Slab over or between stemwalls

[Sendit100]

Hi everyone. I've got a new engineer doing a structural observation report on a job I'm building and he's telling me I can't do a slab between stem walls, and that the slab must be on top of the stem walls or be a monolithic pour of slab and stemwalls together though nothing in our plan specifically calls that out. The slab details in my plans show the slab between the stemwalls (see below) but he's claiming that this is a "drafting error". I've done slabs both between stem walls and poured on top as the circumstances dictate, but this time it makes more sense to me to pour the stemwalls first, then pour the slab between the stemwalls at a later date, not least because the stemwalls are 4' high. Anyone want to weigh in on this? The slab is all going to be covered with floating floors when it's done so the appearance of the concrete finish doesn't matter so long as it's flat, and while I may be dumb, my forms are level in both directions and within a 16th of an inch of the hight where we need the slab to be. #4 rebar extends down the stemwall and bends out into where the slab will go 2" below the tops of my forms so we're covered there. Thoughts?

 

[Enable]

As a contractor I sympathize with your predicament. However, this forum is for practicing engineers to provide other practicing engineers help / guidance / a shoulder to cry on when necessary. It's not really a place where we post designs / opinions from professionals and critique their merit (on what will inevitably be only a subset of the facts).

A couple things I will say:

A) Don't call yourself dumb just because you're not an engineer / have been in trades most of your working life. It takes heart, soul, and a lot of skill to build even the most basic structure in a half decent way (and profitably...don't forget profitably). I'm proud to be a contractor and you should be too.

B) I'm not sure of your local jurisdiction, but from my vantage point it probably makes sense to have the top of the stem wall pinned in some way (especially since that vertical bar layout doesn't give me the warm and fuzzys when thinking about rotational stiffness). Seemingly that could be accomplished either placing the slab overtop the wall, or in-between the walls depending on the reinforcement layout. I wouldn't think it needs to be one way or the other all the time, which seems to accord with your experience. Though it really would depend on local codes and your profile seems to indicate California; they have a bunch of weird seismic things that may prohibit the reliance on bar alone for this purpose. Donnu.

C) Not sure what a structural observation report is but you are obligated to stick to the engineer-of-record's design, unless the EOR tells you otherwise or some other party takes over the job as the new EOR. Is it possible to touch base with the original designer and have them weigh in on, well, their design?

 

[dik]

Just to weigh in a bit... nearly all the SOG work I've done, the slabs are between the stemwalls. They are almost never on top.

Second item:


If the engineer sealed the drawing with the 'drafting error', it's an engineering error!

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[le99]

I guess an observation report is a report made by a junior engineer after reviewing the design done by an engineer who has seniority over the junior.

It is perfectly fine to cast the slab in between stem walls, but I suggest eliminating the hooked bar and adding an expansion/contraction joint alone to the wall-slab interface. The rigid edge connection may cause more headaches than help.

 

[BAretired]

I agree with dik. It could be done either way but between walls is preferable to avoid a horizontal joint on the exterior.

BA

 

[dik]

darn... I knew there was a reason for not doing that...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik

 

[OldDawgNewTricks]

I agree with dik and BAretired. It could be done either way. I have done it both ways many times, the decision mostly depending upon the contractor's preference. I would strongly caution you not to follow the advice about using an expansion joint between the slab and the wall though. That would make the wall into a permanently cantilevered retaining wall and it appears that the footing was probably not designed for that. In fact, you might need to provide temporary construction bracing to prop the wall to keep it plumb during backfill, compaction, and construction of the slab until the slab is strong enough to restrain the wall.

 

[BAretired]

Usually, the top of stem wall is a little higher than the top of slab to permit washing the floor without splashing the drywall, another reason to place the grade slab between stem walls rather than on top. A recess can be made in the stem wall to provide better support to the slab.

BA

 

[dik]

darn... I knew there was a reason for not doing that...

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Do you feel any better?

-Dik