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Re: Weak axis bending of cold-formed steel members

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Bob,
I am not sure if I have fully understood your specific case but for the general case in light gage members, the lateral-torsional buckling stress will be calculated for singly-symmetric members as follows:
- For bending about the symmetry axis: Eq. C3.1.2.1-5
- For bending about the centroidal axis perpendicular to the axis of symmetry: Eq. C3.1.2.1-6
so it's for both the strong and weak axis bending.

In the special case of a single hat channel in bending where the "brims" are in compression and the unbraced length is greater than Lu, the provisions of Section 2, part V will be used. The reason for the special provision is that in such a case (a single hat) the entire section does not rotate as a unit but the compression flange tends to buckle seperately along with the out-of-plane bendng of the web (ie the hat opens up).

HTH,
Reza Dashti P.Eng
Vancouver, BC


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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