Composite Slab on Metal Deck with Rebar

[Guastavino]

My client specifically requested REBAR over WWF and fibers. 5.5" thick slab (3.5" over a 2" VLI composite deck). I was leaning towards #3@12" o/c for longitudinal direction and #4 @ 24" o/c for transverse...but now I'm thinking just a #3@12" o/c mat that is set directly on top of the flutes, but I don't love that. It spans 8 to 9 feet.

Any of you all done one with Rebar?

Obviously it's more reinf. than the WWF provides, but I don't want to space it too far apart either...I'd actually prefer fibers myself, but client wants rebar.

 

[hokie66]

Fiber does nothing flexurally, especially if you are talking about the plastic stuff. Size the bars and chair them so they give you the required negative moment capacity.

 

[KootK]

You may find value in this article: Link

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.

 

[JedClampett]

I always reinforce composite slabs with reinforcing bars. I was trained not to trust the composite action of those decks and have seen nothing to change my mind. Why not put reinforcing bars in the bottom of the pans of the deck? It's pretty cheap insurance. I'd skip the WWF, put flexural reinforcing in the pans and temperature and shrinkage reinforcing perpendicular to it.
The biggest problem I've had with WWF is the contractor can never get the size and spacing I specify. The ACI lists a bunch of sizes and spacings, but if they're not stocked, you're out of luck. Maybe if you're ordering an acre of the stuff, they'll run an order, but my projects are pretty small.
And why fight your client? It's not your money, and like I said, reinforcing is cheap.

 

[Guastavino]

Thanks all. Just as I thought, everyone is different. My tradition has been more like Jed's (IE don't trust the composite decking), but temp/shrinkage steel down 3.5" below the top of slab doesn't do all that much...I'm still debating in my mind.

 

[hokie66]

I do trust composite decking for positive moment reinforcement, at least deck made by one of the reputable manufacturers. Just one caveat, not in a corrosive environment.

Reinforcement laid on the deck 3.5" below the top of the slab will work for crack control in the minor direction (controlling cracks parallel to the ribs) just as well as if placed higher. But it won't do much good for flexural reinforcement. The theory of "just let it crack" sounds like me for slabs on grade, but I don't like cracks in suspended slabs. Cracks will occur, but I do what I can to control them.

 

[Guastavino]

Alright, so I see two options, "A" or "B".

Good About "A": Better chance of controlling shrinkage cracking (albeit Hokie argues depth doesn't matter that much) at top surface.

Good About "B": Will help with flexure and controlling cracking. Also much more constructible.

Either way i'll have bars over the girders for negative moment too. I don't want to overkill this slab design. It's just a mercantile building. Any other thoughts? I'm leaning "B".

 

[hokie66]

I would put the negative flexural bars in the top layer. Chair the transverse bars off the top of the pans.

 

[MotorCity]

The fibers are overkill in my opinion. I have reinforced composite slabs 3 ways: all mesh (my least favorite), all #3 bars, or mesh w/ rebar over the beams only for slab negative reinforcement.

If the owner can live with cracks over the beams, there is no need to provide negative slab reinforcement. The Vulcraft catalog states that by not providing negative slab reinforcement, it will simply act as a series of simple spans.