Concrete floor - footings?

[twosocks90]

This spring we're going to place concrete in 1/3 of our 42x80 pole barn for an office, shop and loft area.

I had some plans drawn up by the architect that did our house. He suggested footings down at the frost line (42") and then placing the slab on stem walls which are resting on the footings, otherwise the slab will heave and we'd have problems where the interior walls meet the exterior walls of the pole barn.

Do I really need footings? It seems like a huge extra expense. I can't decide if the slab might not heave since there's no water inside the building perimeter to freeze or if the ground heaves no matter what once temps drop (it's 4 degrees right now - cold!). What should I do?

 

[bczygan]

Build it per the approved plans, if they are already approved. That's what the jurisdiction will require.

Personally, I don't like a slab to rest on a wall unless it is reinforced. Compaction of the fill material, especially around the perimeter, is critical in this case.

What is standard practice for this in your area?

What are the soil and engineered fill conditions?

Why did the designer design it thus way?

What are the other details of the design?

What are the exterior walls constructed of?

Are there elevation changes that would require a stem wall to accommodate them?

A true pole barn on a flat site with a good base and drainage and no exterior wall weight to speak of, a thickened or turned down edge might be adequate. What else is going on here? You need to find out.

If preliminary drawings, discuss with the designer.

If approved, and you get a new design, it will need re-approval from the jurisdiction.

 

[aar0s]

Yup, it will heave some. Id plan on doing the footings. Look into insulating under the floor with foam before you pour also, it will make it much more livable in the heated area.

 

[ConCretin]

You might be correct when you say there isn't enough water in the soils under your structure to cause frost heaving. The problem is that you won't really know until you're done. If you don't provide for frost protection and you're wrong, it can get expensive. It's better to assume the worst

With that said, your pole structure is presumably frost protected and you should do the same with your slab so everything stays put. Perimeter footings won't do anything to keep the frost out of the soils under the slab. If the finished area is heated, that will help but what if the heat is turned of for some reason.

I'd suggest using rigid insulation. Lay 2" boards under the slab and extend them out 4' around the edges. if you do it yourself, your cost should be about 75 cents a square foot, which os probably a lot cheaper than perimeter footings and will be a lot more effective.

While you're at it, lay some radiant tubing on the insulation and you'll have the start of something.

 

[twosocks90]

I follow the logic of what you're saying but I don't understand -- when someone has a slab in their building or garage and then later decides to go back and build interior walls on top of that slab, there are no footings. How can you get away with that without creating huge interior problems as the slab heaves?

Lay 2" boards under the slab and extend them out 4' around the edges. if you do it yourself, your cost should be about 75 cents a square foot, which os probably a lot cheaper than perimeter footings and will be a lot more effective.

I like this idea but can you explain why it would help? I was trying to think along those same lines. I was considering removing a few inches of dirt and putting compacted gravel down to prevent water from collecting under the concrete, then putting insulation on top of the gravel (as you suggest), then putting the concrete on top of that. Could that work?

 

[Bondo]

I follow the logic of what you're saying but I don't understand -- when someone has a slab in their building or garage and then later decides to go back and build interior walls on top of that slab, there are no footings. How can you get away with that without creating huge interior problems as the slab heaves?

----------------------------------------------------------------------------

I like this idea but can you explain why it would help? I was trying to think along those same lines. I was considering removing a few inches of dirt and putting compacted gravel down to prevent water from collecting under the concrete, then putting insulation on top of the gravel (as you suggest), then putting the concrete on top of that. Could that work?

Ayuh,... To yer 1st point, in that case, the Entire slab is a monolithic slab,...

It'll all move together,...

To yer 2nd point, yer pourin' a floatin' slab, inside the walls anchored to the pilin's holdin' the pole barn stable,...

The floatin' slab will float on the frost below,...
The pilin's for yer barn, I assume are anchored Below the frost level, so don't move, or atleast not as much,...

If yer interior slab mounted buildin' doesn't tie directly to the outer walls, it won't matter,...
If ya use the barn as the 4th wall, it won't move the same as those on the floatin' slab,...

 

[ConCretin]

I follow the logic of what you're saying but I don't understand -- when someone has a slab in their building or garage and then later decides to go back and build interior walls on top of that slab, there are no footings. How can you get away with that without creating huge interior problems as the slab heaves?

I like this idea but can you explain why it would help? I was trying to think along those same lines. I was considering removing a few inches of dirt and putting compacted gravel down to prevent water from collecting under the concrete, then putting insulation on top of the gravel (as you suggest), then putting the concrete on top of that. Could that work?

The goal is to have both the slab and pole structure resistant to movement caused by frost. This prevents problems such as you mentioned regarding interior walls.

The problem with a perimeter frost wall is that it won't stop frost from getting under the slab. Unless the area is heated, the cold will move down through the slab and even if it is, frost can tranfer laterally through the frost wall or haunch into the soils under the slab.

This happens on any frost wall foundation, which is why you are wise to install rigid insulation on one face of the wall. Typically an insulated building will keep the frost out if the heat goes off but a pole structure probably wouldn't.

By laying rigid insulation under the slab and extending it out far enough to keep the frost from reaching in, you ensure that your base material won't freeze and lift your slab. Just lay the boards on a smooth sub base and place concrete right over it.

We usually lay the insulation inside the edge forms and then extend it after the forms are stripped. We've found this is better than building forms on top of the insulation and poking it full of holes with the forming stakes.

The structure is presumably protected by having the posts extend below the frost line. I have seen frost grab hold of a post and lift it so the added protection provided by the insulation is a good thing here too.

Hopefully that all makes sense.

 

[twosocks90]

By laying rigid insulation under the slab and extending it out far enough to keep the frost from reaching in, you ensure that your base material won't freeze and lift your slab. Just lay the boards on a smooth sub base and place concrete right over it.

So just a couple inches of foam board insulation will keep the base material from freezing, even if the area is unheated often? That sounds great to me. I was trying to figure out how this could work since cold would be trying to get in from above the concrete and then I realized that the earth is also trying to send warmth upward from below, to some extent, isn't it?

Yes, the pole building is frost protected. The postholes were dug down 42" and a concrete 'cookie' placed down there. The poles rest on top of that concrete.

LLWillysFan, you wouldn't happen to have any pictures of what you do, would you?

 

[buddyboy]

putting 2" of foam under the slab and extending it out is good advice.

think of it as keeping the 'warmth' of the earth 'in' instead of keeping 'out' the frost.

your slab will be the same temp as the air, the earth below the foam will stay above freezing. it won't matter if you heat the building or not.