Cantilever Beam Bracing
Cantilever Beam Bracing
(OP)
I've got a situation that doesn't seem to fit traditional AISC theory too well...essentially a horizontal "ladder" shape made out of MC18 channels as the primary stringers (ie. the legs of the ladder) spaced about 36" apart, with C6 channel cross members (ie. "rungs" of the ladder) welded to midheight of the flat faces of the stringers at 10' o/c. This ladder shape acts as a cantilever beam, and actually rolls back and forth over it's support frame so the cantilever length is subject to change, with up to 35' cantilever and 17' backspan. My questions are related to how to find the unbraced length of the beam, Lb.
1. Since the frame moves, there won't necessarily be a C6 cross brace aligned with the support, where the maximum moment occurs. The nearest brace might be 5' away. Does this matter? Or can I still call Lb = 10'?
2. The C6 cross braces are welded at midheight of the MC 18 channel...not an ideal compression flange brace, since the compression flange sticks out 6" beyond the welded cross member. Is it reasonable to take the 2% rule (2% of the compression force in the flange) as the force in the theoretical brace, and use some increment of the MC18 web in weak axis bending to justify the flange as being braced at the cross channel location?
I'm analyzing something that's already built so no chance to eliminate the tricky situations. Thanks for your help.
1. Since the frame moves, there won't necessarily be a C6 cross brace aligned with the support, where the maximum moment occurs. The nearest brace might be 5' away. Does this matter? Or can I still call Lb = 10'?
2. The C6 cross braces are welded at midheight of the MC 18 channel...not an ideal compression flange brace, since the compression flange sticks out 6" beyond the welded cross member. Is it reasonable to take the 2% rule (2% of the compression force in the flange) as the force in the theoretical brace, and use some increment of the MC18 web in weak axis bending to justify the flange as being braced at the cross channel location?
I'm analyzing something that's already built so no chance to eliminate the tricky situations. Thanks for your help.






RE: Cantilever Beam Bracing
We had a similar discussion some time ago regarding stair stringers. I am not sure if the C6's will actually brace the larger channels since they are only being braced to each other.
RE: Cantilever Beam Bracing
I think my primary concern is not having bracing at the support - seems like a red flag to me...anyone know how to handle that or if AISC says anything useful about it?
RE: Cantilever Beam Bracing
RE: Cantilever Beam Bracing
RE: Cantilever Beam Bracing
The bigger problem is not having the brace aligned with the support. It's similar to the question of a simple span beam resting with bottom flanges on supports, but only providing bracing at midspan, know what I mean? I think AISC says somewhere that all of its equations assume torsional (and LTB) bracing is provided at the support, but does it make a difference? Thanks again.
RE: Cantilever Beam Bracing
RE: Cantilever Beam Bracing
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