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

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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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