Floating Slab Vs. CMU Stem with Isolation

[Guastavino]

I think this will be a fun discussion here. Do you prefer the floating slab detail where the slab and stem wall are independent? Or do you like them to be tied together like shown below (detail not be me but by an architect I'm working with)?

Personally, with this building being a 3-story load bearing wood structure, I like isolating the wall from the slab to avoid cracking the slab with settlement, etc.

Also, as an aside, I see this detail from time to time by other engineers/architects, and frankly, I don't think I ever like it. The slab will want to shrink to the center and want to pull away from the wall. But maybe I'm biased. Are there situations where the below detail is good that I'm overlooking?

 

[KootK]

I agree completely. I float whenever I can. In my area, it's very common to dowel slabs on grade into the side of frost walls and grade beams. I have clients that insist on the doweling because they think that it will protect against differential settlement/heaving. And I suppose that it does offer some protection. However, I suspect that under any serious expansive clay heaving, the dowels would just spall the SOG concrete. Many geotechnical reports in expansive clay areas will recommend isolation as well.

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.

 

[medeek]

Thanks for posting the detail, I appreciate it when I can study some of these details by other designers and engineers. I apologize for posting a slightly off topic question but I've noticed that there seems to be a number of ways to insulate these slab on grades utilizing rigid foam, see diagram below:

With the original detail posted, would it not make the most sense to place the insulation on the exterior of the stemwall thereby creating a total thermal break. The way it is shown it seems that the heat will escape at the wall edge and flow through the slab to the exterior of the stemwall.

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

http://www.medeek.com

 

[Guastavino]

Medeek. Then you potentially see the insulation from the outside, which is usually unacceptable to the architect. I'm in an area where I think the whole continuous insulation seems to me as the tail wagging the dog. I can't imagine structural engineers were consulted at all in this process when someone said "CI". I just strive to do the best I can without compromising the structure. The idea of CI is great in concept, but structurally poor in reality. Again, I'm not in an extreme climate area, but color me cynical when someone wants to trump good structural details with insulation.

That, and we went from 50 years ago mass masonry walls with no insulation to what we have today. I think the pendulum swung the other way, and I'm hoping we land back in the middle soon

 

[EngineeringEric]

This detail will no longer work with the newest IBC Energy Code that requires continuous insulation. That slab tied together is a massive thermal short through the slab. Some architects don't care, some do.

And i am so glad to hear another person reiterate the tail wagging the dog. I have been in many arguing matches with architects about CI and even a co-worker who is a forensic engineer that really specializes in Building Envelopes... Lets say i care more about my walls cracking then a little thermal short next to a giant curtain wall.

As for the original question, I nearly always isolate for the reasons you mentioned. I do sometimes tie them together for retaining reasons or at doors.

 

[XR250]

FWIW, I have seen many instances where a non-floating slab shrank and pulled in the top course of the CMU. Probably no rebar in the CMU wall, however.