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Rafters are subject to axial loads. I don't have practical calculations in hand, the attached scanned pages from a book may help you. Untitled_29082018_173346.pdf

I am not sure what the reference is but this formula should be for a member being used as column, not as beam. Beam would never have significant axial load (generally speaking) so why use this formula?

@ MA123 This factor will depend on end condition of the member. Increasing to 1 will reduce axial compression capacity for buckling. @ Ayesha I was referring to axial compression capacity for buckling (Ncy) required for checking the member for combined actions.

Yes purlins and girts provide lateral restraint to rafter and column, respectively.. Rafter · For calculating minor axis compression capacity, the effective length of the rafter is taken as the maximum spacing between the purlins. · When the rafter’s top flange is in compression the effective length of the rafter is taken as 0.85 x purlin spacing for calculating out of plane capacity. Column · For calculating minor axis compression capacity, the effective length of the column is taken as the spacing between the girts. · When the column’s outside flange is in compression the effective length of the column is taken as 0.85 x girt spacing for calculating out of plane capacity.