AISC 341 and two-story x bracing

[amendale]

In the AISC 341 provisions for OCBF bracing, a slenderness limit of 4sqrt(E/Fy) is defined for V bracing. When implementing two-story X bracing, are these braces considered as V braces for satisfying this slenderness limit?

 

[jtx]

Would make sense to me. The effective length of the brace is the same whether it's in a chevron configuration or two-story brace configuration.

 

[amendale]

Yes, but the 4sqrt(E/Fy) slenderness limit does not apply for X-bracing, only for chevron. So my question is whether a 2-story X-bracing is considered as chevron bracing for the slenderness check.

 

[JoshPlumSE]

I was going to respond that I didn't think it needed to be considered a V brace. But, then I found this:

AISC 341-2005, section C13.1 figure C-I-13.3 I think you will see your type of configuration listed as an X braced frame. Though the text of the commentary suggests that the behavior should be consistent with a V braced system

To me "behavior consistent with a V braced system" suggests that you need to design it to the V braced provisions inluding that KL/r restriction.

 

[racookpe1978]

Would it not depend entirely on whether or not the "cross" of the "X" (thus turning it into two "V" braces) is actually attached rigidly to the mid-level structural framing between 1st and 2nd floor? No attachment = no V.

 

[jtx]

"Yes, but the 4sqrt(E/Fy) slenderness limit does not apply for X-bracing, only for chevron"

True. But consider how a two-story brace is formed, by mirroring a chevron brace about a beam so that it forms an "X" over two-stories. In an X-braced frame, where the "X" is within each story, the effective length can be cut in half because you could suppose that the braces are supported at their midpoint, assuming that they are connected together. In a two-story configuration, you don't have the ability to do that, so slenderness becomes a concern because you don't want your braces to buckle before they yield. I would say the provision for cheverons would still apply because of this.

 

[amendale]

I think the point of the slenderness provision for chevron bracing is to prevent premature buckling of the compression brace and reduce the unbalanced load onto the supporting beam. However in a two-story X bracing system, even if the compression bracing buckles, there is no unbalanced load onto the beam due to dual tension bracing on both sides of the beam. So this made me think that the slenderness provision for chevron bracing could be disregarded for two-story X. However I wanted to find definitive support for this assumption. Anyone know any documents that refer to this particular case?

 

[SAIL3]

my understanding agrees with amendale...for V-brace, if the compression brace buckles then the tension brace would impart a significant load into the horizontal bm....whereas in the X-bracing this unintended load can be reacted by the bracing above the bm...