Retrofit continuous "L" stem wall to support second story?

[ReverenceEng]

Hello,

I have a client that bought a home in the middle of the build and wants to add a second story. The current engineering (not by us) has a typical raised floor, but the perimeter foundations are only 12" wide, 12" deep, and have a 6" (min.) stem wall. They are of the "L" variety.

I generally reserve the "L" foundation for slab-on-grade applications at perimeter walls (with or without curb), and I would say always use a more typical centered foundation when building a stem wall.

The PLF is still OK, but I am concerned about the eccentricity of load on the "L" foundation given that it is desired to be two stories.

Am I making mountains out of molehills, and if not, has anyone retrofitted a continuous foundation for this issue?

There is a similar thread here:

https://www.eng-tips.com/viewthread.cfm?qid=241252,

but I think it is much more complicated than my situation since that thread deals with a much larger building and loads.

If need to retrofit, what are my options? It might be useful to know that the only work-in-place right now is the concrete.

1. Pour slab on grade over the strip footing similar to slab on grade?
2. Pour mat in-line with top of strip footing to brace?
3. Underpin continuous footing?
4. Pour straps between current grid of posts? This might be easiest...

Help!

 

[JAE]

You might want to post a few sketches of your situation to get better responses.

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[ReverenceEng]

Please see a screenshot of the existing foundation that was engineered for a single story (again, not by us).

Also, please see attached details 1 and 2, which are the stem wall details and what appears to be a cripple wall alternative, though it's not exactly labeled as such. The labels and details don't exactly make sense based on the floor joist arrangement, but that's OK since only the concrete is existing and in place.

No matter whether you consider detail 1 or 2, the question remains the same about the "L" foundation. I wouldn't have done this in the first place (personally), but it's what is in the field.


Over to you JAE et all! Where's koot? haha

 

[txeng91]

So your saying the soil bearing is still good with the extra load, even with accounting for the eccentricity of the load on the footing? If your just worried about the exterior, couldn’t you excavate around the building and dowel in an extra foot or so of concrete to make it symmetrical?

 

[JAE]

I don't like the eccentricity and also don't like the skinny 12" wide footing. Usually 16" is a minimum footing for residential 1 or 2 story (or wider).

You could construct a footing extension on the outside of the footing and dowel into the existing with adhesive anchors. Or you could add helical piling around the perimeter and bracket to the outside vertical side of the existing footing.

But leaving it as is for a second floor - not me.

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