Non Concrete Slab over Metal Deck

[BONILL]

I'm designing a concrete slab over metal deck in which neither the steel beams nor the steel deck are composite with the slab (deck is used as a form). The total thickness of the slab is 6", (2" flute + 4" slab). The span is 8'-6" and the equipment load is 1.00 Ton/m^2. I am thinking of providing reinforcement in the flutes 3/4" clear from bottom parallel to the deck and temperature and shrinkage steel perpendicular to the deck at mid-depth. Thoughts are more than welcome.

 

[hokie66]

My thought is that you are on the right track. If multiple spans, you would also need flexural steel in the top.

 

[delagina]

can the deck support a span of 8'-6" of wet concrete for single span calculation?

i am currently doing a similar elevated deck slab with equipment loads and i already have 0.9 unity ratio for 20 gage vulcraft non-composite deck spanning only 4'-0" for 7" thick slab for 1 span analysis. 2span and 3span analysis is ok.

 

[msquared48]

Yep. Probably will need to be shored centerspan.

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

 

[BONILL]

delagina, 7" thick slab total? What reinforcement are you getting at midspan and for negative moment? Do you conservatively assume the slab above the flutes as your structural slab when designing or do you work with an equivalent slab section?

 

[BAretired]

Why are you not using a composite deck?

BA

 

[rscassar]

For negative moment over the supports I would be calculating an equivalent section which accounts for some of the concrete in the lower flute. I would also allow for some re-distribution back to the mid-span.

 

[BONILL]

BAretired,

I could consider the possibility of using composite deck. This means that the deck provides reinforcement in the longitudinal direction which lowers my positive reinforcement? How do you determine the b width for positive and negative reinforcement calculation? If the beams are non composite with the slab, can it still be assumed that the steel beams are restrained against LTB, for when the deck is running parallel and for when it is running perpendicular to them?

 

[delagina]

@bonill,

i used 7" coz i'll have 2 layers of rebars for positive and negative moments of the slab.

but i am still considering using composite deck so i will only need top bars. vulcraft has VL series composite deck.


can someone explain to me the advantage of using non-composite vulcraft C series metal deck?

 

[SteelPE]

Bonill

You may be able to find some information in this thread:

http://www.eng-tips.com/viewthread.cfm?qid=223271

delagina

I have used C deck over composite deck in instances when I thought there would be a chance of the deck rotting out over time.... or when the chance of the deck rotting out was dire.

In those instances, I used a top and bottom mat of reinforcing to support the loading on the slab, and the deck was there to act as a form only.

 

[msquared48]

If you go non-composite, you will be using a lot more steel and concrete than you have to, $$$$$$$$$$$, but the analysis will be simpler. What's more important?

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

 

[hokie66]

BONILL,

Noncomposite beams are still braced...the deck has to be connected, usually by puddle welds, to the beams. In addition to corrosion, fire rating is another consideration in using composite deck.

 

[wannabeSE]

Around here, most composite decks are designed as simple span. However in the precomposite state, the steel deck is typical designed continuous when acting as a form. You lose a lot of capacity when you shore a comosite deck because the post composite deck must carry the concrete self weight.

We usually design a composite deck by picking something out of a deck manufacture's span tables. Pick your favorite manufacture, and look at there technical literature. If you want to calculate the capacity by hand, it may be worth looking at the Steel Deck Institute's standards

http://www.sdi.org/sdiansi2010.htm

Another reason for a non-composite deck design is when vibrating or moving loads (forklift etc) are expected. These loads can break the bond between the concrete and metal deck.

 

[delagina]

why is composite deck needed for fire rating? cant i have non-composite deck with same fire rating?

i have a 2-hr fire rating requirement.

 

[hokie66]

You misread my comment. When using composite deck, the reinforcement (the deck) is exposed to fire from below, so the fire rating is affected. Different codes handle this in different ways, so you have to investigate your code and/or fire rating agency requirements. One approach is to use the composite deck for gravity loading except in the case of fire, and provide "fire reinforcement" within the slab to provide safety against collapse.

If the deck is just used as permanent formwork, the fire rating requirements would be different. But again, you would need to know whether your fire rating agency has any special requirements for clearance to the reinforcement because of the ribs, and whether the total or an equivalent depth is to be considered for the fire rated element.