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RE: Tube shape LRFD design

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Lateral-torsional buckling (LTB) is a phenomenon of importance for
W-shapes and similar members of open cross-section because there is
comparatively low torsional resistance. In an HSS, however, there is
comparatively high torsional resistance and the warping component is
eliminated almost entirely.

In truth, there is a small reduction for LTB in a rectangular HSS bent
about the strong axis. However, it is noted in the Commentary to the
AISC HSS Specification that the reduction in bending strength is less
than 7 percent for a typical span up to 40 ft on an HSS 20x4, which is
about as bad a strong-to-weak ratio as you could get for an HSS. In view
of this and the likelihood that serviceability limit states will control
beam design anyway, it was decided that LTB need not be a design
consideration as a matter of practicality.

So yes, I think it is acceptable practice to not reduce for LTB in HSS
beams. Local buckling effects may get you below Mp, though.

Charlie

-----Original Message-----
From: richard lewis [mailto:rlewistx(--nospam--at)juno.com] 
Sent: Thursday, May 06, 2004 2:14 PM
To: seaint(--nospam--at)seaint.org
Subject: Tube shape LRFD design

I am designing a HSS18x6x5/16 beam with combined bending and torsion,
simple span.  It spans 30 ft and is unbraced the full length.  I am
looking at AISC Specification for the Design of Steel Hollow Structural
Sections, Sect. 5.1 (pg. 7).  b/t ratio is less than lambda(p) for the
strong axis and greater than lambda(r) for the weak axis.  I have
reduced
the section modulus of the weak section as detailed in the
specification.
 

At the bottom of the page it has a paragraph dealing with unbraced
length. It states Lb (unbraced length) is not limited for HSS shapes.
designed by elastic analysis and is limited by some equations for
plastic
analysis.  Since I have a simple span beam I have an elastic analysis. 
Am I reading this right, there is not unbraced length limit for this HSS
shape?  This shape is designed for Mp in the strong axis and Mn in the
weak axis?

Thanks.

Rich

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