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RE: ASD Weak Axis Bending of Rolled Channel

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The phi*Mp (LRFD) value that Curt had mentioned earlier is fairly equivalent
to 0.75*Fy, as far as idea anyway... phi*Mp/(phi* S.F.) with S.F. = 1.67
(typ. for ASD) = 0.9*Fy (same failure mode as assumed with 0.75*Fy,
yielding).  Based upon that which is fairly well stated in LRFD with
something to the fact of members bent about their weak-axis don't need to
consider lateral torsional buckling...

I have seen Fb=0.75*Fy used for weak-axis bending of channels before.  I
typically would use F2-2, Fb=0.6*Fy based upon its statement ..."For members
not meetin the requirements of compact sections of Sect. B5 and not covered
in Sect. F3 bent about their minor axis..." and the fact that weak-axis
bending of channels is not specifically mentioned as you have stated
previously.

However, it wouldn't take much to convince me that Fb=0.75*Fy would be
appropriate.  The flanges are usually pretty stocky for a channel and don't
seem to have much of a problem with local buckling.

I guess based upon the LRFD (3rd Edition) approach, I would have no problem
in using 0.75*Fy for the allowable weak-axis bending stress on channels.

Hope that helps,
Greg Effland, P.E.
KC MO

-----Original Message-----
From: Les Rhinehart [mailto:l_n_rhinehart(--nospam--at)email.com]
Sent: Monday, June 25, 2001 5:12 PM
To: seaint(--nospam--at)seaint.org
Subject: ASD Weak Axis Bending of Rolled Channel


Does anyone knows if Fb=0.75Fy applies to a rolled channel ( C5x6.7 for
example) bent about the weak axis?  AISC ASD F2.1 does not mention channels
as I read it.  RISA-3D uses F2.1 as applying to a channel.  Can anyone shed
some light on this.  I would appreciate any help.

Thanks,

Les Rhinehart
VITOK Engineers, Inc.
Louisville, KY


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