should indoor slab be tied to the foundation footing/wall

[ptr0]

I have a residential addition project (California, the frost-line is very shallow) where the concrete footing (2' depth and a 2.5' concrete perimeter wall on top) and Slab-on-Grade (SOG, just on top of the footing) will be built on expansive soil. A wooden subfloor will be built on top of this indoor slab and then floor finishes. Currently, structural engineer seems to prefer the indoor slab to be tied to the concrete footing/wall through rebars instead of a floating slab that is separating from the perimeter foundation footing/wall using isolation joints. From the geotech side, 18" of the soil below the SOG will be over-excavated and replaced with class-II baserock to minimize the SOG cracking.
After doing a few researches online, it seems that the standard practice is to have the SOG being floated instead of tying into the foundation footing/wall. Will the current design tying the SOG to the foundation footing/wall cause severe slab cracking, or any other potential problems? Any input will be greatly appreciated!

 

[XR250]

I'd float it if possible. That being said, I honestly have only seen one time in 30 years where tying it to the walls was an issue. Why do a slab at all if you will have a wood subfloor? Why not joists on girders? Also, this is better posted in the SE General Forum.

 

[dik]

I normally don't and add a 1/2" flexcel jointing material.

 

[geotechguy1]

I used to always recommend not tying a slab-on-grade to the concrete wall but I commonly had structural engineers do the doweling in thing anyway (and would tell me about it) or ignoring us and doing it. So I'm not sure one way or the other. As a geotech, floating it in these circumstances seems to make more sense but I can't discount the experience of structural engineers with 30-40 years experience who have always dowelled them in and never had an issue.

 

[ptr0]

I'd float it if possible. That being said, I honestly have only seen one time in 30 years where tying it to the walls was an issue. Why do a slab at all if you will have a wood subfloor? Why not joists on girders? Also, this is better posted in the SE General Forum.

Thanks for your response! Glad to know that tying the SOG to foundation footing/wall hasn't been an issue from your observations. Meanwhile you prefer floating SOG, is it because it will minimize the slab cracking right?

Consider SOG over the crawlspace mainly due to two reasons: 1) the floor (where the SOG is) is about 2' below the exterior grading, and SOG seems to be more typical for this situation likely due to excessive moisture will cause big problems on the wooden joists if go with crawlspace; 2) if go with crawlspace, the footing needs to go much deeper to resist the lateral force from the exterior graded soil (like a retaining wall I guess) and also potentially introduce other complications with the existing house's foundation.

 

[HTURKAK]

Acc. to the sketch posted by OP , the SOG supported on the foundation wall shelf and my opinion it is OK .
In past i also preferred to support the SOG on the perimeter seat to avoid local settlement and seperation of floor finish and never had an issue .
The following excerpt from ( Foundations of Structures by DUNHAM , 1960 )

 

[dik]

Usually bot bot bars are transverse and top bot bars are longitudinal.