2 story x brace

huangyhg

2 story x brace
when using a 2 story x brace (scbf), is there any need for the upper story to be as strong as the lower story.
if only the lower story had a chevron brace, the 2nd floor beam would need to be designed to take the unbalanced load.  when we add an inverted chevron brace on the 2nd floor, then the beam does not need to be designed for the unbalanced load.  so, does the same size brace need to be used on both the 1st and 2nd floors?  do you have any references to back up your opinion?
i'm using the 2005 aisc seismic spec.
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your bracing   
good point, but my unbraced lengths are generally pretty short, 25ft or less.  most of my brace designs are governed by load.  thanks though.