Lateral-torsional Buckling Of Cold-formed Purlins With Two Or Three Anti-sag Bars Under Wind Suction

Cold-formed purlins, as the main supporting members in the lightweight steel roof system, are very important to the roof safety. In recent years, overall collapse accidents of the lightweight steel roof system are frequently reported, which caused the attention of many scholars. Therefore, the lateral-torsional buckling of cold-formed purlins, as the main supporting members, has become one of the urgent topics. Purlins are very prone to fail by the local buckling, distortional buckling, lateral-torsional buckling and their interactive buckling under wind suction. The lateral-torsional buckling of cold-formed purlins is investigated and a practical calculation method is presented in this paper.The common cold-formed purlins are C-or Z-sections. The torsional rigidity is small, so they are very prone to lateral-torsional buckling. The lateral-torsional buckling capacity is related to parameters of purlins and the lateral restraint. The anti-sag bars are frequently adopted to enhance the lateral stiffness of cold-formed purlins. The anti-sag bars are treated as the rigid lateral support in the existing calculation methods, but according to the studies, the efficiency of anti-sag bars may be significantly reduced by the local deformation of purlin web near the location connecting the anti-sag bars. It is necessary to investigate lateral-torsional buckling of cold-formed purlins with anti-sag bars under wind suction using the effective stiffness of anti-sag bar.Lateral-torsional buckling of cold-formed purlins in roof system of self-tapping screw connecting is investigated taking the effective stiffness of anti-sag bar into account. To doing so, the total potential for the lateral-torsional buckling of C-and Z-section purlins with two or three anti-sag bars is developed based on the previous studies and it is verified by ANSYS shell element analysis. Then the calculation solutions for the lateral-torsional buckling load are developed by using Ritz approach. The calculation solutions are excellent agreement with finite element solutions.