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Unbraced Length

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Jeff Udall wrote:
"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."


Tom Hunt wrote:
"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"


I write:
Jeff,
This is a compression flange with elastic support, just like a pony
truss or through-girder bridge.  The web is a continuous spring bracing
the compression flange.  Galambos gives a fairly comprehensive
introduction to this problem on pp. 579-591 of the 5th edition Stability
Guide, with a number of references.  There's also something using
stiffeners on p. 475.  It sounds like you've found all that.  Perhaps
the next step, after thinking about it and formulating questions for
about a month, would be to contact Ted Galambos through the University
of Minnesota Civil Engineering Dept.  He or Dr. Yura are the people who
come to mind.

Tom,
Again referring to Galambos 5th Edition, p. 197-201, they use
tube-shaped stiffeners at the ends of a simple-span beam which, because
of their large torsional stiffness, accomplish exactly what you're
talking about.  Galambos goes through an example with an unbraced W24x55
whose strength was increased 33% by adding these stiffeners.  Kind of
cool.

HTH

Mike Hemstad, P.E.
TKDA
St. Paul, Minnesota
-2 degrees F this morning

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