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Re: AISC Seismic Provisions for SCBFs

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Keep in my the seismic forces you've calculated have already been reduced by Rw
to account for an inelastic response.  For the building to remain truly elastic
at code level seimic events, you would have to use an Rw = 1.0.  Now, should
wind forces still be larger than seismic forces calc'd with Rw=1.0, then I think
you could argue the building is truly designed to remain elastic during a code
level seismic event and elastic detailing could apply (though I still don't
recommend it for high seismic zones).

Daniel J. Huntington
Structural Engineer

KJWW Engineering Consultants, P.C.
623 26th Avenue
Rock Island, IL 61201
PH: (309) 788-0673
Fax: (309) 786-5967


Connor, John A NWK wrote:

> I appreciate all of the responses to this subject.  All of the responses
> helped to clarify some the terms and wording used in the AISC codes.  I
> understand the concept of having the brace yield before the connection,
> however, I don't understand why I have to meet certain criteria when I
> expect an elastic response versus a deformation response.
> For example, for most of the Midwest or areas of low seismic forces, wind
> forces would generally "control" over the seismic forces.  Since my wind
> forces control, my members are designed for elastic responses.  These
> members wouldn't be able to reach deformation levels since my seismic forces
> are so low.  Since I expect the members to remain elastic, I believe these
> members would be considered "force-controlled" members rather than
> "deformation-controlled" members.
> I originally thought my problem was determining the strength of the
> connection ("brace strength" vs. "transferred by the system").  Now that I
> know what "transferred by the system" really means, I find a problem
> accepting the criteria for the limiting KL/R ratios.   The member I'm using
> for the brace was not selected based on its capacity.  The member was
> selected based on the limiting KL/R ratios.  If the KL/R ratios wasn't so
> limiting, I wouldn't need a TS10x6.  A smaller TS size would be satisfactory
> with regard to capacity.
> I've read through the commentaries and the responses to this list concerning
> the concept behind why the ratios are limiting, and I agree with the concept
> behind it.  However, since I'm expecting elastic responses from my system,
> why do I have to meet the KL/R criteria, when the criteria applies to
> inelastic responses?
> Is there some "fine print" somewhere that says the criteria is only
> applicable to deformation/inelastic responses?  At the beginning of AISC's
> seismic provisions, it says that the criteria applies to seismic design
> categories D or greater.  My building happens to be category "D" so the AISC
> criteria apparently applies.
> John Connor, EIT
> Kansas City, MO