Floating slab on bar joists - can it be done? 4

[mistermopar]

This is a new one for me, perhaps this is a really easy question, but I'm striking out with Google so far just trying to find an example of someone else doing this, which concerns me by itself (not going for a ground-breaking design here).

The situation we have is a new 3-story retirement facility (in KY), with a typical steel building frame and barjoist/steel deck floors throughout. However, on the 3rd floor, the architect wants a courtyard area that's open to sky/environment (i.e. below the 3rd floor courtyard walking surface we have bar joists/heated environment). The final finish for the walking surface will be a flexible vinyl product called "Londeck".

The architect has spoke with Dow Building Solutions, who claim you cannot put the deck insulation under the floor deck or the deck will rust/rot away in "just a few years" the condensation will be so bad. They claim it is very common to put the insulation on TOP of the deck, then pour the slab over the insulation (how we arrive at a "floating" condition), but like I said, I'm striking out finding any examples/design aids/etc.

Is this really common to do for elevated exterior slabs? Floating slabs on grade are common enough, but I've never ran across anyone trying to do an elevated floating slab. Has anyone here did this before? Any special considerations? The architect is asking us if this is a 'good idea' or not, I think they're in the same boat having never ran across this situation before. Quite frankly I'm not sure what to say. Right now I'm thinking it would be ok, but I figured I would throw this out to the forum in case someone knows of a design guide or has any advice in general for this case or had troubles to watch out for that I haven't thought of yet.

Any help is greatly appreciated.

 

[hokie66]

Bar joists, steel deck, concrete slab, insulation, concrete slab, waterproofing, protection layer, walking surface.

 

[mistermopar]

Two slabs...THAT hadn't crossed my mind, makes sense though, I believe the architect is planning on varying thickness of slab for drainage, so the upper slab could be for that while the lower slab is flat. Thanks for the info!

 

[a2mfk]

I've read about it but not been involved directly, but I think Hokie has it right. Typically on plaza slabs, green roofs, etc, I believe there is a structural slab, then you can put all sorts of things on top such as soil, insulation and then a topping slab, etc. I would imagine the topping slab is designed as an architectural component and you could get away with something lightweight and thinner than a standard 4" SOG.

You have to at least have deck attached to the roof joists for diaphragm action and bracing. With the loading you are talking about here I would look into wide flange composite slab systems though...

Maybe this is a start for resources?

http://www.greenroofs.com/blog/2010/10/07/green-roof-construction-structural-considerations/

HTH,
Andrew Kester, PE
Florida

 

[Brad805]

Do you need the rigid diaphragm? Assuming you could connect the floating slab to the columns around the courtyard I think it is feasible to construct this with the insulation between the deck and the concrete. Rigid SM is rated for 20psi at 10% compression, so I don't think you have any vertical load problems. Obviously the slab must be tied to something somewhere.

Brad

 

[msquared48]

With the top concrete slab, which will be non-structural, I think a rigid, composite diaphragm would be best.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[msquared48]

Oh, and as a side comment, Hokie obviously likes sandwiches for lunch.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[mistermopar]

Thanks guys, this gives me some confidence in moving forward. This forum is a great resource!

Thanks again to all.

 

[structuresguy]

yep, hokie has it right. that is how we always do exposed decks over living space.