How to calculate load capacity of concrete roof? 3

[MrFurleyEIT]

I need to calculate the vertical load capacity of a 5"-thk. reinforced concrete roof placed on a metal deck that spans 7' between beams. Concrete is the normalweight 3000 psi. I have steel deck manufacturers' catalogs that provide load tables for the allowable vertical load capacity for their deck, but how did they arrive at the numbers? How do I calculate it myself, especially if the nature of the metal deck is obscure, or unknown. Also, I would like to try to calculate the load capacity, as both being supported, and not being supported by the metal deck,

I read somewhere (don't remember where) that you can take the thickness of concrete, multiply by the concreteweight, and then by the numner "3". In my example above, it would be 5"/12*150 pcf * 3 = 187.5 psf. So, the load bearing capacity would be somewhere in the region of 187.5 psf. Does anybody know where this formula came from, and what does the factor "3" mean? I believe it might be some rule of thumb.

Appreciate all yours help.

Thaank you very much.

Furley, EIT

 

[mike20793]

Is it form deck or composite slab? Do you know the deck profile?

 

[jayrod12]

I would expect that 190 psf is high if this has actually been designed as a roof, and not as a future floor system.

5" of concrete on metal deck sounds more like a floor than a roof.

If you don't have any idea of reinforcing in the concrete, composite or non-composite deck, deck manufacturer etc, then you would have to make some conservative assumptions and try to confirm some more details.

I've never heard of that "multiply by 3" rule of thumb.

 

[KootK]

I have steel deck manufacturers' catalogs that provide load tables for the allowable vertical load capacity for their deck, but how did they arrive at the numbers?

It's kinda hard so I usually stick to the manufacturer's tables: Link.

Does anybody know where this formula came from, and what does the factor "3" mean?

I've also never heard of this. It sounds a little sketchy to me.

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.

 

[hawkaz]

That formula doesn't even take span into account- so a 100 for span can support the same as a 2 foot span- I don't think so
Stick to the load tables from the manufacturers

 

[MrFurleyEIT]

Thank you so much for all your replies.

It is a form deck, and not a composite slab. The building is old enough that the original specifications for the form deck is not available anymore, although the profile is clearly visible from below (at about 20 feet high), and the concrete on top of the deck is 4-1/2" to 5" thick, and reinforced with #4 bars at the bottom. I do not know which manufacturer's literature to go by, because the I am not sure who they are.

The roof is located inside a industrial plant, and I believe the engineers and contractors of old would have realized that someday future plant personnel would load the roof/floor in whatever way they can, and they did!

Does anyone know of any "quick and dirty", and even "elegantly professional" way of calculating the load capacity of the roof/floor based on what can be observed, measured or gleaned from as-builts, but missing other information?

 

[KootK]

If it's not a composite slab, then all that you have to work with is the capacity of the concrete and rebar on its own. With the steel deck no longer in play, it's just regular flexure/shear in a one way slab.

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.

 

[MrFurleyEIT]

Can someone please explain the differences, and similarities, between a steel deck concrete roof/floor, and a composite slab/roof? I googled each of those examples, and they look very similar, only that the composite slab/roof (composite decking?) has steel stud that lock the steel deck to the steel beams below. Is this the major difference? How does it affect performance? I may have jumped the gun when I said the roof is a steel deck roof. It may very well be a composite slab, as well!

 

[jayrod12]

Now you're trending into a whole other situation.

When we mentioned composite versus non-composite we are generally talking about the decking/concrete interface only. There are decking profiles that have improved interaction with the concrete that allows you to treat the concrete and metal deck as a single structural system (we call this a composite deck system see canam P-3623 or similar). This allows for lower levels of reinforcing to be put into the concrete (typically). In your case, since it is only form deck, ignore the fact it exists at all, analyze it as a regular reinforced concrete slab. Alternatively, the metal deck could be used as the main structural member and the concrete is there solely for a wear surface and fire protection (non-composite, see canam P-3615 or similar). In this case, the concrete is strictly accounted for as a dead load applied to the deck and you only look at the structural capabilities of the deck.

 

[JAE]

Studs have nothing to do with composite or non-composite decking. The studs create composite beams.

Composite decking will have ripples or other indentations in the deck surface to help bond the metal deck to the concrete slab.
Thus -when it flexes, the deck steel serves as the flexural reinforcement of the concrete deck. The primary failure mechanism in this composite deck system is shear-bond failure between the deck and the slab.

Non-composite form decking is simply a stay-in-place steel form for the reinforced concrete slab system. Once the concrete hardens it serves no other purpose.
KootK is correct that you simply then have a reinforced concrete slab and can derive the capacity based on the depth, d (top of slab to rebar) and compressive strength of concrete.



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[JAE]

jayrod12 beat me to the punch - looks like we agree.

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[jayrod12]

I hit submit that half minute quicker. Better polish up those typing skills, haha.