Cantilever Beam Bracing

[WAstruc10]

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.

 

[Lion06]

I would note the AISC recommends the tension flange be braced at the tips of cantilevers.
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.

 

[WAstruc10]

Braced to each other...so you are saying both compression flanges may buckle in the same direction, correct? I think that buckling translation will induce bending in the transverse brace, as the rectangular shape of the brace is forced into a parallelogram. So it seems to me that if the web of the stringer is locally strong enough to take 2% lateral force applied at the compression flange as a weak axis bending load, it should make for an effective brace for the beam flange. It's kind of how U-shaped pedestrian bridges work, with elastic support of the compression chord, right?

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?

 

[Lion06]

with it being welded to the main channel, I will probably agree that it is more of a rotational restraint than a simple displacement restraint.

 

[JAE]

What if you welded a small angle along the bottom of the channels such that the bottom flange looks more like a "T"....like this:

 

[WAstruc10]

Good idea for stiffening up the flange, but it's already built and the owner has come to the manufacturer of this conveyor support after-the-fact looking for a sealed engineering analysis. The MC18's work fine even at an unbraced length of 20', so I'm getting fairly comfortable with the ability of the transverse channels at 10' o/c to provide enough bracing stiffness as described above.

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.

 

[civilperson]

You have an assembled unit with unbraced length of whatever the cantilever length is. The components making up the unit contribute to the stiffness in each axis. If the loading is in the strong axis, 36" depth, one channel will be in compression. If the loading is in the weak axis, 18" depth, a pair of flanges, one from each channel will be in compression. Analyze the assembly in toto for an accurate prediction of strength.

 

[swearingen]

I would work through Appendix 6 Section 3 of the 13th AISC Manual. I'd check the web to see if it meets the strength AND stiffness criteria of 6.3.1a, Relative Bracing (one flange below the C6 won't affect the one on the other end of the C6). It's much less stringent than your 2% (which shows up in Section 6.3.1b). To be on the safe side, you can go ahead and check it against 6.3.1b as well, which has the 2% strength requirement.



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