Metal Deck Diaphragm

[bpstruct]

If I spec a connection pattern for a metal roof deck, what does that mean for the connection between deck and framing parallel to deck flutes. The pattern covers the connection to perpendicular framing, but not sure how to handle parallel framing. Thinking that I need to spec that separately. I've seen someone else specify a pattern in that direction (36/5). Seems like you would just specify a spacing with framing parallel to deck flutes as the pattern (at least as I understand it) wouldn't make sense in that direction.

 

[steellion]

Roof deck spans in one direction. The connection pattern (e.g. 36/5) refers to the connection of the deck to the structural members below. In the direction of deck span perpendicular to the structural members, a quantity of sidelap fasteners between the structural support members is specified. The quantity of sidelaps required is based on the what is needed for the diaphragm to meet shear strength criteria.

 

[bpstruct]

Not sure I asked this the right way. I understand that the deck spans only one direction, but the framing does not. I have framing in both directions. The 36/5 makes sense to me for framing perpendicular to the flutes - for a 36" wide panel, there are 5 connections per framing member. The vulcraft guide shows the pattern for this connection. It does not (at least not that I could find) say anything about the framing parallel to the flutes.

 

[Gumpmaster]

I personally call out both connections to perpendicular framing and connections to parallel framing. Sidelaps are called out separately from these two.

 

[BadgerPE]

Parallel to the flutes is the strong direction of the panel for diaphragm design. I believe Vulcraft states that to achieve published in-plane (with flutes) shear values that the parallel connection to framing is to meet the sidelap spacing value. I couldn't find this exact statement (it may have been an email w/Vulcraft) but I have attached the diaphragm design guidelines from their manual.

 

[bpstruct]

Thanks to all!

 

[XR250]

It is generally the same spacing as the sidelaps

 

[SteelPE]

Is there a specific reason why you are asking the question? Being a little more specific will get you better results.

I use a bunch of joist and deck framing. The members that frame parallel with the deck flutes would be the supporting girders. In this instance the deck does not attach to the girders as the deck is 2.5" above the girders. The girders are braced by the joists that frame perpendicular to them. In-between the joists I use side lap fasteners in accordance with the diaphragm capacities..... usually with a minimum of 2.

 

[bpstruct]

I asked because I need to know how the diaphragm shear is transferred into the framing - it is a braced frame. For your example (joists on joist girders) how would the diaphragm shear be transferred to the JG if it was a moment frame? Would you be relying on the connection between the joist and JG?

Thanks.

 

[XR250]

@SteelPE
If the diaphragm design REQUIRES sidelaps, then you need the same number of connections at the girder to transfer the shear out. You should be installing 2 1/2" tube steel or similar between the joists at their bearing in order to accomplish this.

 

[structSU10]

I suggest the publication by vulcraft called 'Designing with Vulcraft - Steel Joists, Joist Girders and Steel Deck'. It has a part that discusses diaphragm transfer using the "rollover" capacity of a joist seat. Elastically, the seats calc out to a few hundred pounds or so if rollover resistance, but if you go to plastic they are good for thousands of pounds, with deformations of 1/4" plus. They can be relied upon, but sometimes seperate collectors in between joists for the diaphragm shear can be used.

 

[SteelPE]

Excel,

I was taught to follow the publication that structSU10 refers to. If my capacity is greater than the rollover capacity of the typical seat then I will install the 2-1/2" tubes you talk about. It typically happens on very large and heavy building (masonry bearing). You also have the ability to specify the roll over capacity on your drawings and have the joist manufacturer design the seat for this capacity. Beware though, I have seen hundreds of buildings designed by many different companies, and I have maybe seen the rollover capacity specified once.

bpstruct,

If it is a brace frame or a moment frame it doesn't matter. Shear in the diaphragm still needs to come out and into the LFRS (lateral force resisting system). Usually moment frames are longer (take up more bays of the building) than brace frames (which may only take up a few). In either case, if you have a building that is 400' long and you only utilize 100' for the LFRS, you will end up with collector elements that take the shear load out of the diaphragm and drive it into the LFRS. These elements (which can be beams or joist girders) need to account for the axial load from the diaphragm. If you design the system using joist girders then you need to tell the joist girder manufacturer the load the girder needs to resists...... you also need to make accommodations in the design of the connections to accommodate this load as well.

 

[XR250]

@SteelPE;

You have to have the same number of connections at the girder as the sidelaps. Otherwise you are only counting on one screw at at the end of the bar joist to transfer out the shear. Do the math, it does not generally work out. If you use an edge angle instead of tube steel as the shear collector over the braced girder, then rollover usually needs to be considered.

See attached...