Footers or Slab

[Neil2]

Re: Footers or Slab

If you go the pier route, do not tie an at grade slab to the piers.

This will not contribute to the bearing strength. In frost susceptible soils in particular, a tied slab will tend to move and put stress on the piers.

[BeanAnimal]

Re: Footers or Slab

I have looked at that page several times over the last year or so and similar pages.

This is the applicable image:


But my concern is still the highly plastic and expansive clay soil. It is very tempting to try for a project like this, but I just don't see how it will work due to the poor (total lack of) drainage under the 6" gravel layer that is beneth the foundation.

A parking slab at a filling station and a 6x6 oven are very different

[dmun]

Re: Footers or Slab

and 2" of polystyrene insulation underneath!

Sounds like frost-protected-shallow-foundations to me. If you can't take the structure down to the frost line, bring the frost line up to the structure.

[forno di Lucca]

Re: Footers or Slab

all great points and a good read as I am having the same dilemma with my layout for my oven build...I was actually talking to an engineer friend of mine and he mentioned a floating slab 16" thick ---he said that for gas stations and garbage bin areas behind your local grocery stores thats all they do......heavily reinforced with re-bar of course and he mentioned adding a couple of inches thicker towards the outside edge.....and 2" of polystyrene insulation underneath!

[tusr18a]

Re: Footers or Slab

I live in Western Michigan and grew up in Central Maine. Not wanting to worry about what Mother Nature might throw my way, I put the full foundation under my oven. Dug down through 4 feet of clay. Poured a 1 foot thick footer to support the walls. Cement block to grade and 4" slab on top of that. Everyone that saw the build told me that I was crazy. I told them that I agreed. I rest well knowing that frost will not likely topple the oven. On top of that, I was able to practice my masonry skills, which were non-existant, while building the block wall that is now hidden under ground.

[BeanAnimal]

Re: Footers or Slab

True. I did this, and have a pressure treated floor bolted to brackets on the foundation walls to support my wood storage. One less thing to reinforce, mix, pour, and float.

I do have about 36 sqaure feet of expensive pavers that we decide not to use for our fire pit cap stones

[BeanAnimal]

Re: Footers or Slab

Really? The force at the center of the slab would be up, not down. And in that case, the way you'd increase the load capacity beyond rebar would be post-tensioning, I think.

It would depend on the design of the slab. The slab would have to be rigid enough to transfer the load. If the slab is not strong enough, them the center pier becomes a point load. In the same fashion, if the slab is not strucuraly sound and only has 4 corner supports, they are likely to cause the slab to fail if any of the corners move.

If you did do a full perimeter footing and foundation, the foundation slab becomes obsolete. I would build the stand either as poured wall or standard mortared CMU construction and maybe beef up the hearth slab, but there is zero reason you need a foundation slab with a full and proper foundation. It's exactly like a house at that point.

Yes like a house The block (or poured) walls could rise all the way to the hearth slab and the base slab is simply a floor and not structural.

I need to pour a floor for the wood storage area and will need a ledge for the brick or stone facade. I could of course use the wider foundation block to create the ledge, but still need a floor. I will likely end up just pouring a 4" flour over the whole thing with enough rebar to prevent cracks. Maybe overkill, but on a small structure like this the foundation slab would tie the 4 walls together, more insurance against the poor drainage and frost jacking

I dread the thought of the added hassle, but I think I will sleep better. I still have a week or two to make up my mind though...

[dmun]

Re: Footers or Slab

If you did do a full perimeter footing and foundation, the foundation slab becomes obsolete.

True. I did this, and have a pressure treated floor bolted to brackets on the foundation walls to support my wood storage. One less thing to reinforce, mix, pour, and float.

[splatgirl]

Re: Footers or Slab

The base slab would transfer weight to the center pier via the concrete and rebar.

Really? The force at the center of the slab would be up, not down. And in that case, the way you'd increase the load capacity beyond rebar would be post-tensioning, I think.

In addition, I plan on (overkill) dividing the stand with a center divider. That divider would transfer weight from the hearth slab to the base slab.

In this case, the center pier becomes useful, but there's no point to doing either, IMO.

