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Beam unbraced length

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Charlie Carter wrote:
"With all due respect, and I hate to disagree with you
and the Guide, =
that
detail isn't much better than the stiffeners. The
split tee will only =
create
a torsional rigidity for the width of the beam it
covers. Outside of =
that
zone, there is still as little at there was before,
and the torsionally
stiff zone created is along for the ride if nothing
can provide the
restraint at one flange to be extended down through
the detail to brace =
the
other flange."

I hate to disagree with you, too, Charlie, and mostly
I don't.  The split tubes aren't intended to prevent
lateral movement of a flange by anchoring it to
something; the purpose of these stiffeners as I
understand it is to prevent the lateral motion of one
flange relative to the other.  If you can do this, you
have defeated lateral-torsional buckling (my verbs get
more aggressive when we're at war).  The purpose of
the tube stiffeners is to give the lateral stiffness
of the flanges something to anchor against (the other
flange).  That stiffness still has to be adequate to
prevent the lateral jump of one flange relative to the
other that is LTB.  I personally don't know how to
quantify that stiffness.  Neither, apparently, do
Gunnar Hafsteinn Isleifsson or Ted Galambos (OK, Ted
probably does, but I don't think he told us how in the
Guide).  So, at best, that relegates the technique to
use as the suspenders after you've put the best belt
on the problem that you can.

Mike Hemstad, P.E.
TKDA
St. Paul, Minnesota


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