Insulrock Roof Deck

[bouk715]

We're in the preliminary stages (schematic design) of a building renovation. I have existing drawings for the building dated 1964. The architect is proposing to put large roof-top units on the structure surrounded by a screen. The screen area is large enough that it will increase wind loads into the existing roof structure by more than 10% meaning I will need to review the lateral load resisting system per the 2012 IEBC.

The plans state that the roof deck system is "2-1/2 inch Insulrock on #158 bulb tees @ 32 inches o.c.". I was able to find an Insulrock catalog online from 1970 that provides some vertical load-carrying capacity information. However, I have been unable to find any allowable diaphragm shear information. Can anyone point me in the right direction for that info? Or is the diaphragm capacity relatively low and I should be looking at retrofit options from the get-go?

Also, I'm a bit unfamiliar with the system. How are the bulb tees connected to the roof joists typically (weld, bolt, screw, etc.)? I also did not see any evidence of water staining on the ceiling below the roof structure (doesn't mean something hasn't been fixed at some point); would water damage significantly diminish the diaphragm capacity of the system?

 

[KootK]

One of our members provides this document: Link. It may be the catalogue that you mentioned. It does show some grainy bulb tee connection details though.

I suspect that you're not finding diaphragm values because the product was in use before our profession decided that it was prudent to start seriously worrying about diaphragms. Based on the nature of the product, you might be able to justify treating the individual panels like gypsum shear wall sheathing. You'll need to get a grip on the capacities of the grouted connections between panels and the panel connections to collectors however. Odds are good that there'll be some holes there.

Once you start down the diaphragm retrofit path, it's hard to find a good place to stop. I feel for you (and whomever is footing the bill).





The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[bouk715]

Thanks KootK. The link you posted is where I downloaded the design manual I was referring to.

I did find a thread that seems to discuss the Insulrock system and provides a diaphragm shear of 500-1200 plf. But that sounds like a system where the boards are used as forms and they gypsum concrete is placed over it. I don't believe that is the same system I have.

Thread:

http://www.euken.com/group/seaoc/mailarchive/2005a/msg00035.html

I was afraid that there might not be any data available. At this stage I need to present some options, so my initial thoughts are:

* Remove all the existing deck and install metal decking for the diaphragm shear. The existing lateral load-resisting system (wall x-bracing) would then need to be analyzed for the additional forces. Pretty extreme, to be sure.

* Raise the screen and RTU framing above the roof level to avoid drift, then extend columns down through the building to take the load directly to the foundation. The screen surrounds all 4 sides of the RTU "garden", creating a steel tower. The tower would need some cross-bracing or moment connections of its own and kept independent of the building. Then no additional lateral loads are being transferred to the existing building. Building height is 36'-0" (3 stories). Still pretty extreme, but perhaps less so.

* Put the mechanical equipment on the ground. Much simpler structurally obviously, but I'm not sure what that does to the systems.

Thanks for the feedback KootK. I'm curious to see if anyone else has additional information.

 

[KootK]

Would the roof framing suit a horizontal bracing system beneath the deck? Steel straps perhaps?

The greatest trick that bond stress ever pulled was convincing the world it didn't exist.

 

[bouk715]

Thanks again KootK. Roof framing is bar joists spanning to steel girders spanning to wide-flange columns. So steel straps may be an alternative depending on the diaphragm loads.