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RE: Channels - Weak-axis bending

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For weak-axis (y-y) bending, LRFD Specification Chapter F provides a design
flexural strength of 0.9FyZy. Lateral-torsional buckling does not apply for
weak-axis applications, since it would take more energy to deform the member
in a lateral torsional mode. Flange and/or web local buckling may control
(in general) for weak-axis bending, but I think all of the channels we list
are compact.

Loosely, 0.9FyZ in LRFD translates to 0.66FyS in ASD. I say loosely because
a true translation from LRFD design strength to ASD working strength would
be 0.9FyZ/1.5 = 0.6FyZ, assuming the effective load factor is 1.5. The value
0.66FyS is approximately equal to 0.6FyZ for the specific case of
strong-axis bending of W-shapes, which have a lower-bound strong-axis shape
factor (Z/S) of around 1.1.

In the end, the ASD Specification doesn't directly address weak-axis bending
of channels, but the weak-axis shape factor for channels is higher. I've
seen 0.75Fy used for weak-axis flexure of channels before. Others use
0.66Fy.

So you can either use LRFD where it's covered or use judgement and decide
for yourself at which stress level you feel comfortable if you use ASD.

Hope this helps.

Charlie



-----Original Message-----
From: Frank Griffin [mailto:GRIFFIFS(--nospam--at)c-b.com]
Sent: Friday, September 24, 1999 11:04 AM
To: seaint(--nospam--at)seaint.org
Subject: Channels - Weak-axis bending


There doesn't seem to be a direct reference to weak-axis bending of channels
in ASD.  One P.E. in my office says that the allowable is 0.75 Fy as
referenced in a "cover-all" statement, but we weren't able to find that
statement.  We also noted that Enercalc uses 0.75 Fy in the weak-axis.  Can
someone guide me to the appropriate section of ASD, LRFD, or any other AISC
reference?  Your help would be greatly appreciated.  I suspect that Mr.
Carter at AISC could be helpful in this area as usual.

Frank Griffin
Ft. Worth, Texas