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

• To: seaint-seaosc(--nospam--at)mail-list.com
• Subject: Re: Cb for Channels Bent About Major Axis (AISC 360-10 Section F2)
• Date: Fri, 19 Jul 2013 11:26:32 -0600
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```Hello Bill,

I will attempt to help.

In the AISC 9th Ed of the Manual of Steel Construction (ASD)
their major axis.  It says to use the following eq. for allowable
stress:

Fb = (12 x 10^3 x Cb)/(Ld/Af)  <= 0.6Fy

Where Cb is equal to 1.75 + 1.05(M1/M2) +0.3(M1/M2)^2
below this equation is a paragraph that mentions reverse curvature
bending and what affect that will have on the sign of the M1/M2 ratio.

In Design of Steel Structures by Gaylord, Gaylord and Stallmeyer
they've got a more developed equation for when the moment diagram
within the unbraced length isn't a straight line.  It looks like this:

Cb = (12.5Mmax) / (2.5Mmax + 3Ma + 4Mb + 3Mc)

where Mmax = absolute value of maximum moment in unbraced length
Ma = absolute value of moment at one-quarter point
Mb = absolute value of moment at midpoint
Mc = absolute value of moment at thee-quarter point

I hope that helps.

Take Care,
Lloyd

On 18 Jul 2013 at 18:14, Bill Polhemus wrote:

> Can anyone point out to me the equation or method for computing Cb of
> a Channel bent about the major axis, where there is "reverse curvature
> bending"?
>
> The General Provisions (Section F1) point you to the Commentary, but
> the Commentary seems only to address "singly symmetric I-shapes."
> Perhaps I'm missing something, but I cannot find anything that
>
> Yes, I know I can use Cb = 1.0 conservatively - and likely will - but
> it bothers me that I cannot find anything in the Spec that addresses
> this clearly.
>
> Thanks.
>
> William L. Polhemus, Jr., P.E.
> Sent via Alpine 2.03 Email Client / Fedora Gnu Linux Release 18
> Because I AM that most bizarre of combinations, a THROWBACK-GEEK!

neers.com>
>

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