| Cold-formed C-and Z-purlins are widely used in portal frame roof system. The wind suction often becomes the key gating factor for the design due to the light weight of the steel deck roof. The purlins are often liable to suffer torsional-flexural buckling under wind suction. When analyzing the torsional-flexural buckling of the purlins under wind suction, this paper makes an assumption that the shear stiffness of the sheeting is so strong that the lateral displacement of the top flange is prohibited and also ignores the rotation restraint supplied by the sheeting. The main contents include:(1) Based on the buckling theory of thin-walled members, applying the Ritz approach, the equation for the critical moment of the single span simply supported purlin without anti-sag bars under linear moment is obtained, and is calibrated with the numerical result that is obtained using shell elements of ANSYS with introducing the rigid profile assumption. The obtained formula is found to be in excellent agreement with ANSYS.(2) According to the strength calculation of purlins under wind uplift, applying the Ritz approach, the equation for the critical load of the two-span continuous purlin without anti-sag bars under uniform load is obtained, and is calibrated with numerical result that is obtained using shell elements of ANSYS with introducing the rigid profile assumption. The obtained formula is found to be in excellent agreement with ANSYS.(3) The thus obtained equations for the critical loads of the simply supported purlins under linear moment and two-span continuous purlin under uniform load are further improved by comparing with the results of ANSYS without the rigid profile assumption. The obtained formulae are found to be in excellent agreement with ANSYS.(4) This paper also analyzes the lip’s equivalent flexural rigidity of the channel purlin with ANSYS for the further study of the purlin’s distortional buckling and the calibrated formula is found to be in excellent agreement with ANSYS. |
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