With bracing for steel beam in uplift, is bracing ignored as purlin support for gravity? 1

[thejonster]

I need bracing for my steel beam in uplift and this is bugging me.
I've noticed in PEMB that huge tapered built up plate girders with tiny angles braces to purlins don't seem change the size purlin it's attached to, and the tiny angle doesn't seem like it could carry a big kicker load. In this risa model you can see the connection forces are higher than simply supported loads. It would seem that the load from the purlins could be much higher than the bracing force it's mainly intended for, and that prying force increases the closer the brace is to the end of the beam by the deflecting purlin, possibly amplifying the force the smaller the brace is.

Are the braces usually designed for the bracing force only, how do you guys justify or account for this prying? Is buckling in the brace angle used to justify ignoring this prying?
From what I hear about how PEMB are designed, my guess is that it is ignored since you give the purlin a simply supported load path and call it good. If that's the standard of care I'd like to know your reasoning

I looked for a better picture, but you guys have all seen this tiny 1x1 bracing for huge built up sections

 

[phamENG]

For one thing, those purlins are almost always analyzed as continuous. They lap and nest them and screw them together over the rigid frame, and that provides sufficient continuity. A couple things you can do to test it: include the brace in your model. Not as a support, but the brace itself. That will help account for the axial stiffness of the brace. OR, check the deflection of a continuous purlin at the point the brace connects to it. How much axial deformation would that produce in the brace based on the geometry? What axial load would produce that much deflection? Is it enough to matter?

 

[driftLimiter]

The bracing is provided to brace the moment frame column and beam. It is not typically considered as a support for the purlins, and the purlins are generally designed as continuous across the frames. The AISI d111-09 shows several purlin design examples all of them use CL-CL frame dimensions for supports. I highly recommend reviewing this document and understanding it if your attempting to design CFS Purlins. There are empirical factors (R) used to constrain the problem these account for stability offered by through fastened decking. These members have special design provisions that often result in improved economy.

If you just apply AISI Code directly to it without consider these effects, you will be hard pressed to get RISA code checks to match the metal building manufacturer.