Torsional Bracing of Beams

[Toby43]

Hoping to get others thoughts on the following...

Would you consider beam B2 in the attached sketch to torsionally/rotationally brace beam B1?
Details:
B2 is timber and fixed to main lateral system.
B1 is a steel I-beam and is part of a moment frame, hence bottom flange compression.
My take on the matter is that it will provide rotational/torsional bracing with due consideration for initial twist and bolt slip accounted for in brace forces, similar to the reasoning that slotted holes can be used to laterally brace, as long as the increased brace forces are accounted for. To look at it differently, consider if we had two independent braces coming into the beam spaced at a distance "s" apart ("s" been vert. distance between bolts), then they would create couple that provide torsional bracing.
The counter argument in my head is that using a connection detail that provides a "pin" to B2 should not be used to rotationally restrain B1.
Greatly appreciate any comments on the matter.

Regards
Toby

 

[canwesteng]

The magnitude of rotation required to occur for LTB buckling is relatively high relative to the rotation required to occur for a pinned connection. For typical steel beams, generally less than 1 degree of rotation is required to achieve a pinned connection.

 

[dik]

In your example, I would not consider torsion... the flexural stiffness of the beam is so much greater than the torsional stiffness of the supporting beam. If the beam were loaded on only one side, I would not normally consider torsion.

Dik

 

[WARose]

[blue](Toby43)[/blue]

Would you consider beam B2 in the attached sketch to torsionally/rotationally brace beam B1?

It's not normally done.....but I suppose it is possible. One question I would have about it is: are you trying to brace the beam (as per Appendix 6 in ASIC) or are you trying to get a torque out of B1? If you are trying to do the former.....you will have to run the numbers as per Appendix 6 (the flexural stiffness of B2 will play a role and I doubt you will have the rotational stiffness). I probably wouldn't use it for that.

If it's the latter.....I've done that a few times myself....but you have to limit the fixity of that connection: it can't be zero because it won't "suck in" any torque.....it can't be 100% because obviously it isn't. I've typically just used 10%.

By the way, if it's for bracing.....why not use B2 as a lateral brace? (If it goes to a stiff enough point.)