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# RE: Cb for Channels Bent About Major Axis (AISC 360-10 Section F2)

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• To: seaint-seaosc(--nospam--at)mail-list.com
• Subject: RE: Cb for Channels Bent About Major Axis (AISC 360-10 Section F2)
• From: "Effland, Greg" <Greg.Effland(--nospam--at)BlueScopeBuildingsNA.com>
• Date: Fri, 19 Jul 2013 13:11:12 -0500
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```I don't have most of my references in front of my so speaking off the cuff:

Cb is a modification factor to adjust for non-uniform moment along the
unbraced length of a member.  It is there because of the simplifications in
the Lateral Torsional Buckling (LTB) equations that assumed a uniform
moment along the unbraced length so as to not drive us crazy because of the
math.  The actual response of LTB is a couple of torsion and lateral
bending, i.e. coupled differential equations.  Most of the differences in
handling different cross sections of slenderness types comes from how the
LTB equations are simplified, i.e. does it include J, what terms are
canceled out or insignificant, etc.)

I believe that for you case you would calculate Cb the same as you would
for any other I or C shape.  Now I see you had asked about the different Cb
equations.  Correct that AISC 360-10 has different Cb than the 9th ASD,
and 360-05 had a Rm factor applied to Cb to account for singly symmetric
members subjected to reverse curvature [i.e. same as 360-10 * Rm, where
Rm=0.5+2(Iyc/Iy)^2 ... Iyc = Iy of compression flange, Iy = Iy of section].
I don't know that this is based on physical research as much as it is
looking at the regression data again and redoing the statistics and
reorganizing the equation.  For what it is worth the Cb in 360-10 is not
the latest and greatest research either.  There is newer research, if I
recall correctly by Wong and Driver.  This newer approach is used in the
latest CSA (Canadian) code.  From what I have heard from researchers is
that depending on which test cases you pick, one approach looks better than
the other, i.e. a toss up between which method(s) are better.  Of course
there is also a different variant listed in AISC Design Guide 25, which is
a stress based calculation based on 360-05 (I think it also corresponds to
AASHTO).  So lots of choices on Cb.  Personally I would use the Cb in the
Spec you are using.

If you wanted a second (or third) opinion you could also ask the steel
solutions center for AISC  (<http://www.aisc.org/content.aspx?id=4376>).

Hope this helps,
Greg Effland

-----Original Message-----
From: seaint-seaosc(--nospam--at)mail-list.com [mailto:seaint-seaosc(--nospam--at)mail-list.com] On
Behalf Of Polhemus, William Sent: Friday, July 19, 2013 12:34 PM
To: seaint-seaosc(--nospam--at)mail-list.com
Subject: RE: [SEAINT-SEAOSC] Cb for Channels Bent About Major Axis (AISC
360-10 Section F2)

Lloyd:

Perhaps it does.

However:

The 9th Ed. Was published in 1989. That's 24 years ago.

I don't even know where my copy of that is - in some box somewhere in my
garage, I guess.

There has been a large body of research since then, filling in blanks and
helping us to design steel more efficiently and effectively.

While the equation you reference may still be valid, I suspect it has been
superseded by newer research that has since been incorporated in the AISC
Specification.

That's what I was looking for.

Truncated 1926 characters in the previous message to save energy.

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