Column Design for Special Concentric Braced Frames

[davidfi]

I am working on a plancheck for a SCBF design I did and have a question about column design forces. I designed my columns based on forces developed from a mechanism analysis per AISC 341-10 Section F2.3(I) and F2.3(II). However, I received this plancheck comment:

"Column Designs: Axial load combination per ASCE 7 Section 12.4.3.2 not checked with Omega effect and second order effects".

AISC 341-10 Section F2.3 Exception 2a states: "The required strength of columns need not exceed the least of the following: The forces determined using load combinations stipulated by the applicable building code including the amplified seismic load, applied to a building frame model in which all compression braces have been removed."

I interpret this to mean that I must design the columns for a mechanism analysis, but the design forces need not exceed Omega x seismic analysis load (i.e. design for the lower of the two). Is this correct? If so, I would think my design based on a mechanism analysis alone is conservative. Do you agree?

Thanks!

 

[Deker]

For SCBF, AISC 341 defines the "amplified seismic load" to be the forces corresponding to mechanism analysis. That mechanism analysis involves two separate load cases, one in which the compression brace reaches it's expected strength prior to buckling, and one in which the compression brace reaches it's post-buckled strength (taken as 0.3Pn). The exception you referenced is to simplify the post-buckled analysis. So rather than assigning each compression brace a force of 0.3Pn, you could assume that force is zero i.e. compression brace removed.

As for your last statement, that wouldn't be correct for SCBF because AISC requires a mechanism analysis as described above. However, for other elements like collectors your statement would be correct. The only caveat is that this assumes a reasonably proportioned lateral system. If you analyzed your brace for code-level forces and arbitrarily decided to make your brace 10x stronger than it needed to be, it wouldn't be sufficient to design collectors for an overstrength factor of 2 or 3 even though the code would permit it.

 

[davidfi]

In my original design, I only checked the column using the amplified seismic load based on the mechanism analysis. As I understand it, I do not need to check the columns for an Omega load as the planchecker suggested. Is this correct?

 

[Deker]

Yes, that’s correct because AISC 341 redefines the amplified seismic load Ω[sub]0[/sub]E for SCBF as the force resulting from the mechanism analysis.