Old Gypsum Roof Board

[jimluckn]

I'm trying to determine load capacity for the roof of an indutrial builing that's sheathed with 2" thick gypsum board (no metal or wood decking - just this 2" + 1/2" gyp board ceiling on steel joists). It does not appear to be of the Densdeck or Securerock variety - there's no visible reinforcing fibers in the gypsum, the paper is not visibly reinforced. There is a layer of thin wire mesh (like chicken wire) about 1 1/4" down. The building could be 30 years old. Anybody seen this stuff &/or know how to evaluate it for point loads?

 

[structuresguy]

Lucky (?) coincidence. I am dealing with exactly the same issue right now as well. Doing a building assessment to see if we can bring a 45 year old building up to current code. And it has the 1/2" gyp board + 2" gypsum roof over metal joists. If you find any good info, please post it in here, and I'll do the same. Thanks.

 

[jimluckn]

Lucky! So far, i've come up totaly empty-handed. If i find anything, i'll certainly let you know.

 

[Ron]

What you have described is a "poured-in-place" gypsum deck. It has a gypsum form board on the bottom, sitting on the flange of a bulb tee. "Chicken wire" is draped over the bulb of the bulb tee, then the gypsum concrete is poured over the top. What you see from the bottom appears to be like a drywall material. Every two feet or so, you should see the bottom flange of the bulb tee running perpendicular to the open web joists.

This was a common deck type in the 1950's, 1960's and even into the 1970's, for commercial and industrial applications.

A variation on this that came about in the late '60's, early 70's was the use of fiberglass insulation formboard on the bottom, instead of the gyp board.

Design info is hard to come by. Generally the gypsum material had a compressive strength ranging from about 500 to 1500 psi. The spans are short (2 feet), so distributed loads were not a big deal. Point loads should be checked as punching shear, since bending is likely not going to prevail.

 

[structuresguy]

Mine is exactly as Ron described it. I did find this PDF file, at the National Roof Deck Contractors Association website. No design info, but definitely describes the system and how to repair/replace it.

http://nrdca.org/webfiles/NRDCA 500.pdf

http://nrdca.org/webfiles/GypsumRepairPictures.pdf

Here is an article that would be nice to get, but I can't find it anywhere online:

http://cedb.asce.org/cgi/

http://WWWdisplay.cgi?14415

 

[jimluckn]

Thanks, Ron, i am looking at the same thing. I had searched under various terms i thought described it, but "gypsum roof deck" opened the whole thing up. I got the folowing link to an old handbook from one of several previous threads on this site that addressed these decks:

http://files.engineering.com/getfil...76c692e394d&file=GA-300-73_Gyp_Roof_Decks.pdf

A lot of good info in those thresads!

My joists are 6' oc with the builb tees about 3' oc. At this point i am feeling pretty leery about adding significant load to this stuff & will figure out a solution that loads directy above the joists.

 

[Ron]

Guys...keep in mind that those bulb tees are fairly robust structural members. For various equipment supports, we have tied back into those with good success.

 

[Momgineer]

I'm am adding a new opening for an RTU duct on an existing commercial building that I just found out have this old gypsum roof deck. My opening is more than 12" and I know that I have to add angles to support the new openings. My question is this:

How are the new angle members connected to the roof? I know that there the the bulb tees, but what about the opening perpendicular to that?

 

[Ron]

Your largest opening bounded by steel is currently the bulb tee spacing, usually 24 inches, by the open web joist spacing, usually 4 to 6 feet. Let's say you have need of an 18 inch square opening in there.

Span the bulb tees with angle sufficient to carry the load spaced 18 inches apart. So you have two parallel angles, spanning 24 inches with an 18 inch space between them. Now span two more angles perpendicular to the first two, spanning the 18 inches between the first two angles, and spaced 18 inches apart. You now have an 18 inch square frame supported by the bulb tees.

 

[StructuralJoe]

Does anyone have any design info for how resistant this product is to wind uplift?

My project requires a change of occupancy which changes the Importance Value from 1.0 to 1.15.

My major concern is how can I calculate the capacity of the gypsum when it really isn't attached to the "bulb tee"?

What prevents the gyp from being completely pulled free from the bulb tee?

Any ideas for engineering a system to transfer this load?

 

[Ron]

The gypsum encapsulates the "bulb" of the bulb tee. The reinforcing mesh holds it together across the tees. For uplift, this is usually sufficient, even in relatively high wind areas. The issue is usually the attachment of the roof membrane against uplift, not the decking itself.