Connections for torsion.

[3doorsdwn]

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?

 

[DaveAtkins]

I have wrestled with this issue before. Usually, I assume that the torsion is taken by two rectangles (the outstanding angle legs). Each rectangle takes 1/2 of the torsion. The shear stress is equal to (3*Mt)/(d*t^2), where Mt = 1/2 of the torsion, as stated above, d = the depth of the angle leg, and t = the thickness of the angle leg.

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

 

[csd72]

The torsion reaction of the beam will be a horizontal force couple with the forces at the top and bottom flanges.

 

[DaveAtkins]

csd72,

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

 

[csd72]

Dave,

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.

 

[3doorsdwn]

"Dave,

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.