Addition next to Slab on Grade

[medeek]

I'm looking at an addition for a detached garage. The garage is a small 24'x27' hip roof structure built on a slab on grade foundation. The customer wants to build a two story mother-in-law apartment next to it. Since this addition will be living space I would rather it be on a stemwall foundation with crawlspace rather than slab on grade. My experience has been that slab on grade dwellings tend to be less insulated and prone for moisture problems.

The problem I am seeing with this is where the two foundations meet. I've got a thickened edge slab of the garage butting up to the new stemwall and footing. I'm not seeing how this is supposed to work since I would like both floors to be at the same level.

I've never seen something like this done before so maybe there is a reason for that (design and structural). Has anyone ever seen this sort of thing before or would my best option be to just keep it all slab on grade?

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

http://www.medeek.com

 

[XR250]

Dig under the thickened edge and form up a foundation wall up to the bottom.

 

[jayrod12]

Design the stemwall footing to be eccentric?

 

[RHTPE]

This topic was also discussed in [link]http://www.eng-tips.com/viewthread.cfm?qid=401701[/url]

What may be required by code will depend on the locale - you have not identified the location.

Anything is possible I suppose - it's just a question of time & money.


Ralph
Structures Consulting
Northeast USA

 

[EdStainless]

You are usually required to have a step up from the garage into occupied space. this is to prevent fluid (gasoline) from running in. and a fire rated wall as well separating them.
Do you know what the footings for the garage look like? This might be a good place to start.

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P.E. Metallurgy, Plymouth Tube

 

[KootK]

You could:

1) underpin the existing slab edge thickening down to the desired new building footing elevation.
2) drill in dowels into the existing slab edge that would later be embedded in the new stem wall.
3) pour the new stem wall up against the existing slab edge and underpinning.
4) use an eccentric footing as Jayrod suggests.

Expect some whining about the cost of the underpinning. Wold the new and old structures wind up sharing a common roof?



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.