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Stitch plate in double angles

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I designed a double angle knee brace.  The one supplied was one thickness size smaller than the one I specified.  The brace has already been erected.  The contractor has asked me if he could add stitch plates to reduce the unbraced length so that the smaller on will work.  As I see it, the stitch plate doesn’t do anything for increasing the strength of this member.  The buckling strength of the double angles (3x3x1/4) based on the overall length of the 2 angles together in the weak axis, not the individual angles (page 1-77 of AISC 9th ed.).  The stitch plates do not restrain the buckling behavior of the overall member.  It is still pinned at each end and will buckle in a single mode shape about the weak axis.  I see stitch plates as being spaced at whatever spacing is necessary to force the overall member to buckle before an individual angle will buckle.

 

Am I looking at this right?

 

 

Rich Lewis