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Lateral brace for a SCBF beam 1

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ash060

Structural
Joined
Nov 16, 2006
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Working on a project that uses special concentric braced frames, and trying to figure out a way to avoid putting a lateral brace at a beam that has chevron bracing coming into it. The provisions require that a brace be provided at a minimum where the braces intersect the beam, unless the beam has sufficient out-of-plane stiffness and strength to not need it. There is a user note that explains how to demonstrate whether the beam has the sufficient out-of-plane stiffness to not need bracing. It states to apply the bracing force defined in Appendix 6 of the steel code to each flange and create a couple. Then the stiffness of the beam with this torsional load and the flexural load should be sufficient to satisfy Equation A-6-8 in the code, which applies to nodal bracing. Not sure why it would be applied as a torsional load and that compare to equation that is looking at the flexural stiffness of the beam. Anyone have some guidance it would be much appreciated.
 
Could it be that the torsional couple is to be adjusted such that both flange forces point the same way prior to comparison with weak axis flexural stiffness? I don't have access to the literature today buy that would seem rational to me. Analogous to checking beam stresses under torsion with the bi-moment method.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.
 
You are miss reading A-6-8, it is not flexural stiffness of the beam but stiffness of the brace. Your beam now has to have sufficient stiffness to act as a brace for both torsional and flexural loading. For some further guidance see example A6.6 from AISC Steel Construction Manual Design Examples v14.2, downloadable from AISC.
 
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