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Pinning apron to large foundation?

Pinning apron to large foundation?

Pinning apron to large foundation?

(OP)
Is there any guidance or rules of thumb for "pinning" an outdoor apron or small slab (for door opening, stair landing, etc.) to a large foundation? We are placing a large mat foundation for a silo in an area with a frost line depth of 4.5ft. Placed against the mat will be small lightly-loaded slabs. We don't want to take the slabs below the frost line and also don't want the slabs moving around over time due to frost heave. Is there any guidance or rules of thumb for pinning the slabs to the mat to disallow movement and to limit cracking?

RE: Pinning apron to large foundation?

I don't feel you can have it both ways. You can't place a pad above frost, and also expect that it doesn't move with the frost.

What we do for small apron slabs at doorways is dowel into our foundation and provide cardboard voidform for the first few feet and thickened edge at the outer extent. This typically keeps the slab area within the door swing from heaving to the point of jamming the doors.

RE: Pinning apron to large foundation?

(OP)
Thanks jayrod12. Do you mean you provide void form around the "dowels" for the first few feet inside the apron and thicken the slab edge that abuts the mat foundation?

I imagine there are also other things you can do, such as a well draining subgrade below the slab, to reduce frost heave.

RE: Pinning apron to large foundation?

No I mean we dowel the apron slab directly to the foundation, then provide void form for the first few feet below the apron slab (as if it were a structural slab) then provide a thickened edge at the outer extent of the apron.

There are ways to mitigate the frost heave, but without providing foundations below frost, you can never get rid of it completely. And if you're going to go so far as to remove and replace a bunch of the sub-grade, you might as well provide a proper foundation below frost.

RE: Pinning apron to large foundation?

(OP)
Thank you jayrod12. Do you add any additional reinforcement around the dowels in either the apron slab or the mat foundation to lower the possibility of cracking?

RE: Pinning apron to large foundation?

What's the area/perimeter of your apron slabs here?

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Pinning apron to large foundation?

Other possible methods include:

1)Place extruded insulation beneath the stoop slab and extend horizontally the same distance as frost depth around 3 sides. Slope insulation to drain properly.

2) Use well graded non-frost susceptible fill below the stoop slab (to depth of frost) and extend out horizontally 3 sides and provide means to drain subgrade.

3) Provide grade beams cantilevered off main structure to support stoop slab. Provide 6" void under stoop slab (can use different methods).



RE: Pinning apron to large foundation?

(OP)
Thanks jike: For 1. If the frost depth is 5ft below the apron, what is the purpose of having the insulation (sloped away from the apron) extend 5 feet past the apron on three sides? Would this not increase the potential for differential settlement? - the 5'x5' apron foot print is now 15'x10'. Maybe I am misunderstanding your recommendation.

KootK: The largest apron is 7'x3'.

In a related scenario; if we have a 5'x16' 1.5'thick slab on grade supporting lightweight equipment that is sensitive to settlement; is placing 5ft deep walls below the slab along each 5' edge a useful method for controlling settlement?

RE: Pinning apron to large foundation?

Quote (SM44)

Maybe I am misunderstanding your recommendation.

The insulation is about creating a shallow frost protection system to deal with frost heave: Link.

Quote (SM44)

KootK: The largest apron is 7'x3'.

4) You could drop a couple of helical piles out at the corners of the aprons. Those are dirt cheap in my area but that's probably not the case everywhere.

Quote (SM44)

In a related scenario; if we have a 5'x16' 1.5'thick slab on grade supporting lightweight equipment that is sensitive to settlement; is placing 5ft deep walls below the slab along each 5' edge a useful method for controlling settlement?

True downwards settlement or frost heave / clay swelling uplift? Probably not helpful for the former; would help for the latter but then your slab on grade would become a suspended slab.

I mostly use Jayrod's method for apron-ish stuff.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: Pinning apron to large foundation?

(OP)
Thanks KootK: I was thinking the 5ft deep walls on the edges would limit upward or downward movement, but you make a good point that this might result in the center of the slab being unsupported. A 16' long unsupported slab isn't too bad.

RE: Pinning apron to large foundation?

For your information non-frost susceptible material is any material (usually sand or gravel) where the amount passing the number 200 sieve (that's 200 wires per inch)is less than 5 percent. The usual gravel you would get from a ready-mix plant meets that spec. Clean beach sand probably also does it.

RE: Pinning apron to large foundation?

StructureMan44 and others

Extending the insulation horizontally on 3 sides will prevent frost from getting under the edge of the stoop. You may also use a combination of vertical and horizontal extension equal to frost depth.

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