unbraced length 1

[RainT]

In truss, the top chord, I have 1.2M sp of purlins, can I use that as my UNL=1.2 and Ly=1.2? or my Ly is the default length?

 

[RockEngineer]

I usually like to separate my K and L when figuring buckling factors but you don't have to. I put in my L as the actual unbraced lenth of the item which for a truss is probably different for LY and LZ. This becomes a real problem when you have multiple nodes in between your bracing points because the program defaults to LY and LZ as the actual distance between nodes unless you tell it different. You add in your end constraints by KY and KZ

 

[structuresguy]

Well, depending on the loading, I usually define unbraced length for top flange and bottom separately. For instance, UNT=1.2 and UNB=span. For roof beams, or top chords of trusses, uplift may govern the design due to longer unbraced lengths of the bottom flange of the beam. THe purlins will brace the top flange, but unless kickers are used, they do nothing to brace the bottom flange. So if you have a net uplift, or end fixity at panel points (like a continuous top chord of truss), I suggest using UNT & UNB instead of UNL.

Also, if you are going to use your purlins to brace the top chord, you will have to design the connection and the purlin itself to handle the axial force needed to provide bracing. Traditionally this has been 2% of the compression force. However, I think I have heard more recent studies which show that less force (say 1%) would also be acceptable. I always use 2%, since that is what I know.