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Re: Weak axis bending of cold-formed steel members

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Bob,
 
Are the compression portions of your hat stiffened?  i.e. is it a U-shaped section or do the points on the U have lips to stiffen it?  Without being stiffened, I would think that LTB could possibly be a concern, but local buckling of the compression elements would be more of a problem.
 
Check out this free downloadable program for finite strip analysis by Ben Schafer at Johns Hopkins http://www.ce.jhu.edu/bschafer/.  I used it once for column loading on a light-gage U-shaped member to back up some hand calcs.  It took maybe an hour or two to figure out the program, but it gave me some great results that I was able to back up and feel very comfortable with.
 
Jim Wilson
wilsonengineers(--nospam--at)yahoo.com
Stroudsburg, PA

Bob Riley <bob(--nospam--at)m-me.com> wrote:

List,

I was wondering if anyone could point me in the right direction concerning lateral-torsional buckling and weak axis bending for light gage members.  My case is for weak axis bending of a hat section, but I was also wondering about the general case.  I have the three AISI 2001 manuals and I believe I understand the different strengths that need to be calculated, however, the specification manual section C3.1.2 is for ?I-, Z-, C- and other symmetric section flexural members? and not for ?U- sections?; however there is a Supplementary Part V Section 2 which does include ?U-sections?.  It further states that the provisions of this section do not apply to laterally un-braced compression flanges of otherwise laterally stable sections.  I can?t find anything that specifically states that you do not need to check this limit state, but I vaguely remember from my steel design courses (maybe incorrectly) that this does not occur for weak axis bending in general. 

 

My questions are:

 

1)  Is the limit state for Lateral-Torsional Buckling limited to major axis bending as it is in the AISC LRFD Manual section F1.2?

 

2)  Is weak axis bending considered a laterally stable section and therefore you don?t have lateral-torsional buckling?  For example, a C-section in weak axis bending would not need to use the procedures of C3.1.2 or C3.1.3 (One Flange through-fastened to Deck or Sheathing).

 

3)  The Supplementary Part V Section 2 concerning hat sections has exclusions for 1) roof panels and 2) for a system comprised of flexural members and panels.  Does the 2nd exclusion occur only if you have more than 1 hat and therefore you have a system?

 

Thanks,

Bob

 

Bob Riley, P.E.

Mesiti-Miller Engineering, Inc.
224 Walnut Avenue, Suite B
Santa Cruz, CA  95060
voice: 831.426.3186 x105

fax: 831.426.6607

www.m-me.com

 

 

 


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