Composite Metal Deck Moment Capacity

[kc9508]

Most manufacturers list capacities for composite metal deck for “no” studs and “full” studs with interpolation allowed between the values. In the past I’ve used (perhaps conservatively in some cases) the no studs value even when studs are provided at the steel beams. My question is if the “full” stud value is used (or some percentage if less than the number of studs per foot to obtain full capacity is provided), how do the studs increases the capacity of the deck?

*Are they improving bond/load transfer? Usually the studs are welded through the deck to the beam, so it wouldn’t seem to help that much.
*If the beams are partially composite, it would seem the stud capacity is also being utilized for the beam capacity. Is it double-dipping to then assume the studs are also assisting to increase deck capacity?

Any feedback is appreciated!

 

[JAE]

Primary failure mode in composite metal deck is shear bond failure between the concrete and deck. So studs at the beam would conceivably increase load capacity.



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[KootK]

Can you point us to any deck catalogs where the impact of the studs is included like this? I can't remember the last time that I saw one and a brief scan of SDI's latest deck spec doesn't seem to offer up any options for including the studs in the flexural capacity.

Are they improving bond/load transfer? Usually the studs are welded through the deck to the beam, so it wouldn’t seem to help that much.

As JAE mentioned, they are (or were) thought to improve shear bond between deck and concrete which is often an important failure mode. Even if the studs are welded through to the beam and burnt through the deck, you still have dowel action of sorts like with bolts.

f the beams are partially composite, it would seem the stud capacity is also being utilized for the beam capacity. Is it double-dipping to then assume the studs are also assisting to increase deck capacity?

Interesting question. I've never known any one to consider this. Nor have I seen this kind of consideration mandated anywhere. That said, our obligations don't end with what has been set out in codes. Beam flexure and deck flexure will use the studs differently. For beam flexure, you're taxing the studs in shear, tension, and flexure. For deck flexure, you're really just taxing the studs in shear. So there's definitely some interaction when it comes to the shear capacity of the studs and the welds but not much else. I suspect that stud and weld shear capacities are generally pretty high compared to demand so that might explain the lack of concern for interaction.



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[Hokie93]

Vulcraft publishes a "Composite Steel Deck" catalog in which the deck capacities are based on no studs being present (the conservative option) or shear studs installed at 12", 24", or 36" on center. My copy is dated 2010. As KootK mentions in his last paragraph, my opinion is that the interaction of forces between demands from the deck and the beam/girder would likely be negligible. There are papers and articles that have been written about interaction forces on shear studs where a composite beam/girder also acts as a collector element and, if I recall correctly, it was shown that the collector forces did not appreciably reduce the shear stud capacity for beam/girder flexure. One of the articles was in the December 2008 issue of Modern Steel Construction magazine by Burmeister and Jacobs.

 

[kc9508]

Canam lists different values for no studs versus full studs (2010 was the latest catalog I could find).

https://www.canam-construction.com/wp-content/uploads/2014/11/canam-steel-deck.pdf

The difference can be somewhat significant. For instance, I was looking at 5.25” LW on 18 ga. 2” deck and the full stud value is 91 k” versus around 65 k”.

 

[KootK]

I looked into my 1997 edition of SDI's composite deck manual.

- There's a factor on Mn for the studs that ranges linearly from 0.6 at no studs to 0.75 at full studs.
- There are some tables indication what full studs means for various decks.
- I could find anything allowing one to determine the full stud value themselves.

Maybe there's more info in later editions.

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.