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Lateral brace for a SCBF beam

Lateral brace for a SCBF beam

(OP)
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.

RE: Lateral brace for a SCBF beam

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.

RE: Lateral brace for a SCBF beam

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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