Is stacked X-brace same as K-brace

[jechols]

thread507-422360

I have a one-story structure with stacked X-bracing same as the referenced thread above. I am not understanding how it is not a K-brace. The definition from AISC Seismic Design Manual (5.2) states: where a brace frames to a column at a location where there is no out-of-plane support. Is it a K-brace condition?

 

[EDub24]

I don’t think so. A K-brace is a problem because the horizontal component of the brace goes directly into the column creating a bending moment. In a stacked X-frame the horizontal component goes to the brace below which basically provides the out-of-plane support.

 

[JAE]

Agree with Edub24 - a K brace is a column with axial load and a lateral force resisted in bending. When the column goes inelastic, it buckles and collapses the structure.

A double-stack X brace is simply a vertical truss - when it goes inelastic it can still achieve post-yield stability unlike the singular column.

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[jechols]

That is what I was thinking until I found this article.

 

[JoshPlumSE]

My thoughts on this. Some of it is semantics.
1) We're talking about R = 3 vs R = 3.25, right? Because we're thinking OCBF vs a non-341 system. For industrial structures I usually just start out assuming R=3.
2) Is it the height limits that are causing concern? If it's a non-building structure then the normal height limits are relaxed a good amount depending on what you call it.
3) I don't 100% agree with the interpretation in that article. But, I understand where it's combing from. If you had a beam at the intermediate levels then there is no question that it's an X brace. If not, then it may not qualify as an OCBF. I would never personally call it a K brace, but I understand why he takes the crappy code definition of a K brace and applies it to that situation.... Rather I would say that the column isn't sufficiently braced at that level to qualify for OCBF.

 

[jechols]

Thanks to everyone for your responses!

 

[sandman21]

The article is incorrect, the configuration is a multi-tier BF not a K-frame. Design criteria for multi-tier BF; OCBF. SCBF and BRBF can be found in the latest edition of the AISC 341.