Based on the buckling theory of thin-walled members and the total potentials of cold-formed Z and C purlins whose lateral restraints are ignored,the formulae of rotation and lateral displacement of the purlins were obtained under wind suction on the principle of variation.Considering the nonlinear strain energy generated during flexural-torsional buckling by the stress before buckling,the expression of critical moment of Z and C purlins was obtained on the basis of stationary value of total potential energy.The rotation,lateral displacement and buckling moment were compared with numerical results using shell elements of ANSYS and found to be in excellent agreement with ANSYS.
