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RE: x brace question

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In a typical x-brace where one brace is in compression and one in tension, it would seem that the tension member could "brace" the out of plane buckling of the compression member.  In order to buckle over its entire length, the compression member must displace at its midpoint.  In order to displace, because of deformation compatibility with the tension brace (assuming they are connected), wouldn't the effective radius of be the combination of both members, i.e., ry1 + ry2?  That would have the same effect as using a k=0.5 out of plane (if the members are the same size which they usually are).

In the theoretical case where compression members are fixed-fixed, the LRFD uses k=0.65 rather than k=0.5 as a recommended effective length factor.  By the same reasoning, it would make sense to use k=0.65 for out of plane buckling of concentric x-braces.

Maybe this is where Naeim gets his k=0.69 ( he actually uses 2/3 which could be interpreted as a rough approximation of 0.65)

In any case, if one were to use the out of plane bracing ability of the tension member, be sure the connection between members can handle the forces generated. 

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