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# Re: Fw: Torsion on a steel tube and AISC WINTORQ program

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• To: "?" <seaint(--nospam--at)seaint.org>
• Subject: Re: Fw: Torsion on a steel tube and AISC WINTORQ program
• From: Christopher Wright <chrisw(--nospam--at)skypoint.com>
• Date: Tue, 18 Mar 03 21:30:47 -0600

```>The program appears to give an incorrect value for shear stress, unless I
>am missing something.
I guess I still don't get it. What is somewhat higher? more than 10%?  What kind of tube are analyzing? Is it a closed section?  the section complicated enough so that WINTORQ has to do some numerical approximation. This would likely be a multi-celled shape or one with a non-uniform wall.

Your items 1 and 2 are respectively
torsional stress in a thin-walled single cell tube of any shape
torsional tress in a square tube where b = the length of one side

Your item 4 formula isn't dimensionally consistent unless the theta' value is actually rotation (in radians) per unit length, Ø/L. I suspect the relationship 4. comes from eliminating the Torque, T, between the relationship for angular displacement, Ø, (=TL/JG) and shearing stress, tau (=Tc/J) with J as the polar moment of inertia. This works for round and elliptical tubes because the J is the polar moment of inertia in both relationships. For other shapes (you can work it out for yourself) the J in the stress formula is different from the J in the angular displacement formula and neither is the polar moment of inertia.

Looks like 1-3 apply to a square tube and 4 and 5 apply to a circular tube. Probably why the results don't agree. If I've guessed what your problem is you might want to clear it up by going through the section on torsion in Blodgett's _Design of Welded Structures_. He shows the general relationships for torsional stress and deformation in closed thin wall tubes

Christopher Wright P.E.    |"They couldn't hit an elephant at
chrisw(--nospam--at)skypoint.com        | this distance"   (last words of Gen.
___________________________| John Sedgwick, Spotsylvania 1864)
http://www.skypoint.com/~chrisw

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