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RE: Torsion in Wide Flange Beam

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Thanks for all who responded to this question.

I'm still confused.  I understand Warren's comment regarding developing the
forces in the flange.  That was helpful.  I still have a hard time picturing
a pinned end connection for torsion.  ASIC Design Guide 9 shows one, and has
the data for designing it.  Like Bill Sherman, it seems to me the end needs
to be fixed, or ideally should be.  I assume the shear connection
illustrated in the Design Guide should state that it needs to be designed
for both vertical shear and a torsional force couple.

If I have a uniform load that is eccentric, and is torsionally pinned at the
end, it seems to me the maximum torsion then occurs at the mid span,
analogous to a simply supported beam bending moment.  But if there is a
force couple at the end, then there is a torsional moment at the end, or is
it not there?  A sum of the forces would say there has to be an end torsion.
So, is a torsionally pinned connection one that has torsional restraint,
except for warping stresses?  I don't understand why the Design Guide calls
it "free torsional end condition".  To me, that implies zero torsion.

Why do all the strength books stop short of illustrating this?


Rich 
 

-----Original Message-----
From: Jones, Mark [mailto:MJones(--nospam--at)ssoe.com] 
Sent: Wednesday, May 25, 2005 11:55 AM
To: seaint(--nospam--at)seaint.org
Subject: RE: Torsion in Wide Flange Beam

Uniform loads do not produce uniform torsional moments just like they do
not produce uniform bending moments.  Only a pure moment, either
torsional or bending, will produce a uniform moment.  You are correct, a
uniform load will produce a torsional moment curve just the same as a
bending moment curve and point loads will produce linearly varying
moments.

The parallel between bending and torsional moments hold.  Each goes from
a load to a shear to a moment. (assuming you don't have pure shears and
pure moments.)  While we typically think about torsion as just a moment,
there is an induced torsional shear.

A load inducing a torsion reaches equilibrium similarly to one in
bending.  Remember that when the load starts it is a pure load resisted
by the restraints in the X, Y, Z directions.  The moment is then induced
INTERNALLY within the member, progressively resisted by the member
properties.  Assuming you have a bolted framed beam connection, then the
load on the beam's flange will be externally resisted by a axial forces
in the bolts acting as a couple.  If you weld just the web of the beam
then the resistance is axial forces in each of the welds.

If one connects the flanges of the beam then a fixed (or semi-fixed) end
is created and all of the above changes similarly to how a fixed end
changes bending reactions.


Mark


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