Minimum Composite Deck Thickness

[Loui1]

Just looking for a rule of thumb for a typical 100psf corridor commercial application. What is the minimum concrete thickness above the deck you like to use? When there's only 2" of concrete above the flutes I kinda get worried because I've been told by other engineers not to go below 2 1/2". Wondering if it is a valid point. Obviously Vulcraft isnt going to list a product that wont work, but.....

 

[whyun]

Concrete thickness over deck and whether to use hard rock or lightweight concrete is driven by the required fire rating.

 

[Loui1]

No rating is reqired. Would you use the thinnest deck?

 

[jike]

I have found that when fire rating is not a consideration then often the thickness is controled by the need to provide sufficient mass to minimze transient vibrations. I do not recommend less than 2 1/2". I often use 3 1/2" to control vibration. If you have composite beams with headed shear studs, you need a minimum stud height above the top of the deck of 1 1/2" plus perhaps 3/4 to 1" of cover.

 

[whyun]

I also agree with jike on the vibration issue with thinner decks. Verco catalog has span tables for 2" over metal deck (for example, 4" total thickness for W2 and 5" total thickness for W3 deck).

One question: Is stud length of 1 1/2" minimum above the top of deck required by code?

 

[lkjh345]

AISC Manual of Steel Construction, Chapter I, Section 5.1.4 (Page 5-60 of the green ASD Manual) requires the stud length to be 1 1/2 above the top of the metal deck.

Also, total slab thickness shall not be less than 2" above the top of the deck. I.5.1.5.

 

[jike]

If the slab is too thin (say 2"), it will often produce a hollow sound which gives a "cheap" feel to the building rather than a solid feel.

 

[lkjh345]

Jike:

I agree. I was giving the code miminums above (at least the AISC minimums). In the absence of a required fire rating, I usually use 3" above the deck, just for that reason, and vibration control.

 

[whyun]

Thank you for the code reference. I also would not use a 2" over deck for the reasons you mention. But if one was to use 2" over deck profile (as per deck manufacturers' catalogs) is it acceptable to design beams/girders as non-composite?

Another reason to use a thicker concrete is for installation of expansion anchors at hanger supports. Many product ICC reports require 3 1/4" minimum over deck.

 

[Loui1]

What process do you use to check for vibration of the deck itself? I know how to use the AISC design guide for the steel, but never for the deck.

 

[lkjh345]

Whyun:

IMHO, it is should be acceptable to design steel beams and girders as non-composite if that fits the particular application better. A local steel erector has told me that if you have approximately 500 or less studs on a job, the savings in steel using compsite action probably does not pay for bringing in the stud welder machine, getting it calibrated for the job, and having the special inspector doing ring and bend tests. So, for small buildings, we tend to go with non-composite beams if we can.

Good point about the expansion anchors, though be carefull of expansion anchors in an overhead application.

Loui1: Good question. I've always checked the entire 'system' ( beams, girders, deck, etc). Never the indivual elements.

 

[jike]

There is no vibration check for the deck that I am aware of. The slab thickness may have to be increased to add mass to change the vibrational response of the "system".