Basement Bearing Walls Positioned On The Edge Of Footing (residential 3 stories)

[Mark_J]

Within cities, this condition is very common. The 10" foundation wall sits on the edge of a 24" to 30" footing. Almost always this overstresses the allowable soil bearing pressure provided by the geotechnical report (via straight calculation).
What are your thoughts on tying the slab into the footing to help disperse the building loads? 4" slab, use #4 rebar @ 16 instead of wire mesh?

 

[jhnblgr]

The issue is the eccentricity of the loads. The reason I have heard from not pouring the footing with the wall is water infiltration and finishing difficulties

 

[DaveAtkins]

I have designed this type of footing in the past. You assume uniform bearing under the thickened portion of the slab, which means the resisting upward force is eccentric from the downward force. The moment is resisted by the reinforced slab on grade. The top of the slab will be in tension.

 

[Greenalleycat]

This type of construction is done all the time in residential and commercial. Usually people don't want footings sticking outside the building line so you're stuck with the wall on the edge of the footing regardless.
I've never personally checked the eccentricity and resolved it into the slab etc, but my detailing would basically take care of it by default. I always tie the slab into the wall footing.

If it's a retaining wall then it probably won't matter so much as the retaining forces will presumably be pushing the opposite direction, so the gravity eccentricity is actually favourable

 

[Mark_J]

Yes, there would be a cold joint into livable space every time. Certainly a consideration, may just make the case for always using double dowel bars regardless what's in the wall.
So, in your opinion, (any and Greenalley), would you consider this detail to be the more appropriate method to follow by default? I'm looking to pitch as the "preferred" detail. Not just a one and done, hundreds of buildings changing to this method rather than footing, wall loading the edge, slab poured after and not connected.
Besides the water, working in the city is usually very tight. Sometimes, out of necessity, footings need to be poured asap to stabilize the excavation. Hundred year old buildings, underpinning, water issues, etc all lead to wanting to pour concrete quickly.

Thanks for the feedback above, much appreciated.

 

[Ron247]

I have designed this type of footing in the past. You assume uniform bearing under the thickened portion of the slab, which means the resisting upward force is eccentric from the downward force. The moment is resisted by the reinforced slab on grade. The top of the slab will be in tension.

I guess I am not following the statics on this description. I don't doubt you have done this successfully, but can you clarify some of your description?

1. When you say the top of the slab is in tension, do you mean concrete or where the reinforcing steel is?
2. Do you also mean the slab has a couple in it such as tension and compression?
3. If the slab is only in tension, where is the negating horizontal force so that Sum x = 0?
4. Lastly, I am not familiar with "assuming soil bearing" has any profile other than is or is not present. Soil bearing is a reaction, not an applied load, so wouldn't statics be what tells you what its' profile is?

I have seen this detail at times, but it is not that common where I am except in 1 story SOG homes, not 3-story ones, so I find this interesting and a chance to learn.