Composite Deck - Reverse Bending

[Skazak]

I am analysising a composite floor deck (2" type B steel deck 4" conc.) which has some fairly high reverse bending due to point loads.

Am I allowed to take any bending strength from the inherent strength of the steel deck by itself?

ie if my analysis resultes in a moment capicity of X for the full section, and the mfgr listed strength of the steel deck is Y, is my total Moment capicity X+Y, or does Y get ignored?

Thanks

 

[271828]

You'll have to be a little clearer. Does your slab have a typical continuous beam bending moment diagram with tension on top, over supports? Do you have some weird uplift loading that causes top tension out in the middle?

 

[271828]

"Am I allowed to take any bending strength from the inherent strength of the steel deck by itself?"

Yes, if you can somehow arrive at the flexural strength of the deck only. You probably can, using the manufacturer's literature and the appropriate effective elastic section modulus.

"ie if my analysis resultes in a moment capicity of X for the full section, and the mfgr listed strength of the steel deck is Y, is my total Moment capicity X+Y, or does Y get ignored?"

No. For positive moments, the section acts like a concrete beam with the deck acting like bottom rebar. Therefore, you can't add this strength to the strength of the deck acting alone. FOr negative moments, your deck does act alone because the concrete is cracked, or assumed cracked.

 

[JAE]

It will crack in those negative bending moment regions.

In many cases, WWF is included in the concrete and if you can get them to chair it up at the beams, you effectively have negative moment reinforcing and can count on that.

 

[Skazak]

My slab has reinforcing (#4 @ 12" oc ew) which, assuming installed correctly, would give me at least some strength in negative bending.

Would I be ok to take the allowable moment from my conc. and reinf. and add it to the allowable bending from the steel deck?

And yes, my negative moment is occurring over a support and my deck is installed in a 3-span condition.

 

[hokie66]

The strength at the supports depends largely on where the reinforcement is placed. If it lies on the deck, it doesn't help much. If it is chaired to the top with only the cover required for the exposure, then it assists greatly. Adding the deck component at this location is probably not a good idea, as in order for the deck to contribute much in negative bending, the reinforcement would have already yielded, cracking the concrete at the top.

 

[JAE]

Would I be ok to take the allowable moment from my conc. and reinf. and add it to the allowable bending from the steel deck?

I wouldn't do that. The compression/tension couple you are using with the concrete and the rebar (assuming its chaired as hokie66 states) is what you want to use. The deck would have some bending capacity, but how they interact would be tough to calculate - not worth my time I know that.

 

[VTPE]

Unless you can determine that the deck is developed by its interaction with the concrete and that it has yeilded, you can not expect the deck to work as reinforcement. Even if the deck has deformations, it would be difficult to provde it is composite. Further - I think ACI has limits on the type of steel to use (44 KSI min) and its ductility. If the deck and not meet the ductility requirements and the minimum yeild strength, what code will you use?