Roof Deck 2

[JD2]

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.

 

[Ron]

Those are not "straps" welded to the open web joists. Those are the bottoms of "bulb tees". The system you describe is a gypsum roof deck using bulb tees, formboard and cast-in-place gypsum material. The gypsum has a welded wire fabric, slightly better than "chicken wire" to keep shrinkage cracks in the gypsum relatively tight.

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.

 

[rowingengineer]

JD2,
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?

 

[Ron]

RowingEngineer...they are the bottoms of bulb tees.

 

[rowingengineer]

Ron,
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.

 

[dik]

Just a caution... there may have been some significant changes to design loads and codes since the original construction.

 

[rowingengineer]

Good point, Technically which do you have to comply with, if it was in my neighbourhood it would only have to meet the original design requirements, and not current codes.

When in doubt, just take the next small step.

 

[Ron]

It depends on the amount (% of building) and cost (% of replacement cost)in many juridictions. For instance in one code, if 25% or more of a particular feature is replaced, it must meet current code.

 

[JD2]

Ron, Thank you very much for the information. Once I had the term (name) "bulb tee", I searched within the Structural Forum and was able to find several other discussions concerning the same system.

Thanks to all others who responded.

 

[dik]

In many jurisdictions, it depends on the extent of the renovation or if there's a change in use or occupancy. You can talk to the local building authority to determine if this is an issue.

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.