Experimental And Theoretical Study On Distortional Buckling Of Cold-Formed Thin Wall Steel Lipped Channel Columns Under Compression

With the cold-formed thin-wall steel members tending to higher strength, thinner and more complicate shaped sections, distortional buckling has become an important research topic recently in the field of international research of cold-formed steel structures. However, up to now it is far more from mature and systematical for the researches on the problems such as the occurring conditions of distortional buckling, the simplified calculating method of critical elastic distortional buckling stress and the interaction between distortional-local buckling and distortional-overall buckling.Few specific experimental as well as theoretical studies were carried out on the distortional buckling behavior of axially compressed columns in China and so the corresponding references are very scarce. Therefore no relative design rules exist in current Chinese Specification GB50018.With the emergence of distortional buckling, a new calculating method for cold-formed steel members' bearing capacities, Direct Strength Method (DSM) is studied significantly abroad. But the corresponding researches in China on DSM just starts and few people concentrate on the problem of the difference between effective width method and DSM and whether DSM can be used to clarify the occurring conditions of columns' distortional buckling.In the light of the above research status, in this paper a series of valuable problems /issues are analyzed and studied by test, numerical simulations and theoretical analysis systematically and profoundly, such as the behavior and calculating method of the columns' distortional buckling, the occurring conditions of the columns' distortional buckling and the simplified formula for calculating the elastic critical distortional buckling stress etc.In order to study distortional buckling and its interactions with local and overall buckling, a total of 16 specimens with 550MPa high strength steel lipped channel cold-formed thin wall columns under axially compression were tested between fixed ends. Three different flange-distortional buckling modes which influence the columns' bearing capacities are found. These phenomena and results of the test thus provide the clues and verifications for a proper calculating formula of columns' distortional buckling strength.A further finite element analysis of the distortional buckling behavior of specimens is carried out. First of all, a finite element model was established using ANSYS which could consider geometric and material non-linearity. Then, the phenomena and results of the test are used to verify the correctness of the finite element simulation model. Finally, lots of finite element parameters are analyzed to investigate whether the factors such as the three different flange-distortional buckling modes, the size and shapes of the initial geometric imperfections and the length of the members etc might impose an effect on the bearing capacities of the columns failed from the distortional buckling.Based on the comparisons of test and finite element analysis results with calculated results by DSM, it is found that the distortional buckling does have an interaction with overall or local buckling and such interaction has an adverse effect on columns' distortional buckling strength. The formula D+E in DSM could be used to consider the interaction of distortional and overall buckling, but it is unconserative for the interaction of distortional and local buckling. Therefore, a calculating formula for the interaction of distortional and local buckling is proposed in this paper. The formula is verified by the results of test and finite element analysis.The occurring conditions of columns' distortional buckling are analyzed and studied thoroughly by DSM and finite strip software CUFSM. The new concepts of sectional distortional sector and distortional degree are proposed and used to judge whether the column distortional buckling may occur. It is pointed out that the members' distortional buckling conditions lie on the section firstly and then the length of members. The difference of f_ol and f_od namelyΔ_ld=f_ol-f_od is used to evaluate the sectional distortional degree and the factors influencing the differenceΔ_ld=f_ol-f_od are also analyzed. The conditions forΔ_λ= 0 are put forward and the three most possible cases that the columns' distortional buckling may occur are specified as: low a/t and h/b, outward inclined lip and the small stiffeners in web or flange.Finally, the formula which is similar to the formula calculating elastic local buckling stress is proposed to calculate the elastic distortional buckling stress. And it is found that the flange distortional buckling coefficient k_d^f in the formula has a linear relation with a/t and h/b. So a simplified formula for calculating the elastic distortional buckling stress is provided by the analyzing results of the finite strip software CUFSM.