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# RE: Unbraced Length

• To: <seaint(--nospam--at)seaint.org>
• Subject: RE: Unbraced Length
• From: "Udall, Jeffrey D" <JDUdall(--nospam--at)babcock.com>
• Date: Thu, 11 Dec 2003 10:34:37 -0500
• Disposition-notification-to: "Udall, Jeffrey D" <JDUdall(--nospam--at)babcock.com>

Thomas, The math that was used to develop the lateral torsional buckling formulas assumes that there is no distortion of the cross section.  Therefore, there is no consideration for stiffeners without external bracing.  I suppose in reality, there may be some small effect gained, but I suspect that the effect is proportional to the thickness of the flange to the span of the beam.  In that 1/2" of plate thickness, you've changed the cross sectional properties of the beam (ie. I and S).  But away from that, you've got the same beam.  I posed the same question to the AISC Steel Solutions web site (great resource if you've never used it!) and their response was that there is NO effect.  Perhaps "NO" is a bit strong, but it's damn close.

Charlie, I found the section in the LRFD manual.  That's what I was looking for.  They've even got some references at the back for the literature that it's based on.  Off to the library to see what I can dig up!

Thanks everyone that replied.
Jeff
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

There are provisions in the 1999 AISC LRFD Specification for torsional bracing in Section C3.4b. They are indended for exactly this case.

Charlie