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

Mark_J

Mark_J

Structural
Joined
Feb 10, 2024
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Location
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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?
 

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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
 
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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.
 
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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
 
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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.
 
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DaveAtkins said:
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.
Click to expand...
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.
 
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I'm just going to post these links because I likely won't find them again:
design62713

Thread 'Wall Footing Eccentricity'

I am trying to find out which section in ACI-318 talks about L/6 eccentricity limit for designing wall footings? Thanks in advance for help.
Phillip001

Thread 'Property line eccentric strip footing'

Hi,

I have allways wondered if eccentric footings create moments in foudation walls. Unfortunately, my bosses would only make me feel like I make to much off nothing and answer vaguely, no mather how many times I would say '' BUT THAT MOMENT HAS TO GO SOMEWHERE!?''. So I have allways secured myself by adding rebars...

Here is a particular case where I am adding load to the building and demolishing the old foundation wall. The contractor wants to keep the old foundation wall head to make the underpinning easier, and drilling the old wall head from under doesn't seem like a good idea.

You...

I see it as introducing tension in the slab as well. I see it distributing more evenly to the soil, max pressure at the corners of the building. Check the slab for shear and moment based on the weight of the entire building, including a lateral event. In my mind, edge loading as I showed in first post, helps the load find the soil over a greater area, and evenly. 6" slab was to replace a 12" mat slab design. That led me to consider using it for all property line buildings, with the normal 4" slab poured monolithically.

I can't find a calculation that suggests it won't perform better. If it is an independent footing, there is only 30" (+/-) to consider, and always leads to a less desirable result.
 
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What’s your fbd look like? How much tension has to be developed so that the soil pressure doesn’t exceed the bearing? Also will it create uplift causing failure in the slab.
 
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DaveAtkins said:
See attachment for how I design these. Keep in mind for concrete design you must factor the load.
Click to expand...
Thanks. This gives me something to visualize.
I assume then that there is no passive soil bearing from the left nor horizontal "basement wall" loadings involved in your solution.
 
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What I am showing is the design of a one story building on grade. Not a basement wall.

If you have a concrete basement wall you could tie it into the slab like a retaining wall.
 
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DaveAtkins said:
See attachment for how I design these. Keep in mind for concrete design you must factor the load.
Click to expand...
I am mentally applying deformations to your sketch to help visualize it. I am now trying to understand at the location of the moment in the slab (M=Pe), why there is no shear and no axial other than statics says they must be zero due to the FBD you have. Moment can exist without Shear when moment is a maximum or a minimum, or you are in pure bending; but how do you know that occurs?

I hope these questions aren't getting aggravating, just trying to understand something I have seen before but never designed one in this manner.
 
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DaveAtkins said:
See attachment for how I design these. Keep in mind for concrete design you must factor the load.
Click to expand...
Assuming that type of eccentricity won't make a 4" slab work. One thing to keep in mind is most residential footings are 12", you don't get great embed on dowel bars with that length. Turning down gets to a 16" footing with almost no difference in volume of concrete. Where does eccentricity really fall and how much soil overstress are you at to begin with? And this is a slab on grade. I think, without stretching too far, it can be justified fairly easily to turn down a 4" slab and it perform well.

6" slab seems to be no problem with your sketches.

There are drawbacks, if concrete needs to be poured quickly a slab turn down is not the option (sub-grade and under slab plumbing is needed). And there is that cold joint issue.
 
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You can use crushed stone under the concrete to increase the bearing width of the footing at an angle of 30 degrees from vertical.
 
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