Double Angle Connection with Torsion

[Valstone]

The EOR has requested that I provide a calculation showing the torsional capacity of a bolted-bolted double angle connection between a W8 beam and a W8 girder. My thought was to resolve the torsion through bolt tension in the bolts on the beam side. My concern now is how I design the angle leg for this torsion. Should I check the angle leg for torsional stresses in addition to the weak axis flexure induced by the bolt force from the torsion OR should I ignore the the torsional stress in the angle leg and design the angle leg for the weak axis flexure only? It would seem that the angle thickness would have to be pretty thick to resist any type of torsional forces. Any input would be greatly appreciated.

Thanks,

Val

 

[hokie66]

Dave,

I'm still curious as to why you say that. Why not a rigid support? I don't think we know enough about the rest of the structure to define how a given connection should work.

 

[DaveAtkins]

I will admit that if you connected the beam flanges to the girder flanges, by welding or bolted splice plates, you could take out more torsion. But then you would be introducing some partial end moment from the beam to the girder, which I suspect is not the intent of the EOR.

So if you wanted to keep the support pinned, and not connect flanges to flanges, you could still take some torsion while maintaining the flexibility in the double angles which is required by AISC.

DaveAtkins

 

[hokie66]

I understand now. Your reluctance to make the connection rigid is because of a code provision, not practicalities. Anyway, if the connection is stiff, that end moment is taken by the beam in bending, not the girder in torsion.

 

[ToadJones]

I'd try like hell to torsionally brace the beam at the point of application of the load causing torsion.
I guess I should assume this was attempted, but not possible.

 

[vandede427]

I agree with a few of you on this; the detailer should post an RFI to the EOR about the desired intent of the connection. Possibly the EOR forgot to detail it specifically, thinks they can get torsion out of this connection, or doesn't know better.

 

[Valstone]

This connection occurs at a pipeway framing plan in an industrial plant and due to the lateral forces induced by the pipes a torsion is induced at the ends of the beams. There is grating that bears on the top of steel so a flange plate would not work. I would be concerned about connecting the flanges of the beam directly to the girder flanges because it would definitely change the behavior of the beam. I know for a fact the beam web will need to be reinforced, but my main reason for asking was to see if anyone else has run into this type of problem. This is not a typical connection let alone a normal special connection. I have spoken with AISC and they were not much help other than saying that the coped out beam web acts like a rectangular section in torsion and that there is no documentation for this type of connection. I personally think the EOR hasn't fully thought this through. I think sometimes the EOR has so much pressure to get member sizes and drawings complete that they don't think about all forces involved at the connection. If I were designing the framing myself I would definitely resolve the torsion outside of the connection. I will definitely be talking to the EOR about this connection because this affects a good amount of the connections, which will be costly to the fabricator and owner.

 

[DaveAtkins]

I don't agree with AISC that there is no documentation on this type of connection. The torsional shear stress in a rectangle is (3*M)/(b*t^2), where M is the torsional moment, b is the long dimension of the rectangle, and t is the short dimension of the rectangle.

DaveAtkins

 

[BAretired]

You could put an end plate on the beam outside the girder flange, then weld a 5x5 HSS stub with a 5 x 10 end plate with four bolts through the girder web.

BA

 

[fabengr]

Just to be clear about the comment about the AISC requirement that a clip angle has to be 5/8" to ensure ductility. That is a comment in the manual and not the spec. The same section in the manual says that ductility checks in Part 9 of the 13th Ed can be used to justify other angle thickness.

 

[DaveAtkins]

On second thought, in this particular case, the angle flexibility doesn't matter very much. Even if you were to rigidly connect flanges to flanges, what would happen is the girder would twist slightly, thus creating the pinned end condition for the beam. The slight twist of the girder would not result in any appreciable torsion in the girder.

DaveAtkins

 

[hokie66]

Now that BA has designed the connection, all that is left is to implement it.

 

[BAretired]

Thanks, hokie.

BA