Shear Transfer in Acoustic Deck

[bbartlet]

In a typical metal deck diaphragm, the ends of each deck sheet are overlapped over a supporting member to create deck/diaphragm continuity. When acoustic deck is used, the perforated sheet on the underside of the deck makes it impossible to overlap the deck, so sheets are simply butted together. When the supporting member is not one solid piece, (e.g. - steel bar joist top chord made up of two angles with a gap in between) the two deck sheets are often fastened on opposite sides of this gap. How is the shear transferred across the gap?

Is it safe to assume that the shear is safely transferred from one chord angle, through the welds of the web members to the top chord, and into the other angle and back into the other deck? Or should a thin gage steel plate be placed over the top of the deck flutes to create diaphragm continuity?

We have been discussing this in-office for a while and with another engineer in the area, but haven’t been able to come up with a consensus on how to remedy this, or whether or not we're making a bigger deal of this than it needs.

Any info would be appreciated. Thanks.

 

[MiketheEngineer]

Is the metal deck securely fastened to the supporting members? Is it a small deck - say 12'x12' or less.

If so, not sure I would worry about it.

 

[bbartlet]

I honestly can’t say about the project that it was noticed on. But regardless of any current project, this is a question about future projects as much as any. Acoustic deck is often used over rooms larger than 12’x12’ (e.g. - gymnasiums, choir rooms, etc.) where the deck is also counted on for diaphragm capacity. And in this instance, we're assuming that each deck sheet is securely fastened to the supporting member. However, each sheet is fastened to separate angles on opposite sides of the joist (see attached sketch).

When contacted, the deck manufacturer's response was that the shear force goes through the supporting member. And when the supporting member is a steel beam, this isn’t an issue. But is this an issue when the supporting member is a bar joist with a gap between the two top chord angles? The joist manufacturer's comment was that the welds of the web members to the joist chords are not designed to transfer lateral shear from one chord member to the other.

A conservative approach would be to fasten a steel plate(matching the deck gage) over the butted ends. Or, while the joist welds aren’t specifically designed for the loads, is it safe to assume that they can handle the lateral loads? We’re in the upper midwest where wind controls in 99% of our projects and a typical diaphragm force is roughly 250-500 PLF.

I’m just curious how others have handled this or if they’ve even come across this issue.

 

[shobroco]

Your acoustic deck is different than any that I have used. In Ontario, acoustic deck is simply regular deck with perforated sides to the deck flutes so the laps and installation are the same as any deck installation, although the diaphragm value is less because of the perforations.

That aside, I would say you are worrying needlessly because most joist top chords are angles which would have far more lateral bending resistance in the length between the web attachments than the decking and puddle welds would be able to apply.