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RE: Torsion in Wide Flange Beam

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 I would say that a beam with torsion must always be "fixed" against
rotation at the ends - otherwise it is not stable against rotation.
However, I agree with Warren's description as relates to steel beams -
the "pinned condition" is a bit of a misnomer - it still acts to resist
torsion, but the way a wide flange resists torsion is affected by the
connection type. 

I don't think that the torsion diagram along the beam length changes for
each end connection, but the predominant method of torsion resistance -
warping vs St.Venant - does change, which can also affect the amount of
beam rotation.  Thus the connection only affects the way the beam is
designed but does not change the value of torsion along the beam. 


William C. Sherman, PE
(Bill Sherman)
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)cdm.com

________________________________

	From: Foy, Warren [mailto:Warren.Foy(--nospam--at)mhgrp.com] 
	Sent: Wednesday, May 25, 2005 9:52 AM
	To: seaint(--nospam--at)seaint.org
	Subject: RE: Torsion in Wide Flange Beam
	
	
	A torsionally pinned condition means that the supports cannot
develop axial stresses in the beam flanges, compression on one side of
the flange and tension on the other side, due to warping torsion. Simple
support connections for gravity loads are, I think, necessarily also
torsionaly simply supported.  For a beam to be torsionally fixed, there
needs to be some means of developing axial stresses in the beam flanges.
A fixed end, moment connection, can do this but the stresses due to
torsion are additive/deductive to the stresses due to flexure.
	 
	HTH,
	Warren Foy

		-----Original Message-----
		From: Rich Lewis [mailto:seaint02(--nospam--at)lewisengineering.com]
		Sent: Wednesday, May 25, 2005 11:31 AM
		To: seaint(--nospam--at)seaint.org
		Subject: Torsion in Wide Flange Beam
		
		This is probably more a strength of materials type
question rather than a steel question.  I have searched through my
Strength and Statics book but haven't found this described or
illustrated.  Torsion is always shown with a fixed end.

		I am investigating a wide flange beam that is supporting
a uniform eccentric load.  It is an existing condition, not a new
design.  I have AISC Design Guide 9 on torsion in steel.  I am trying to
understand what a torsionally pinned connection is.  I believe this is
what I have since I do not see any torsional restraints at the ends of
the beam.

		If I have a uniform torsional moment along the beam
(caused by a uniform eccentric load) with pinned ends, is the maximum
torsional moment at the center of the beam?  Is it zero at the ends? If
it's pinned I would think so.   I guess I would picture the torsion
loading diagram like a uniform load diagram.  I would picture the
torsion diagram like a moment diagram for a simply supported beam with a
peak in the center and zero at the ends.  I'm having a hard time
picturing an equivalent shear diagram to the torsion.  This would have a
maximum value at the ends and a zero value at the loads.  If there is no
torsional restraint how could I have a force at the ends?  I was
thinking that there should be diagrams like load, shear and moment where
you can get the value of one by integrating the area under the curve for
the previous one.

		If there is a torsional force on the beam, and it has
pinned ends, how is it in equilibrium?  What produces the equal and
opposite force of the torsion?

		I guess if I answer these questions I can better
understand how to apply Design Guide 9.

		Thanks for any insight you may give.

		Rich Lewis


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