PEMB Zee and Cee Purlins

[NedHead]

I'm trying to wrap my head around the design of light gauge Zee and Cee purlins in pre-engineered metal building design. I have the metal building systems book by Alexander Newman as well as the AISI manual and spec (newest edition of spec and the 2008 edition of the manual). Here's the set-up: fairly typical PEMB style building, 4:12 gable roof, 50'x90' long, steel frames at ~23' O.C. with Zee purlins spanning between frames. Standing seam insulated metal panel for the roofing, so I believe that means no diaphragm action (at least none you could really put numbers to).

With the sloping of the roof, you end up with weak axis bending in all the purlins, which normally would be taken out by diaphragm action, but in this case needs to be taken out to the steel frames through the weak axis alone. Even with really solid bracing between purlins (cold form channels at a reasonable spacing) you'd still be loading the purlins in the weak axis. As such I can't seem to get reasonable purlins to calc out well. I've seen 12" zee purlins spec'd at 20+' lengths and 4' spacing with higher snow loads than I'm currently working with on plenty of PEMB drawings, how do they do it? Are their systems using some diaphragm action they've arrived at through testing? Or diagonal metal strapping across the bays to take the load back the main steel frames (not a detail I've seen on any of the PEMB shops I have). Or is this just ignored when the roof slopes are low-ish

 

[jhnblgr]

Wouldn’t the pre insulated panel act as a diaphragm

 

[wayne_dwops]

Not really an answer but a comment. A lot of smaller metal buildings are not designed or certified by engineers, just "building designers" for companies like Mitek or others with proprietary metal building software. These structures get pawned off to the lowest bidder in such a way that all liability related to design criteria is removed from said designer. So, if you are wondering about how a metal building works, the answer is it probably doesn't.

That being said, rationally approaching the structure means that the hundreds of fasteners in the tin can 29 gauge diaphragm in conjunction with weak axis bending of the purlins must be the only way that load parallel to the frames get transferred from the roof , through the clips on the frame, into the frame and then through the moment frame to the (hopefully) engineered foundation.

When I design metal buildings I use a metal roof deck with actual capacities and analyze the connections from the deck to the frame. Of course, this is probably why I don't have a lot of metal building clients.