Existing Poured Gypcrete Roof Deck

[Rweave]

I am working on this building from around 1960 and it has an existing poured gypcrete roof deck system that spans to bulb tees that span to the steel structure. The client wants to close a bunch of existing roof penerations that had existing duck work with a steel plate bearing on the existing deck. Does anyone know what the allowable bearing capacity would be that could act on the existing roof deck? My concern is they want to close like 4 foot square openings with a steel plate. The loads acting on the plate are dead load, snow and some areas of high snow drift. I just don't want the plate crushing the existing roof deck????

 

[JAE]

I would try to run some steel angles across the sides of the openings - welding them to any existing roof joists or members. This would shore up the edges of the openings and allow you to place a new metal deck infill (or plate if it works) over the angles as well.

I'm always suspicious of these old gypsum decks - especially near edges of old openings as those are areas that typically leak - so the gypsum could be mush.



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[wannabeSE]

Do you plan to have the plate over a bulb tee or other structural member so the gypsum is only in compression? Take a look at your loads. Assume you have a big drift and 180 psf total load on a 4' span. The reactions are 360 plf or 30 pounds per inch. Assume you have 3" of bearing. Then, the compressive force is 10 psi.

Typical gyp board has 300-400 psi compressive strength. Or look at the specification at the end of this document: GA-300-73_Gyp_Roof_Decks.pdf. It list the compressive strength as 500 or 1000 psi.

I have only seen a few of these installation (exposed in mechanical penthouses) and they looked good without any signs of water damage. If there is water damage, more may need to be done than fill holes.

 

[Ron]

Agree with JAE...frame all penetrations. My rule of thumb is anything 12" or over needs framing.

Gyp deck on bulb tees is a special character. See if you can find some of the older literature on it. Some used gypsum formboard and some used fiberglass formboard.

 

[Rweave]

Thank you everyone for the quick replies. There currently is supplemental angle framing that is picking up the edge of openings that they want to just leave in place. So my only concern really was the bearing on that existing poured gyp deck. WannabeSE thank you for the pdf for the gyp deck I will have to add that to the library.