Suspending ceiling from untopped roof deck

[jdgengineer]

We are working on a small commercial TI for a building that was constructed in the mid 60s (I think). The existing roof framing is W12x steel beams at 7'-0" oc with 1 1/2" roof deck with a 1 1/2" rigid insulation above (no concrete topping). I don't know the specifics of the exact deck but assuming it is a 22 gauge 1 1/2" roof deck at a 7'-0" span a current Verco catalog would give a 45 PSF single span rating (L/180), or 77 PSF double span rating (stress).

As part of the project the architect is looking at putting in 2 skylights which we will provide new channels framing between the existing beams for support. Additionally, they are looking at putting in a suspended gypsum ceiling (US Gypsum or similar suspended product. It is similar to an accoustical ceiling grid with wire ties @ 4'-0" oc max each direction). Each wire tie would have approximately 40# of load on it (2.5 PSF).

Would it be typical to try and hang this directly from the metal deck (assuming it has the capacity)? I'm not thrilled about using a TEK screw in withdrawal for the condition. Would there be other alternatives (some sort of toggle bolt?) Or would it be more common to frame out the supports with light gauge steel framing?

 

[XR250]

I think they make grid that can span 7 ft.

 

[wannabeSE]

Consider using screws in the side of the flute. This way the screw is mostly in shear.

 

[Guastavino]

I would say it is typical. Doesn't make it right, but I probably wouldn't sweat that too much based on the PSF loads you show. I don't think I would sweat the withdrawal load from a Tek screw if it was installed correctly (whether vertical or into side of flute). I do think there may be some proprietary connectors too.

Remember, there is a lot of redundancy in AT ceilings

 

[MrHershey]

Fee like I see this all the time.

I probably wouldn't be concerned much for the deck. If it can handle the 2.5 psf extra load then I wouldn't worry about the concentrated. Vulcraft seems like they may agree, in their 'Designing with Vulcraft' publication they note that loads with regular connections of less than 150# each could be considered to be 'distributed' (page 12). To be fair, they aren't 100% clear in stating this is for decks as the document also covers joists and joist girders. But 40# is so light that I wouldn't really be worried about it as a concentrated load. Keep in mind this deck is probably being walked on by workers during construction and maintenance workers, all of whom weight a lot more than 40#.

 

[hawkaz]

Suspended ceilings (with T grid) are usually supported at 4' o.c. each way. Many owners don't like people attaching to deck directly since there is a risk of damaging the roof membrane and creating leaks.
I would suggest running unistrut between the beams and hanging from that.

 

[XR250]

Span table attached...