Preventing Torsion
Preventing Torsion
(OP)
I am designing a bent C channel spanning 14'-0". The channel is loaded by hat channels on the bottom flange. See attached drawing. Since I am not loading the C channel through the shear center the loading will create torsion in the C. I think I can resist the torsion by welding the hat channel to the C. What do you think?
Thanks
DHK
Thanks
DHK






RE: Preventing Torsion
RE: Preventing Torsion
RE: Preventing Torsion
RE: Preventing Torsion
RE: Preventing Torsion
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An example from Design of Weldments by Blodgett:
A pair of C6x8.2 connected with cross members like ]_[ subject to 1000in-lb or torque have an angle of twist of something like 2 degrees.
Put those same two channels back to back like this ][ and subject them to the same torque and your angle of twist drops to like 0.8 degrees.
Put the same two angles toe to toe like this [] and subject them to the same torque and the angle of twist drops to like 0.02 degrees.
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So the basic idea is you want to approximate a "box" shape as much as possible with your torsion members. If you can't do that then you need to attach your left and right side channels with diagonal cross bracing, not straight bracing that resembles a bookshelf. The reason being that diagonal braces transform the twisting of the sidewalls into compressive and tensile loads much like beam in bending or members of a truss.
In your picture the top flange of your 10" channel is basically unrestrained and welding the bottom 3" of the web to a straight cross brace isn't going to improve the situation much. If you added another hat channel to the underside of the top flange you would get an approximate "box" section and stiffen the frame quite a bit as well.
RE: Preventing Torsion
Unfortunately I cannot install a hat channel at the top flange of the C channel.
RE: Preventing Torsion
RE: Preventing Torsion
RE: Preventing Torsion
Normally what results is a small rotation of the beam to a point where biaxial bending resists the force. In your case the fact that the load is hanging from the bottom of the beam would tend to resist the rotation and minimise the twisting.
RE: Preventing Torsion
As long as you're not cantilevering off the open section, you're OK.
RE: Preventing Torsion
I was just trying to give a scientific explanation.
RE: Preventing Torsion
RE: Preventing Torsion
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Preventing Torsion
RE: Preventing Torsion
What others didn't perhaps say clearly, in stating that there is no need to account for torsion of C beams, is that this is true because the torsional deflection is generally resisted by cross members (of course not true for a lifting beam, as an example).
So my answer to DHKpeWI will be: if you don't weld your cross members (or hat channels as you call them, don't personally know the meaning of this nomenclature), you are at a risk of having the cross beam not well supported on the lower flange, because of the torsional deflection: with high or limit loads, the failure mechanism here could be with the cross beams losing their support and falling down.
If on the contrary you weld the cross beam to the main ones, then you can forget about torsion and shear center.
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RE: Preventing Torsion
Are you saying that a C-shaped weld has a "shear center" in a similar location to that of the Channel? I'm not sure if I believe that. If that is true then the C-shaped weld tables for AISC are all wrong, no? If that were true then the eccentricity would need to be measured to the shear center and not to the centroid of the weld group, but the tables all measure the eccentricity to the centroid of the weld group.
RE: Preventing Torsion
A side effect is that the flanges rotate horizontally about the vertical axis at the ends, a warping distortion, as in torsional analysis, so welding changes the end conditions.
This tendency for the flanges to distort in opposite directions is a function of the bending moment, the instantaneous curvature is a function of the instantaneous bending moment, so there is a difference in the effects of point and uniform loads.
Michael.
Timing has a lot to do with the outcome of a rain dance.
RE: Preventing Torsion
Turning the channel around as suggested by hokie is the best solution, but I assume there is some reason why this cannot be done.
I do not agree with Jed that the torsional moment is a secondary effect. It is a primary effect and a moment connection is necessary for stability if the channel is not capable of accepting the torsional moment. A secondary effect is one where torsional moment is not needed for stability such as the torsion in a spandrel beam caused by joists framing into one side.
BA
RE: Preventing Torsion
We are Virginia Tech
Go HOKIES
RE: Preventing Torsion
There are not answers to everything.
RE: Preventing Torsion
I ran a calculation for the warping torsion on the channel assuming the hat channels do not resist the torsion. In this application the C channel is restrained against rotation at the ends. For the load and span used here the warping stresses are larger than the yield stress of the steel (AISC Design Guide #9). I added a stiffener welded to the top of the hat channel and the web and flange of the C channel to retrain the torsion.
God Bless.
DHK
RE: Preventing Torsion
Michael.
Timing has a lot to do with the outcome of a rain dance.