Nodal Bracing of Back-to-Back Channels

[jdonville]

I would like to design nodal bracing for a pair of channels oriented back to back with a large gap between them to make a built-up beam member. The intention is to provide enough bracing to maintain the full flexural capacity of the built-up section along the entire length.

I have the maximum moment that the beam needs to support.

I have used full-depth plates welded to the backs in the past to resist torsion on back-to-back channel beams subjected to discrete point loads. Can I do something similar in this instance when the loads are distributed and the beam will bend through at least 2 points of contraflexure?

Do the provisions of AISC 360-05 App6.3.2a apply? The discussion for this Appendix that I have seen deals with doubly-symmetric sections and I am not sure how or if it applies to channels.

Any responses appreciated.

J

 

[ishvaaag]

In my view if your pair of channels plus nodal bracing are a stand-alone system to port the loads to support, such behaviour won't be warranted without reliable torsional bracing, because the tying plates themselves may be unable to properly restrain the overall rotation when in LTB even if able to port the required bracing load to a nonexistent rigid point; a house of cards failure is feasible and this is where the torsional restraint appears as a need. So for this standalone behaviour the torsional restraint is required, apart than that ar support points. So the section you quote applies, in my view.

If your top flanges were solidary to reliable diaphragm action able to prevent LTB, you would have the issue resolved for the length of beam where only compression affects the top flanges, but still would need to warrant the lower flanges are properly braced against LTB any time and where they are in compression.