Cb for Roof in Uplift 1

[abusementpark]

I have read in various places that you can use a Cb = 2.0 for a typical roof beam in uplift that is braced on the top flange by a roof deck. I believe it is based on work by Yura. In the places that I read/heard about this, it was presented such that structural engineers can use this value for Cb in actual design.

However, I have never seen this mentioned any type of governing document. Basically, I need something I can easily reference to justify my use if ever questioned. Even if I find the technical paper where this was first presented, this doesn't mean that the work is officially accepted by the governing code.

Thoughts???

 

[Lion06]

For the use of Cb in your case, I would use equations that account for the location of the load application. For wind uplift, even though the load is applied to the top flange, you can assume it is applied to the bottom flange of a gravity loaded beam (basically just loaded on the tension flange). This creates a stabilizing force. The AISC equation assumes the load is applied at midheight of the beam and is, therefore, unconservative in some cases. Even though AISC doesn't explicitly recognize this equation, I would feel comfortable using it (and being able to justify its use). The one I am referencing is in "Structural Stability of Steel" by Galambos and Surovek. The edition I have is copyrighted in 2008.

 

[271828]

The Yura/Helwig bracing seminar notes include an equation for Cb that applies to this situation. It's on Page 1-15 of their 2006 notes. The basic idea is that the tension flange can't translate at all, and Cb can far exceed 2.0. In their example on Page 1-16, Cb=5.67.

 

[Lion06]

Does anyone know where I can get my hands on a copy of those Yura notes?

 

[271828]

I don't know if AISC will send them nowadays. The best bet is to just ask around. Every office I've ever worked in had a copy of these because the seminars have been given so many times over the years.

Honestly, overall, I don't think they're all that useful. I've tried over the years to get a firm grasp of stability bracing using these notes and am still unable to really apply App. 6. I've been a steel and stability guy for some time, so I have to think that most folks are in the same boat. There are some good nuggets of info in there, like this Cb business, though.

 

[abusementpark]

Well, I think the notes would be worth having just for the Cb stuff. I have sized many a heavy roof beam due to it being unbraced on the bottom flange. Being able to use a higher Cb would help out a lot with the economy of those sections.

Unfortunately, I don't know anyone with these notes.

 

[slickdeals]

Does anyone have these notes to share? I am sure many of us would benefit from them. I have tried in vain with AISC.

 

[271828]

Might contact Helwig. He's very helpful and a nice guy. He might send a copy or perhaps he sells them at a nominal charge.