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In the calculation model of equivalent stabilizing force qd (shorterimperfection force) for both bracing system and purlins consistent with EN 1993-1-1 it has been conservatively assumed that the member to be restrained is uniformly compressed within its length by an axial force. This is incorrect when the actual distribution of the axial force is considered as nonuniform, having a parabolic shape and signvariable characteristic (compression and tension). Thus a different imperfection force is generated in comparison with this given in EN 1993-1-1. The actual imperfection loads of the upper flange of the restrained roof rafter being fully fixed to columns have been analyzed. The axial force in the restrained member changes parabolically along its length from tension at the support zone to compression at the central part. Values of tension axial forces Nsup and compression axial forces Nspan depend, among others, on the rigidity of the roof rafter, rigidity of columns and stiffness classification of the rafter to column connection. Parametrical analyzes of the imperfection forces together with evaluation of strains of purlins and bracing system versus axial forces Nsup, Nspan acting in the restrained member have been done. The result of presented analyzes is determination of the safe imperfection force and he strain of purlins and bracing system in different, real design situations and a comparison of obtained results with these based on the conservative model given by EN 1993-1-1.

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