-2013, 08:35 PM
Laborer
 
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Default Re: foundation depth confusion

Dear elder forum statesman.

Though I am a babe in the woods as far as this WFO thing is concerned, I still get the feeling that I am not making myself clear and that no-one is understanding the gist of my question.

Oh, that I were a better writer, I shouldst be better able to make my meaning truly ring out clear like a bell tone. Let me try one more time to calm this tempest which may prove to exist solely in a teapot.

1. If you have a floating slab on grade, it is going to be affected by frost (to one extent or another. (probably to only a tiny extent, or probably to no extent). The oven (the "structure") is directly on the slab, so the oven will be affected as the slab moves, or cracks, probably, at the worst, simply going out of level a tiny bit.

2. In a house or a garage or shed, the structure is slightly DIFFERENT because it is NOT on the slab. It is directly on top of the footings. The slab in the basement floats on grade between the footings. The slab is affected similarly, but since there is nothing structural on top of it, it is not a big deal. Besides that, there is ambient heat from the house that keeps the frost a bit at bay.

3. The Pompeii plans dictate the structure of the oven NOT over the footings, but four inches towards the center away from the edges of the slab. How are footings under a WFO slab going to prevent the slab itself from moving? Most of the slab is on grade between the footings or piers, so frost will push up on it, and push it right OFF THE FOOTINGS.

4. It seems to me that it doesn't matter if you have footings or not. Frost will be an issue (perhaps a minimal or non-existent one) UNLESS you build the walls of the oven stand directly over the footings and have essentially no slab, (or a non-structural one) which is NOT what the Pompeii plans proscribe.

5. Where is my logic wrong here?

Sorry to be such a dope about this.

Last edited by ronwass; 01-15-2013 at 08:37 PM.
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  #16  
Old 01-16-2013, 04:18 AM
Peasant
 
Join Date: Oct 2012
Location: Minnesota
Posts: 43
Default Re: foundation depth confusion

In cold climates with significant frost depth the ground can heave several inches especialy if you go into winter with saturated soil. Your floating slab will raise but will be straight up if site prep was good and slab reinforced. Your oven will never feel any stress. If you are against another structure such as the back of your house you have more to worry about as the temperature until slab and thus the frost heave will not be consistant. If that is your situation a foundation below the frost line becomes important. The slab should be pinned to existing structure and pinned to foundation with rebar. The frost will not be able to pull the foundation up from four feet of ground. Again, post footings will save you much excavating and concrete.
A post hole digger will get the job done if you don't hit rock. Don't forget to call diggers hot line to mark utilities.
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  #17  
Old 01-16-2013, 09:42 AM
Laborer
 
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Default Re: foundation depth confusion

OK, my respected forum-mate from Minnesota, I now see what exactly I don't understand about the answers I am getting.

You say that, "the frost will not be able to pull the foundation up from four feet of ground" I assume you mean the footings. The foundation is the slab on grade.

But HOW does the footing prevent the slab on grade from frost moving. Footings aren't anchorages like for a suspension bridge. They are meant to AVOID frost completely because the ground they are on is not affected by frost. The slab on grade on top of footings will push up and if it is tied strongly enough to the footing will either pull the footing up with it, or crack in the middle because the edges are held down by the footing.

This isn't a problem with a house because the whole structure is built directly on the footings. The slab in the basement of a house floats between the footings. The Pompeii design has the structure directly on the slab NOT the footings, so would be affected.

PS: I totally get what you are saying about having just a slab which will rise and fall fairly levelly, which won't be much of a problem. My conjecture is that footings don't prevent this, and may make it worse. The only SLIGHT improvement to building on a slab would be to build ONLY on footings, no slab.

Last edited by ronwass; 01-16-2013 at 09:45 AM.
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  #18  
Old 01-16-2013, 10:27 AM
Serf
 
Join Date: Dec 2012
Location: pa
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Default Re: foundation depth confusion

This is what I would do, I live in PA not too far from you. Install 4" stone try to put corrugated drain pipe a foot in from edge of stone and allow to slope away from slab, maybe do this in two places if you can. Install plastic and re bar then pour 5" or more concrete. In my opinion having a slab pinned into footings will still crack as slab will want to move and footings will try to hold it and cause cracking. A single slab will move some but if you can keep it as dry as possible underneath it will be minimal.
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  #19  
Old 01-16-2013, 10:34 AM
Faith In Virginia's Avatar
Master Builder
 
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Location: Virginia
Posts: 720
Default Re: foundation depth confusion

Ronwass, From what i have seen I think you are missing what a frost heave is and how that affects footers foundations and the like.

Frost heave and frost damage is caused by freezing moisture/water in the soil. As water freezes it expands. If it does not have the room to expand then it takes the path of least resistance which is usually up.

So the frost line is determined by the depth at which the ground will freeze solid.

So by placing your footers below the frost line you are getting below the depth in which moisture in the soil will freeze and expand... so no lifting.

Now with a continuous footer and a foundation wall on top you are preventing any moisture from getting inside of this structure. so the footers are below the frost line and if you back filled the center with soil ...it should be dry because of the outer walls...so if the soil in that area does freeze there is no moisture to expand and push things.

If your concerned about frost heaving you could always do a mono-slab/monolithic slab this is where the weight of the load is distributed throughout the entire slab and the outer perimeter drops below the frost line (but not a big as wide or thick as a footer.) Google (monolithic slab) the edges that drop low also prevent moisture from getting under the slab portion thereby preventing any frost heaving.

hope that helps
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  #20  
Old 01-16-2013, 12:47 PM
Laborer
 
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Default Re: foundation depth confusion

I totally understand the first four paragraphs of your post, Faith.

Fifth paragraph seems to imply that there is only moisture movement horizontally not vertically. Moisture can certainly come up from below the slab to the middle and freeze, can't it? It's not that there is no moisture below that depth, it is that it doesn't freeze at that depth. Moisture does go up and down, especially with temperature changes.

Googled monolithic slab. Seems that that only works in the Northeast if there is ambient heat coming down from the structure from a house. Wouldn't apply for a WFO.

At the risk of sounding like a complete dope, I'm posting two more pictures and asking the question as simply as I can make it:

In the Pompeii oven design, where the legs of the stand are 4 inches in from the edges of the slab,

How does A (footing below frost line) prevent B (soil frost movement on floating slab on grade)?

My answer is: It doesn't.

In the other design, where the legs of the stand are directly built up from the footings, and which is closer to a house structure, and there is no slab, my answer is,

Slab B doesn't exist so nothing to prevent.

Refutations?

Maybe I should have called this thread, Oven stand foundation, alternate possibly better design.
Attached Thumbnails
foundation depth confusion-how-does-prevent-b.jpeg   foundation depth confusion-alternate-foundation-design.jpeg  

Last edited by ronwass; 01-16-2013 at 12:55 PM.
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