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column capacity with and without flange stiffeners

column capacity with and without flange stiffeners

column capacity with and without flange stiffeners

(OP)
Scenario: water tower composed of braced frames. There are 4 columns, beams at 20ft in height, x-bracing at every level.
Beam to column connections are simple shear connections. Brace connections are vertical gusset plates connecting beam and brace only (this is just for discussion purposes. no brace-column connection, only relying on beam-column connection for bracing of column flange)

Question: can you develop full plastic capacity of the columns without column flange stiffeners?

assume Lp of column is > 20ft.
The beams and bracing will provide global lateral support, but the "exterior" flange does not anything connected to it. Can the "exterior" flange go plastic in strong axis bending?
Would you consider the beam-column connection to the column flange on the "inside" flange considered braced so it can go plastic in strong axis bending?
Or do I need to provide flange stiffeners at all beam locations to obtain column plastic capacity?

RE: column capacity with and without flange stiffeners

This is a high seismic application where you're expected to develop some moment fame action after the bracing gives? If that's the case, I don't feel great about what you've described. Flange to flange web stiffeners and a beam connection capable of transferring significant weak axis bending to the beam sounds better to me.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: column capacity with and without flange stiffeners

(OP)
fair point. and I agree, in seismic application, would definitely put in stiffeners.

what are your thoughts if this were not a seismic location?

RE: column capacity with and without flange stiffeners

Without seismic, I'd be a lot less concerned. With columns w14 or less and conventional double angle connections coming in both directions etc, the joint should be fairly well braced. It makes me a bit nervous that you seem to be wanting to take your columns all the plastic under axial load only though. That's hopighly unusual column design as most columns will buckle long before As x fy. Can you explain that a bit more?




I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

RE: column capacity with and without flange stiffeners

(OP)
This is purely a theoretical discussion; thus switching from a water tower to a billboard or whatever else I could dream up. It was to get opinions on whether clip angles act as flange restraints. Majority of my designs do not go into plastic column, typically some combination of axial-moment. In any event, I do have to calculate the moment capacity of the column. If, for whatever reason, I get the beams framing into the column frequent enough, I could theoretically achieve plastic capacity of the column (as opposed to only LTB). This discussion was to see if it was theoretically reasonable to assume the beams framing in to the column provide column flange restraint to resist LTB, and consequently achieve plastic yielding of the column cross section. I was particularly the billboard example since it does not directly restrain the flange.

Thanks for discussing.

Does anyone else have any additional thoughts?

RE: column capacity with and without flange stiffeners

To conclude my contribution, if I were actually expecting to take my columns plastic/near plastic axially, I would want to check the bracing situation out more rigorously than the usual practice of merely applying judgement to conventional scenarios. AISC appendix six has bracing provisions that may well be adaptable to this.

It might also be worth examining AISC's design guide on end plate moment connections. Many of those are unstiffened and develop heavy column moments. They offer more restraint than clip angles but not tonnes more.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

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