Composite Slab Diaphragm Design

[abusementpark]

For a fully composite steel floor system, how do you typically determine diaphragm capacity?

SDI publishes composite deck diaphragm design values based on welded metal deck connections and an unspecified amount of lightweight or normalweight concrete above the deck. This is a little questionable to me. Shouldn't the thickness of the concrete be a parameter? As well as the fact that there are shear studs present? I get the impression that the methodology here relies solely on the contribution from the metal deck.

Is there any reason why you can't simply argue that you have a concrete diaphragm based on the thickness of concrete above the flutes and design per the ACI shear provisions. That seems a lot more realistic.

Thoughts??

 

[Lion06]

SDI has a diaphragm manual thta has equations that includes the thickness of the concrete. There are two components to the diaphragm capacity - the metal deck and the concrete. The floor framing really just helps to provide stability to the diaphragm for what would be buckling. It doesn't add capacity to the diaphragm.

 

[abusementpark]

OK, reviewing the tables I was looking at, I can see that they do stipulate a min. concrete thickness of 2-1/2". However, another thing I don't understand is why the capacity changes based on whether the concrete is normalweight or lightweight. Shouldn't the 28-day compressive strength be the more critical parameter?

Also, I still don't understand why they say the support fasteners are 5/8" puddle welds (see attached table)? Wouldn't the shear studs be the support fasteners?

 

[ToadJones]

Is this deck table for composite decking of the deformed type?

 

[abusementpark]

Do you mean deformed as in ribbed with flutes? If so, yes.

 

[ToadJones]

No, I mean is the decking embossed....in other words, the decking + concrete can be a "composite" and the decking + floor framing + concrete can be "composite".

Unless I am mistaken, you do not need shear studs to make a "composite" slab only.

 

[abusementpark]

See the attachment from the Verco Floor Deck catalog, where they give you diaphragm capacity based on the contribution from the shear studs, instead of the welds only capacity that Vulcraft gives you. Is there any reason why these values can be used for a composite floor system?

 

[Lion06]

They say support fasteners are 5/8" puddle welds because that's what is used to determine the metal deck contribution to the diaphragm capacity. The shear studs don't transfer load between the metal deck panels (at least any contribution they might provide is not counted on)

 

[WillisV]

The tables as you noted are based on 2.5" of concrete above the flutes (which is typical of most manufacturer's tables). In general, the concrete contribution is much greater than the contribution of the metal deck below.

If the values for 2.5" work, then you are done, otherwise you need to calculate the diaphragm value for the actual thickness using the diaphragm design guide (3rd edition found here:

http://www.sdi.org/manuals.htm).

Actually I have found that Hilti DF diaphragm program is an excellent tool for calculating these capacities (they let you use puddle welds vs. hilti products for comparison:

http://www.us.hilti.com/holus/page/...?lang=en&nodeId=-114925&selectedOid=-116114).

Of course it changes depending on the strength of the concrete - take a look in ACI - there is a LW reduction factor for shear in reinforced concrete as well.