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RE: Torsion in Wide Flange Beam

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Rich - 

"So, is a torsionally pinned connection one that has torsional restraint,
except for warping stresses?"  

Yes, a torsionally pinned connection is one that is only capable of
resisting a torque at the support.  However, the restraint of the flanges is
not great enough to resist warping of the cross section at the support
point.

The classic example of this is a double clip-angle shear connection.  The
support is capable of resisting some torsion, however there is little or no
effort to restrain the flanges.



At the risk of complicating things even more, I'll point out that the
torsionally FIXED connection that AISC has in their design guide is very,
very rare.  Most moment connections do not have anywhere near as much flange
restraint as is required to develop the full warping restraint in those
locations.... Maybe some of the "blast resistant" moment connections that
extend vertical plates on either side of the flanges for a good distance
beyond the supports.  

What I'm getting at is that even out standard moment connections are much
closer to the idealized torsional "pin" than they are to the torsional
"fixed" support.  


Josh Plummer, SE
RISA Technologies

-----Original Message-----
From: Rich Lewis [mailto:seaint02(--nospam--at)lewisengineering.com] 
Sent: Thursday, May 26, 2005 9:20 PM
To: seaint(--nospam--at)seaint.org
Subject: RE: Torsion in Wide Flange Beam

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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