Design of double back to back channel in staad

[gorgik]

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), Y is weak axis.
Any advise?

 

[a2mfk]

K depends on the axis you are considering, so it depends if you are supplying out-of-plane bracing for the truss. Usually for a truss, the chord member in-plane bracing is supplied by the web members and out-of-plane may be supplied by a diaphragm (ceiling, roof) or by individual bracing members. Take a step away from the computer program and think about it like basic column design. And you have to consider the whole structural system, not just the truss, but also the bracing system.