GWB (sheetrock) as horizontal diaphragm on cold-formed steel joists?

[Geoffre14]

Can anyone provide a reference that shows the allowable horizontal floor diaphragm capacity of 1/2" or 5/8" Gypsum Wallboard (GWB) as a floor diaphragm on light-gauge cold-formed metal joists?
[ul]
[li]We found the IBC table which lists GWB diaphragm on wood joists (70plf).[/li]
[li]We also found the AISI tables listing GWB as vertical shear wall capacities.[/li]
[li]We have separately found similar cementitious products like backerboard or structural panels which list horizontal diaphragm capacities.[/li]
[/ul]

But we can't find anything for regular GWB on metal joists. FYI This is for a redundant ceiling below a shallow attic crawl space.

Would appreciate any help you maybe can provide.

 

[jayrod12]

I guess my question would be, is this a good idea? What do you want to consider it a diaphragm for.

I've been a part of many renovations where the GWB ceiling is removed in full, and a dropped acoustic tile ceiling installed.

 

[JAE]

Can you use fire treated plywood instead?

 

[Geoffre14]

jayrod12: The ceiling is just a decorative element but must be braced seismically (and documented as such). It is not hung: it is framed with a secondary layer of joists. Therefore instead of wire bracing, it felt like the sheetrock should be sufficient for the very small 40plf diaphragm shear demand.

JAE: possibly but I imagine in this instance, wire bracing would be cheaper. Nonetheless we had hoped to be able to document the GWB would be sufficient as is. But we are having trouble finding adequate document listing GWB on CFS joist diaphragm capacity.

 

[phamENG]

For seismic, I don't believe GWB is permissible as a diaphragm. It's tenuous enough for wind loading (one leak and your done), but apply cyclic loading and the gypsum turns to powder if it's the stiffest thing in the building. If it's only self weight, what about looking at individual joists? 1.33"o/c * 5psf = 6.65plf applied to the joist in the weak axis. I would imagine it could take it without acting as a diaphragm. It only needs to be a diaphragm if it's taking the seismic loads from other elements and directing them to the shear walls.

 

[Geoffre14]

phamENG: thanks. We also went with that same approach of weak-axis bending.