Connections for torsion.
Connections for torsion.
(OP)
I’ve got a situation where I have torsion on an I-beam. At the end of the beams I have simple shear connections (i.e. angles and bolts). What I’m having a hard time deciding is: what to design the angles for (in terms of the load from torsion; throwing the (block) shear demands aside for a moment). Intuitively, you’d design for a bearing force (tension or compression) on the angle (and the bolts) for the couple created by the torque. BUT this type of connection is torsionally “pinned” is it not? So, theoretically, it shouldn’t see any load from the torque. (But it actuality (as we all know) there really isn’t any such thing as a connection that is perfectly “pinned” or “rigid”).
Thoughts?
Thoughts?






RE: Connections for torsion.
RE: Connections for torsion.
I would try sizing an angle at the top and bottom flange for the torsion moment.
RE: Connections for torsion.
RE: Connections for torsion.
virtually all the torsion resistance of an I beam is in the flanges, the web plays very little part. So any connection that does not connect rigidly to the flanges is not a torsion connection.
If you are saying that this is connected to a perpendicular beam then this is a difficult detail. You can either detail it so that there is negligible torsion transferred into the supporting beam, or you can brace the supporting beam (with fly braces e.t.c.)to take any torsion.
A flexible end plate is sometimes used that can transfer torsion (in plane) but is too flexible to transfer out of plane loads.
I would suggest you read the AISC design guide on this as this really is a complex topic.
csd
RE: Connections for torsion.
RE: Connections for torsion.
Actually, I have the AISC design guide on this topic and (like most publications) doesn't specifically address this topic (accept to address what constitutes torsionally fixed and torsionally pinned connections (i.e. on p.9 & 107)). I have not addressed torsion connections (in the past) without some type of restraint for the flange (and it appears that that may be necessary for this case; perhaps (since it’s beam to beam) a full pen weld on each flange may be required).
RE: Connections for torsion.
If the connection is fully fixed, it's easy. If the member is OK, the connection is OK. But for a torsionally pinned connection, you can either check the angles or shear plates for the torsional shear forces or design a special connection. You'll find out pretty quickly that the angles or shear plates have to be very thick to carry any significant torsion. So we would put plates top and bottom with shear tabs on each side of the flange to carry the torsion. The top and bottom plates (only welded to the main member) can develop the couple. Not too expensive, but not free either.
You didn't ask, but be sure to get your AISC Torsional Design Guide to analyze the warping stresses on the member.
Once you design some of these, you'll see the advantage of avoiding the torsion all together with kickers or some either method.
RE: Connections for torsion.
RE: Connections for torsion.
I hope I am not muddying the waters.
DaveAtkins
RE: Connections for torsion.
Note that torsion doesn't transfer from the twisting beam into it's support as torsion, usually. Twisting in the beam usually becomes flexure in the supporting beam or column when it's orthogonal to the twisting beam. You get a concentrated moment into your support.
RE: Connections for torsion.
If you can treat the two angles as pure shear members acting as a couple and keep the shear through them under the allowable, you'll be OK. But the moment arm is so small, that's going to be tough.
RE: Connections for torsion.
RE: Connections for torsion.
DaveAtkins
RE: Connections for torsion.
"I'm away from my Enercalc machine at work. I don't think it analyzes warping stresses, however.
If you can treat the two angles as pure shear members acting as a couple and keep the shear through them under the allowable, you'll be OK. But the moment arm is so small, that's going to be tough."
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My thinking was somewhat similar but I was planning on treating the end force like a couple that would put forces on the angle as if it was a seated connection. It's a 2-bolt connection so the "d" of the twist would be about 3". Since 4 bolts connect the sub beam to the beam it is framing into (the torque is on the sub) that gives me 4 bolts to resist the T/C couple. With that accounted for (and the force on the angles) I should be covered.
RE: Connections for torsion.
http://www.webcivil.com/thintorsion.aspx
RE: Connections for torsion.
As I said before, in Nuclear Plant design when we tracked all the forces through their various paths, we learned a lot about the connection behavior. And that torsion was a bear to design for.
RE: Connections for torsion.
I ask because I've seen crane beam assemblies writhe under their own weight while being hoisted, and I'd have to describe the torsional and lateral deflections as 'scary', at least to a civilian.
Mike Halloran
Pembroke Pines, FL, USA
RE: Connections for torsion.
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But that is the part I'm unsure of: Are you *sure* that is how the transfer would take place (i.e. with the T/C couple happening vertically)? It may seem intuitive that is how it would occur (considering the shear flow in the web is vertical). But you would think that the beam's rotation would carry the couple directly into the angles (i.e. making them act as a seat as I said above, thus making the force on the bolts horizontal).
RE: Connections for torsion.
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Yes, rotation and deflections are minimal/tolerable.
RE: Connections for torsion.
You will end up with thick angle legs (but don't make them too thick--AISC limits angle thickness to 5/8" max (I think) in order to assume the connection is pinned for gravity loads).
DaveAtkins
RE: Connections for torsion.
RE: Connections for torsion.
You are correct if you connect the top and bottom flanges to the support. 3doorsdwn is trying to take the torsion through a pair of clip angles.
DaveAtkins
RE: Connections for torsion.
Then how does the force get from the flanges to the clip angles?through bending of the thin web?
If you only connect the web, then the effective torsion resistance of the connection will be miniscule.
RE: Connections for torsion.
Then how does the force get from the flanges to the clip angles?through bending of the thin web?
If you only connect the web, then the effective torsion resistance of the connection will be miniscule."
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I don't know how it gets there but it does. I made an FEA model of the beam as plate elements only. I restrained the nodes along the web only (at each end). As I expected, the torque exited the "beam" as shears perpendicular to the web face. (And the equilibrium matched what I got on paper.) It appears that a combination of shear and bending in the web is what makes the force transfer.