Roof Deck
Roof Deck
(OP)
I have a remodel job (one story building approximately 16,000 sf, original drawings are dated 1971) where the drawings indicate a metal roof deck with a lightweight fill. The Architect is going to locate approximately 9 relatively large skylights as part of the remodel. My concern was the reduction in allowable roof diaphragm capacity. When I went out to the job site for an initial visit, I found the roof deck to be a 2" thick orange fibered board topped with approximately 2-1/2" of a light weight gypsum material topped with a built-up gravel roof. Metal straps were welded to the top chord of the joists at 24"o.c. similar to top chord bridging (I assume this provides a brace point for the top chord of the joist and also helps to support the fiber board - see attached photograph), but I couldn't tell how, or if, the metal straps were attached to the fibered board. I was wondering if anyone had ever used or analyzed a similar system, how it is attached to the metal straps and where I might be able to locate some representative values for diaphragm shear to attempt an analysis. My alternative was to place in plane x-bracing wherever a skylight hole was cut to transfer the amount of force that the cut out area would have resisted. Any ideas or thoughts would be appreciated.






RE: Roof Deck
If you are concerned about diaphragm action, the bulb tees welded to the joists are good for that. They look like small crane rails or railroad rails. and are quite capable of carrying significant load.
This system was common for the era.
The way the system is installed is this:
1. The bulb tees are welded to the joists, perpendicular at about 24" spacing
2. The form board in placed in from the top. It will pass the "bulb" and stop on the bottom flange of the bulb tees.
3. Then the "chicken wire" is draped across the bulb tees, then the gypsum is poured and allowed to cure.
4. A built-up roof system was then installed on top of the deck.
RE: Roof Deck
There are many publications like: "Manual of Stress Skin Diaphragm Design" that would give guidance on how to analysis this type of structure.
My question is, are you sure they are metal strips that we can see in the photo and not beam flanges?
RE: Roof Deck
RE: Roof Deck
We both must have been writing at the same time, and you beat me to the punch. bulb tees sounds right, and for what it is worth I agree with all of Ron's comments.
RE: Roof Deck
RE: Roof Deck
When in doubt, just take the next small step.
RE: Roof Deck
RE: Roof Deck
Thanks to all others who responded.
RE: Roof Deck
In addition, for diligence, the decision not to bring the work up to current code, even if it's not required should come from others and act on their instructions. Just to CYA.