Purlin design: Failing under slenderness ratio 3

[hamy08]

I am analyzing shed design on staad pro. I have released moments at the start and end of roof purlins. But still purlins are failing under slenderness. May be because the force is acting at the ends and the purlins are behaving like columns. Which is also true in real case as the purlins are supported between the rafters with bolts at the web of purlin and at upper flange of rafter beam.
Can any one explain me which type of end supports should be considered for purlins?
why it is failing under slenderness, however it should act like a beam?
Can we still use purlins even failing under slenderness ration?

 

[ajh1]

Ron247, the details you provided above are from Butler. The small angles were replaced a number of years ago by a 5" deep channel that braces both flanges at the same time. The channels have tabs that project through those same holes in the purlin web and have a drop pin that connects one channel to the next in line.

 

[Ron247]

Thanks ajh1, I remember years ago the braces as shown could be knocked out fairly easily and could fall out if someone bent the tabs up instead of down.