Study On The Elastic Distortional Buckling Of Cold-Formed Thin-Walled Lipped Channels With Inclined Simple Edge Stiffeners

The distortional buckling is one of the important buckling modes for high strength cold-formed thin-walled steel open cross-section members. It may become the dominating buckling mode and control the ultimate load-carrying capacity of members in some conditions. With the cold-formed thin-walled steel members tending to higher strength, thinner thickness and more complicate section shapes, the distortional buckling is receiving more attentions in recent years.Reseachers worldwide have studied the local buckling and the overall buckling for a long time and reached a significant conclusion. Howerver it is a relatively short period of time since the distortional buckling was recognized. And deeper study of the distortional buckling remains to be made. Mature computation methods for the distortional buckling are of rare occurrence in the present. There are no specific clauses for the calculation of distortional buckling in current Chinese national specification'Technical Code for Cold-Formed Thin-Walled Steel Structures" (GB50018-2002, abbr. GB50018). The Direct Strength Method (DSM) is a new design method of the cold-formed thin-walled steel and has been adopted in North America and Australia region. First the value of elastic distortional buckling of element must be calculated. Then the design value of distortional buckling is solved by introducing the value of elastic distortional buckling into the formulae of DSM. As a kind of computing method to be perfected DSM merits further investigation. Its major feature is to calculate the distortional buckling.The calculation of the elastic distortional buckling is the key for DSM and is the important subject in this paper. The current good method of calculating the elastic distortional buckling is numerical method such as the finite strip program of CUFSM. Proposed by scholars to calculate the elastic distortional buckling, approximation model method has been adopted by some foreign standards. As the computation process is iterative, approximate model method is inconvenient.Generalized Beam Theory (GBT) can be used for solving the elastic distortional buckling. But the calculation is a bit complicated, which holdbacks the application of GBT.The cold-formed thin-walled lipped channel steel has been applied widely in practical engineering.Based on GBT and related research data, this paper studies the elastic distortional buckling of cold-formed thin-walled lipped channels. The main contents are as follows:(1) The formulae of the elastic distortional buckling for cold-formed thin-walled channel columns with inclined simple edge stiffeners under axial pressure are derived. Except the calculating model of rigid body displacement not including the flange transverse bending,two new kinds of calculating model with the inclusion of the flange transverse bending are given, ie, the calculating model of hinged support and the calculating model of restrained support. The intersection of flange and stiffener is viewed as hinged support in the calculating model of hinged support. The intersection of flange and stiffener is regarded as restrained support in the calculating model of restrained support. Based on the three kinds of calculating model, the calculation formulae for the elastic distortional buckling of cold-formed thin-walled inclined lipped channel columns have been established respectively.The validity, accuracy and efficiency of the method are demonstrated by comparing the results from the presented formulae and the ones from existing data, other computing methods.It shows that the formulae are accurate and suitable for engineering application. The smaller value obtained from the calculating model of hinged support considering transverse bending and the calculating model of rigid body displacement is proposed in terms of safety.(2) The formulae of the elastic distortional buckling for cold-formed thin-walled pure bending channel beams with inclined simple edge stiffeners acted by moment in the weak plane are derived. Except the calculating model of rigid body displacement, this paper presents two new kinds of calculating model of elastic distortional buckling of cold-formed thin-walled inclined lipped channel beams, ie, the calculating model of hinged support and the calculating model of restrained support. And the related calculation formulae have been derived respectively.It is shown through the compared calculation that the value obtained from the calculating model of rigid body displacement is a bit larger than that obtained from other methods. The value obtained from other two calculating models is very close to that obtained from other calculation methods.So it is recommended that the calculating model of hinged support considering transverse bending be applicable for the elastic distortional buckling of pure bending channel beams acted by moment in the weak plane.(3) The linear formulae of the elastic distortional buckling for cold-formed thin-walled channels with inclined simple edge stiffeners subjected to eccentric loading in the symmetry plane are derived. Based on the formulae described above, the linear formulae are obtained. The calculated results indicate that the linear formulae are higher precision.The correctness of the above formulae is verified through the channel sections of the Appendix B.1.1-4 of GB50018. The results show that the presented formulae suit China's national conditions.The simple formulae in this paper are applicable to calculate by hand. The formulae are not only independent on the dedicated computer program but also avoid the redundant calculations. There is no need for the iterative calculation by the formulae. They are easy to apply, graspable easily for the disigner and may be directly used in practical design and incorporated into future design codes and guidelines. Therefore the formulae have the practical significance and will accelerate the utilization of DSM.Based on GBT and related research data, the corresponding mechanics computational models have been proposed, the practical formulae under different loading states have been developed and analyzed in this paper. Compared with other methods, the calculation method proposed in this paper is proved to be reasonable and verified.