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Unbraced Length for Cantilever Beams 1

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EngDM

EngDM

Structural
Aug 10, 2021
461
Hey all,

For a cantilever beam, what would the unbraced length for bottom flange be considered as? Would you consider the entire length between columns/to the end of the cantilever (if the T/O column is braced), or would it be acceptable to take the length until the inflection point, as past this you don't have bottom flange compression anyways. I just need justification to go one way or the other with my designs. I've been providing bottom flange bracing to cut down the unbraced length, but I just don't know if this is necessary.

If anyone has a CSA reference that covers this I'd be forever grateful.

Thanks,
 
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Why are you bracing the bottom flange of a cantilever? You brace the top, tension flange, for cantilevers to prevent LTB buckling. There is no CSA reference - I recommend Guide to Stability Design Criteria for Metal Structures for an approach to this. The old version had a really simple check for unbraced length, I think the new one has some method of accounting for backspan stiffness that is a bit more complicated though.
 
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Inflection point is no longer considered a bracing point. So generally we would brace it at the column, at the end of the cantilever, and at the inflection point. And then my unbraced length for moment would be the larger of inflection point to column, or column to end of cantilever.
 
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The bottom flange of a cantilever beam goes into compression near the columns. Moment resistance is based on compression unbraced length. Whereas the top flange is typically braced by the deck or joist seats.
 
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@jayrod12 Where/when did inflection point stop being considered a bracing point? I just want to have a source to bring up with my boss.
 
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The inflection point is not a point braced against lateral torsional buckling. You generally want to restrain the tension flange not the compression flange for a cantilever to prevent LTB. Though benefit can be gained by doing both.

EDIT - Oops, I got interrupted and 5 people answered before I finish reply.

ManitobaStructural said:
@jayrod12 Where/when did inflection point stop being considered a bracing point? I just want to have a source to bring up with my boss.
Click to expand...
I can't comment on what your code says. But a inflection point has never been a braced point against LTB buckling. If a code has ever used this approach it has been wrong.

ManitobaStructural said:
Moment resistance is based on compression unbraced length.
Click to expand...
Not for a cantilever, the critical flange is generally considered the top flange. (I say generally as different codes might have different approaches.)
 
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Inflection point stopped before considered a bracing point since before I was in school - we were taught only to use stability bracing as brace points. You probably won't find when exactly it stopped being used as a brace point but it must go back to the 90s when it was changed... maybe in use a little longer with AISC than CSA though.
 
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Yeah I don't have a reference for when. All I can tell you is the old boys in the office still say the inflection point, but there's no basis for that in the literature anywhere either to back up that it can be used.

Back in the day up until I would bet the late 90s many people used the inflection point as a brace point. But I've never seen that codified anywhere. And I'm going to try paging KootK to help on a reference thread. A few of the big hitters around here went through a fairly in depth discussion about bracing of cantilevers that may answer a bunch of your questions.
 
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I feel awkward being the only person who doesn't understand the inflection point. What's the loading?
 
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The inflection point is the point on the backspan of the beam where the moment switches from negative to positive. I.e. the bottom flange of the beam goes from compression to tension.

There was a time when these points of inflection on the moment diagram were considered braced points in considering unbraced length of the cantilever.
 
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Yes that situation. Thanks steveh49.

Still would like if someone knows of the thread where there was a whole bunch of fea modelling done regarding beams and bracing and inflection points. I believe they used Yura's paper linked by steveh49 as guidance for the example problems.

 
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You mean the infamous "Rafter without fly brace?" thread? thread507-459248
 
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I was confused about the "inflection point" of a "cantilever beam" as well. Anyway, I started practice in the 80's but do not recall having ever seen a code provision that allows the use of the inflection point as a brace point.
 
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I think the NDS still allows it. I know Wood Works Sizer has an option to recognize inflection point bracing.
 
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Eurocode is generally not at the cutting edge for design codes, but the fact they would let you use the inflection point as bracing in 2012 is a bit troubling... although if you don't use the moment gradient modifier, I think it's only an issue in some situations
 
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@phamENG,

Can you bring up the NDS provision for reference?
 
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Can't find it in the NDS, so disregard that. WoodWorks Sizer does have this option, which I think should be removed:

Screenshot_2022-06-23_112533_pe2vgn.png
 
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