Foundation Blockout for Garage Doors 1

[medeek]

I'm trying to determine which is the preferred method of blocking out a stemwall foundation for garage door where a slab will be poured in the garage as shown in the images below. I've seen it done both ways and I've actually drawn details showing both these options. Option 1 does not typically block all of the stemwall out and seems to be more common in locations where the frostline is deeper (24" or deeper, ie. Utah). Option 2 completely blocks out the stemwall at the garage door and then a thicken edge is poured where the slab meets the footing at the garage door (this seems to be the preferred method in Western Washington). From a structural point of view which is the better method and also from a construction point of view what are the pros and cons?

Option 1:

Option 2:

View model here:

https://3dwarehouse.sketchup.com/model.html?id=9052ff81-b3c2-4aef-b40a-39a8be9e520b

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[medeek]

Most garage designs I work on have portal frame or shearwalls on each side of the garage door so my thinking is a continuous stemwall might be stronger and better resist the bending moments imposed on the foundation by the lateral loading of the shearwalls.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[jayrod12]

Option 1 is the only way I'll do it. And there's times where I'll drop the soffit to match the top of the wall where I need the capacity.

 

[KootK]

It depends on the wall/frost depth for me. At four feet below grade, I like number one for the reasons that you mentioned. At the proportions shown here, I don't feel that it matters much.

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.

 

[medeek]

I should point out I've also seen a sort of a combination of the two options where the blockout does not extend to the footing but a thickened edge is employed at the slab/door interface and the thickened edge is typically 2x the slab thickness, or called out as a minimum dimension of 8".

Option 3:

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[KootK]

Yeah, I do option three with the 8" drop often with frost walls that go down 4-ish feet. The interior and exterior slabs then meet over the wall which make for attractive detailing from a differential settlement perspective.

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.

 

[medeek]

This would be a typical application with portal frames each side of the garage door:

I have shown Option 3 but given the 24" stemwall and the 12" projection of the stemwall above the 4" slab it would seem more practical to go with Option 2.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[jayrod12]

Ya with only a 4" of stemwall remaining (our even 8" with option 1 detailing) it seems like a waste of labour and you might as well do option 2.

Why the 12" projection? Around here 8" is typical.

 

[KootK]

When you see it like this, it becomes apparent how unfavourable the shear wall overturning situation becomes. I'm surprised to hear myself say this but I pity your shallow frost depth here. It would be nice to have some more grade beam meat to work with. Can you dowel/couple rebar from the stem walls into the slab downturn? It's 24" from top of slab to top of footing, right? It looks like less here graphically.

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.

 

[jayrod12]

I think he's saying 24" stemwall total, top of upstand to top of footing. With a 12" projection above the slab, therefore 12" from t/o slab to t/o footing.

 

[KootK]

Ah, that makes sense. Thanks Jayrod.

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.

 

[jayrod12]

That's what we're here for.

I agree that although we complain about our deep frost depths constantly, they sure then themselves to beefier foundations. Perhaps we should stop complaining.

 

[boo1]

Medeek why show the garage door header span the entire wall?

 

[medeek]

Back to the portal frame vs. shearwall argument...

In this particular case I would not portal frame (PFH) it since my aspect ratio is 3.33 and is less than the 3.5 maximum requirement by the SDPWS. I would just use two segmented shear walls on each side of the door.

The one flaw with portal framed walls is the hinge created at the header and pony wall, that is the reason for the limitation on the pony wall height, at least that is my understanding of the rationale behind the IRC/IBC on this matter. Other engineers hate these things and strongly prefer a conventional shear wall (segmented) design. Moving the header to the underside of the top plate certainly helps but there is still a hinge issue between the header and the portal frame shear panel which is a weakness for out of plane loads (ie. wind perpendicular to the wall).

I would certainly argue that a conventionally framed header with two shear panels is better, but most designs I encounter don't have the wall length that allow for it.

The one positive of the portal frame is the reduction in the holdown forces. I wasn't able to fully quantify this until I created my portal frame spreadsheet/calculator but it is significant.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[medeek]

@KootK

Based on Terry Malone's book I've written a spreadsheet for looking at the overturning, shear and moments in the foundation at locations like this where there are concentrated shear wall loads. A lot of these designs don't work, they fail, you really do need a better grade beam. The problem is the designers and local contractors push back really hard when I get too creative with this stuff.

A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com

 

[medeek]

This spreadsheet now contains two sheets. The first considers a shearwall(s) on top of a stemwall foundation. The contribution of the footing is neglected and the bending moments are analyzed treating the stemwall as a beam. The second sheet is for a similar situation however in this case I am considering option 2 where the stem wall has been blocked out and a thickened edge slab spans the gap created by the garage door. I am treating the footing and the thickened edge as a single grade beam, similar to as shown in this typical detail:

The spreadsheet is here:

http://design.medeek.com/resources/shearwall/2016-XXX_STEMWALL_SHEARWALL_FOOTING_SWLX.xls

PDF preview:

http://design.medeek.com/resources/shearwall/2016-XXX_STEMWALL_SHEARWALL_FOOTING_SWLX.pdf

Any feedback or corrections are much appreciated. My comfort level with concrete is still not where I would like it to be.



A confused student is a good student.
Nathaniel P. Wilkerson, PE

http://www.medeek.com