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RE: Beam unbraced length

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Mike,

I now understand a little better what you're getting at.

To have a meaningful effect on LTB of a beam, the restraint preventing
lateral movement of the flanges must be as close to the point of maximum
flange slope in the deformed shape under LTB (this would be a lot easier to
say if I could attach a picture). As an example, if you place such a split
HSS at the center of a beam with fixed ends subject to reverse curvature
bending, it will contribute almost nothing. The shear deformation the split
HSS would see is zero there as the roll of the flanges relative to each
other is maximum at that point (making the slope and relative displacement
zero). You would get some benefit from the bending stiffness, just like for
stiffeners, but only if the connection into the diaphragm can restrain it.

If I move the split HSS away from that point, I will start to pick up some
shear in it. So if I place properly sized split HSS in pairs symmetrically
about the centerline of my beam, I can restrain the segment in between them
to roll as a unit. Of course, the proper strength and stiffness could be
pretty tough to calculate. Might even be that the detail really needs pretty
extensive boxing plates making it into a semi-tubular member to accomplish
this. And then, those plates would likely be more useful if applied from the
ends in rather than along the middle.

In any case, I don't think this approach creates braced points in the member
because the one flange at that location can still move laterally relative to
the other (it rotates). Rather, I think what this detail may be doing is
changing cross-sectional behavior by restraining warping, thereby increasing
Mcr by some amount I'd have to think about how to calculate -- nerdily as
Gunnar noted. (-:

The stiffener approach is different. It is based upon preventing the flange
from moving laterally. The required strength and stiffness to do that are
calculable according to the equations provided in LRFD Specification Section
C3. As you pointed out, the stiffeners and their connection to the diaphragm
must be suitable to provide for that required strength and stiffness. A
heavier beam or direct bracing will often be more economical.

So perhaps we are not disagreeing after all.

Charlie




-----Original Message-----
From: Michael Hemstad [mailto:mlhemstad(--nospam--at)yahoo.com]
Sent: Thursday, March 20, 2003 1:19 PM
To: seaint(--nospam--at)seaint.org
Subject: Beam unbraced length


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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