If you did do a full perimeter footing and foundation, the foundation slab becomes obsolete. I would build the stand either as poured wall or standard mortared CMU construction and maybe beef up the hearth slab, but there is zero reason you need a foundation slab with a full and proper foundation. It's exactly like a house at that point.

[BeanAnimal]

Re: Footers or Slab

The base slab would transfer weight to the center pier via the concrete and rebar. In addition, I plan on (overkill) dividing the stand with a center divider. That divider would transfer weight from the hearth slab to the base slab.

I have done a lot more thinking this evening and going back and forth but am now leaning towards a full foundation again. While I think the piers will work, I am not sure I am willing to take the gamble even though it would be FAR simpler and less costly.

[splatgirl]

Re: Footers or Slab

What about the weight distribution? If the load above is spread via the hearth slab to the U or square perimeter block stand, is a central pier in your foundation really going to have any benefit?
A full depth perimeter footing and foundation wall would be a box without anything in the center, so it's not clear to me why a central pier is helpful in the sonotube situation. Basically all it's doing is supporting the middle of the foundation slab itself.
I'm certainly not the expert here...just wondering out loud.

[BeanAnimal]

Re: Footers or Slab

I may have just talked myself into sonotube footers... lets say worst case 15,000 pound oven. With 5 legs, (one in the middle) that would be 3,000 pounds per leg. With a 2' x 2' footer the load would be 750 pounds per square foot.

The bearing capacity of clay is listed someplace in the neighborhood if 2000 PSF. So it would appear that the legs would not pile in any further as long as they were below the frost line.

I suppose the next question would be concerning the intregity of the slab resting on 5 points of contact. I guess extra thick and extra rebar will come into play, begging the question if it would just be easier and safer to pour a traditional footer and build walls?

I have a headache!

[BeanAnimal]

Re: Footers or Slab

I use tube forms to ensure that my piers are smooth walled to prevent ice from grabbing on to them. After the deck and shed (21 for the deck, 9 for the shed) and 15 or so odd fence posts, i am pretty quick about digging a 2' by 3' hole with a shovel and posthole digger I suspend the tube form (using a simple frame) about 10" off of the bottom and fill the tube and hole in one shot. If you only knew how many 80# bags of concrete I have hand mixed over the last 3 years!

I can put a drain tile around the oven base, but can only get it down maybe 1' and still keep enough pitch to get the water to drain down away from the patio.

I am still a bit concerned about the weight per pier and the limited bearing surface that it will rest on. Brick, slab, and finish stone are going to come in at maybe 10,000 - 15,000 pounds.

You figure 60-80 #80 bags of concrete ~6,000 pounds. Maybe 3,000 pounds of brick and another 2,000 - 5,000 pounds of finish material depending on choice.

That is ~3,000 pounds per pier so even with a 2' x 2' base, you are 750 pounds per square foot!

Your success has given me at least some hope that the slab may be an option. Maybe I can go down 1' or so and backfill with compacted stone over geotextile cloth. The cloth will prevent the stone from migrating downward into the clay and the drain tile will keep it somewhat drained.


When all is said and done, I suppose most engineers would have me dig a traditional foundation and build or pour a wall.

[dmun]

Re: Footers or Slab

I did try to mushroom them at the bottom to approximate a true footing and a bit more anchoring.

I agree that tubular footings are more or less useless unless they are flared at the bottom. They make specific forms for casting these in combination with the sonotubes, but of course that means a lot more excavation.

[splatgirl]

Re: Footers or Slab

I did not use tube forms. I find that with digging by hand, a post hole digger makes a perfect earth formed hole and I can never find small enough tube form anyway. So I suppose my cylindrical holes are roughly 6-7" dia. and I did try to mushroom them at the bottom to approximate a true footing and a bit more anchoring. Four piers total with a few sticks of rebar in each. I left one stick long on each footer and then bent it over to tie into the slab grid.

For comparison, around here, doing any outdoor flatwork the right way would mean digging out and replacing down about 2'. I have no idea how/if that would have changed things for my slabs--I'm sure it would have helped but, as you say, you've still got water stuck under the drainage base with nowhere to go.
If that's your sticking point I guess you could consider drain tiling around it. It'd mean more digging but be cheap and easy after that.