How to define the LZ value for the Rafter shown below?

Question

My first question is, do I need to define the value of LZ for the rafters since they will experience compression? 
 If so, what should be the correct value for LZ? Is it going to be 12.948m and 11.558m respectively or add 12.948 + 11.558 for the rafters? 
 By the way, I'm using AISC LRFD as my design code.

Sye · Accepted Answer

The LZ would be the rafter length. I would think using 12.948 m or 11.558 m, as appropriate, should be OK. LY would be spacing between purlins or lateral braces if any. The LY and LZ are unsupported lengths used for flexural buckling and would affect the axial capacity. However to specify the laterally unsupported lengths for the compression flanges (LTB), you need to assign UNT or UNB parameters in addition.

Madhan Elumalai · Answer

Hi Ian, The LZ value is required to calculate the slenderness ratio and you have to specify the total span length as the value of LZ. In addition to this you have to use the LY (unsupported length of the compression flange in the lateral direction, usually the spacing of purlins for rafters) as well as UNT and UNB values for calculating the bending strength as well. regards, Madhan

Sye · Answer

Just bumped into this last question when trying to refer an user to this forum post. If the beam is horizontal, definitely we should use the column to column distance. However for a gable frame like the one in question, I would think, it would be very conservative to consider the LZ as the distance between the columns. Again this is my opinion and practicing engineers, who deal with structures like these on a day to day basis, will be able to offer more insight.