Stoop with no Foundation Wall

[carnahanad]

I'm in the Midwest and we typically detail stoops with foundation walls that are extensions of the building foundation wall. I'm currently working on a project with 1000's of feet of storefront where a door could be inserted anywhere along the storefront. We are trying to figure out a way to have a stoop without the additional 4ft tall foundation wall parallel to the building foundation wall. I will be doweling the stoop slab into the building foundation wall, I'm just worried that will act like a hinge. I've heard that on the east coast they use a detail similar to this with no stoop walls. Anybody have a suggestion?

Thanks!

 

[jike]

Use 4 feet of non-frost susceptible (well graded granular with less than 6% passing a no. 200 sieve) fill below the stoop slab. An alternate would be to provide 2" Styrofoam under slab. Keep water from building up under the slab by providing drain tile in either case. This will prevent creation of ice lenses under the slab. Dowel the stoop slab to the foundation wall.

Have the GC coordinate the joints in the slab placement as information becomes available.

I hope this helps.

 

[JAE]

I've seen retail centers where the entire front area was a structural slab with a void below (using metal deck or cardboard void forms).



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[KootK]

I got started in the midwest and "grew up" with the same detail which I miss dearly, truth be told. I subsequently moved north to Canada where heave us more of an issue and several variants are used:

1) Shallow frost protected apron slab with edge thickening (non-expansive soli).
2) Thin suspended slab with a grade beam and screw piles out front and void form below (expansive soils).

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.

 

[TehMightyEngineer]

In Maine I see a lot of option 1 that KootK posted with an option 3 being drilled holes below grade with precast frost posts or poured concrete posts using sonotubes (more common in residential).

Professional and Structural Engineer (ME, NH)
American Concrete Industries

http://www.americanconcrete.com

 

[jike]

I misread your question. With the solution that I had previously suggested, I would still have a continuous frost depth foundation wall parallel with the building foundation. Having a parallel frost depth foundation wall and a slab on grade does not necessarily prevent slab heave unless it is bridged with a structural slab with a void underneath (as JAE suggests) or an insulated slab or non-frost susceptible fill with drainage below the slab on grade. Slab should also slope away from the building.

 

[FoxSE14]

Another option that may/may not be cost effective: a few scoops with a backhoe down to frost...fill with lean concrete to elevation less the slab thickness. "Sidewalk slab" poured on top. You've got a nice mass of concrete bearing at frost elevation, supporting your sidewalk slab. Doubt that would go anywhere. Smooth trowel on the lean concrete with a layer of visqueen between the two allows initial shrinkage of the sidewalk slab. Dowel into bldg side foundation as you stated.

I also like option 1 suggested by KootK so long as the subgrade is fill not susceptible to frost (verify composition with geotech?). I like the idea of extending the "non-frost fill" down to 4' and beyond the plan extents of thickened edge stoop slab by a few feet (give contractor a min dimension).