Composite Deck Capacity 3

[onetimedeal]

How to calculate the cross-sectional moment capacity of a composite metal deck with concete and wire mesh?

I have a renovation project which needs to evaluate the existing composite deck with new mechanical units hanging underneath the deck.

After comparing with some catalogs, it has been identified as Vulcraft 2VLI19 or similar. It is a corrugated mental deck with concrete slab. The total slab depth is 5-1/4" with Light weight concrete. The span is 8'-0". The existing wire mesh size is 6"X6" #10/#10.

I can get the uniform load from Vulcraft (260 PSF). However, the mechanical units are using rods to hang from the bottom of the deck as concentrated loads.

What is the proper way to calculate the capacity of the deck for resisting 60 PSF live load plus these concentrated mechanical unit loads?

I am thinking to calculate the cross-sentional moment capacity the same way as culculating concrete beams and slabs. However, the corrugated metal deck stops my thought. It may be too complicate to do so.

What do you think? Please help.

Thank you for your input.

 

[Lion06]

Calc the moment that is in the deck/slab based on their allowable load of 260 psf. Then check your actual moment with the loading that you describe above and make sure that you are below theirs. They base the psf on all LL, so if you have some dead load, you can use the appropriate load factors and get a little more capacity out of it compared to what the tables say.

e.g. Your phiMn is 260*1.6*(8^2)/8 = 3328 lb-ft
Your LL Mu is 60*1.6*(8^2)/8 = 768 lb-ft
Your availble DL moment is now 3328-768 = 2560 lb-ft
This will be your available factored (at 1.2 though, not 1.6) DL moment.
You didn't say where they are located, so I can't help any further, but that is the approach I would take.

 

[SlideRuleEra]

Since the slab does not have structural reinforcement (wire mesh does not count), I would not rely on the concrete for any support. Treat the concrete as a dead load.

Do the math to convert each concentrated load into an equivalent UDL.

Then sum everything - dead loads, live loads, and equivalent live loads. Don't use the 260 PSF from the catalog as the acceptance critera for the following reason:

If the total is less than 200 PSF (as recommended by Nucor), you are done. The 200 PSF (comes form Note 2 at the bottom of page 47 on the Vulcraft Catalog, see this link:

http://www.nuconsteel.com/pdf/vulcraft_steel_deck.pdf

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[kslee1000]

I will tend to do the following:

1. calculate Mmax for the mechanical unit alone.
2. convert Mmax to an uniform load that would causing the same mangnitude of moment at the location Mmax occurs.
3. check the resulting and actual uniform loads against the known capacity.
4. make sure the slab will not crash by the concentrated load at the hangers (punching shear).

 

[tedamiao]

What I did before was same as kslee1000 said. Note allowable capacity given by catalog is superimposed load. You do not need consider slab self weight.

 

[Lion06]

kslee's method will work fine (I agree that the slab weight does NOT need to be included since the allowable load is a superimposed load), but is conservative since for strength calculations the entire super imposed load is assumed to be LL. If you need to squeeze some extra capacity out of it, you can do so by using the appropriate load factors for things such as mechanical equipment (1.2 load factor instead of 1.6). This can make a pretty big difference.

In the OP's situation - a LL of 60psf means the additional load would be 200 psf (260 - 60), but if you apply the proper load factors and you need to know the allowable DL if you only have 60 psf LL, you will get 267 instead of 200 (This comes from 200*1.6/1.2).
Like I said, this can be very helpful.

 

[onetimedeal]

Wow....you gys are great.

Now, I have a better idea what to do.

Thank you guys. You are very helpful.

 

[kslee1000]

structuralEIT's method is appropriate and desirable for the result falls on the margin or below. Rather than change the design, load factors can be manipulated to justify the capacity. After all, isn't that's the purpose to apply load factors - for uncertainties, which have been reduced to a certain point after the original project was done.

 

[Galambos]

How 'bout you follow SDI's guide for concentrated loads?

 

[JAE]

Also - on the connection between the units and the deck, make VERY SURE that the connection is to the concrete, and not just the metal deck.

If they tack weld or something just to the deck, the load could possibly peel off the deck bond to the concrete above at the localized hanger points and the composite action between deck and slab might be altered.

Also, the points above are OK - but keep in mind that the primary failure mode for composite deck slabs is loss of shear bond, not flexural failure.

Focusing exclusively on moment is incorrect. You also should look at the shear envelope across the span. Point loads alter this much more than the moment envelope.

 

[kslee1000]

I have very limit exposure to composite deck design, or evaluation. However, the method (mine & others) we used are to ensure the resulting uniform load is "comparable" to the given by the manufacturer, and the end reactioons as well. In such case, the shear bond is not likely to be broken by the addition load. However, local two way shear needs to be evaluated using convention method, ignoring the effect of the steel deck.

 

[southard2]

I'm assuming the deck is supported by steel beams or joist. Alternatively you might also think about attaching the hangers to some HSS tube steel and simply welding the tube steel to the beams. This way you don't have to worry about punching shear, bad installation, existing condition of the deck, false ID on deck type, previous construction quality, etc...

Welding of course can be a problem in an existing ceiling space. Thru bolting with a bolt between joist bottom chord could work.Or try cold formed steel studs and TEK/5 screws.

I agree with total PSF methodology when checking bending. Shear probably won't be a problem. Just check the the concrete only slab thickness for punching shear and shear failure modes. Keep redundancy in mind if the load is heavy.

 

[Galambos]

Or you could follow USD/SDI's guide for concentrated loads.