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RE: Unbraced Length

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Jeff,

Another case that has always intrigued me, that may be interesting, is the effect of full depth stiffener plates on the effective unbraced length of a simple beam without any physical bracing of the flanges.

During lateral torsional buckling the beam experiences three phenomena:

1.  The beam translates to the side.
2.  The beam rotates about it's center
3.  The beam flanges become un-parallel

I would think that the stiffeners alone would provide some (may be very small) resistance to torsional buckling by keeping the flanges parallel.  Is this 1 percent, 5 percent, etc.  Inquiring minds want to know.

Thomas Hunt, S.E.
ABS Consulting




"Carter, Charlie" <carter(--nospam--at)aisc.org>

12/10/2003 12:54 PM

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RE: Unbraced Length





There are provisions in the 1999 AISC LRFD Specification for torsional bracing in Section C3.4b. They are indended for exactly this case.
 
Charlie
 
 
 
 
 -----Original Message-----
From:
Udall, Jeffrey D [mailto:JDUdall(--nospam--at)babcock.com]
Sent:
Wednesday, December 10, 2003 10:36 AM
To:
seaint(--nospam--at)seaint.org
Subject:
Unbraced Length

I'm presently doing a literature search as part of a university thesis project.  I'm hoping this list can give me some assistance.

I'm trying to determine the critical bending moment of a beam when it is subjected to a uniform load.  The beam is continuously supported laterally and torsionally on the tension (bottom) flange.  The top flange (compression) is not restrained.  Current practice is to consider the unbraced length of the compression flange as the full span of the member.  In order to reduce this span, stiffeners are added at appropriate intervals that essentially tie the compression flange back to the laterally braced tension flange.  

However, these stiffeners are costly in terms of labour, especially when there are large numbers of them and the spans are significant.  I am trying to determine the effective span of the compression flange given that it really is attached to the tension flange through the web. The stiffener connects the two flanges together by treating the stiffener as a column, and is therefore quite stiff. The web serves the same purpose but acts only in bending in the weak axis.  This is not nearly as effective, but it does provide some degree of strength that is being ignored. I am hypothesizing that the effective unbraced length of the compression flange is less than the full span length, and determination of such will reduce 1. the need for stiffeners, and/or 2. the size of the beam.

I'm reviewing some of the texts by Nethercott, Bleich, Galambos, and a few others, but I can't seem to find my specific situation.  Galambos (in his Stability book 5th ed.) makes mention of it but does not elaborate.

Has anyone looked at this arragement before?  I'm looking for references if you've got them.

Thanks.
Jeff Udall

Cambridge, Ontario (Canada)