Want to Count the effect of purlin in PED steel shed design with out draw the purlin into the model.

Dear sir,

I model a PEB shed in staad. i don't model the purlin into this staad 3d model, but in design i want to count the effect of purlin.

i used AISC (ASD) method for designing. how i input the command in staad. i have to use UNT and UNB ?? or LY.

I also use flange braced into the rufter 2.6m interval for support the purlin. Purlin interval is 1.3m. please explain this issue.

Thanks

shafiq