Design of double back to back channel in staad

Hi,

I'm modeling aluminum truss member-diagonal in STAAD. I'm using double back to back channel with bolt in the middle of the length.
First, i'm not sure whether k is 0.5 and in which direction(strong or weak) i have to apply it . Second, in the design input for aluminum design i'm getting very close allowable stress no matter i modified KY=1(allowable stress=4.77ksi) or KY=0.5(allowable stress=4.99ksi).
Any advise?