AISC 341 SCBF Slenderness Requirements

[ReginaldBoigard]

AISC has limits on slenderness for braces in Specieal Concentrically Braced Frames. Below is section 13.2a.

"Bracing members shall have Kl/r < 4*(E/Fy)^.5

Exception: 4*(E/Fy)^.5 < Kl/r < 200 are permitted in frames in which the available strength of the column is at least equal to the maximum load transferred to the column considering Ry (LRFD) or (1.1.5)Ry (ASD), as appropriate, times the nominal strengthes of the connecting brace elements of the building. Column forces need not exceed those determined by inelastic analysis, nor the maximum load effects that can be developed by the system."

I am looking to use the exception but am having a hard time interpretting the paragraph. I take it that the columns must be designed for the column loads plus some resultant force that would come from the brace failing. The load that would make the brace fail must take into account an overstrength factor and does not use a phi factor. Is this interpretation correct, and if so, would we use the tension or compression brace failure force?

Thanks.

 

[ishvaaag]

The last line you quote is some waiver for the column and a `precision for the simplified method at the same time. In the column, it is said, you need not sum brute force the effects of all the braces at overstrength, just to the extent determined by "inelastic analysis". Would have to read the pertinent section to see if consideration of inelasticity by some of the simplified methods that account for material and geometrical nonlinearities is enough, but SHOULD be the case for SHOULD be safe substitute for the more exact methods.

And if column forces need not to exceed those determined by "inelastic" analysis, they can be directly taken from one such analyisis, or when used, the exception becomes a general waiver.

For, in the second part, is indicated that you may follow the hypothesis when evaluating the maximum load effects, compression at the brace when compression, and tensile action when tensile, then ... that is what a set of hypotheses does, locate the worst case, at which moment you'll have identified "the maximum loads that can be developed by the system".

So as soon as you analyze inelastically with a proper set of hypotheses, the waiver is general. And if "direct analysis-like" Appendix 7 of 360-05 methods are considered in a per member and general basis as a safe substitute of the more exact "inelastic" methods, what is not unreasonable since of general applicability, the waiver would be BEING general.

However, it is likely the note is asking for the specific requirements in Appendix 1 of 360-05 for members and other structural parts be present to enable the waiver effective.

 

[Gumpmaster]

I think it's refering to the tension capacity of the brace RyFyAg as shown in 13.2.b. Take the vertical component of this and use it as a column load.

If you look in the commentary, it says that an overstrength factor times the actual applied load isn't good enough, so you have to apply Ry times the capacity, FyAg.