Can you please explain with an example as to how one should define the unsupported lengths of members ( Ly and Lz)
In general, if the unsupported length is different than the length of the analytical member, one needs to define Ly, Lz (or Ky, Kz) accordingly.
For demonstration purposes a truss model is attached. The bottom chord of the truss is broken into segments by the in-plane ties/struts. So there is no need to define the unsupported lengths for the in-plane bending (Lz) as the segment lengths correctly represent the unsupported lengths. If defined, the LZ should be set to the segment length which is 7.5 ft ( not required as that is the default anyway ).
As far as out of plane bending of the bottom chord is concerned, let us assume that there are restraints at the end and at the midpoint of the bottom chord. However since the restraint normal to the plane of the truss is NOT modeled, this information needs to be communicated to the software through the design parameter LY so that design correctly accounts for that. The entire length of the bottom chord ( consisting of members 20 to 23 ) is 30 ft. So all the bottom chord members need to be assigned a LY of 15 ft as shown next.
LOAD LIST 3
LY 15 MEMB 20 TO 23
TRACK 2 ALL
CHECK CODE MEMB 20 TO 23