Interaction Buckling and Postbuckling in the Distortional Mode of Thin-Walled Sections (No. R870)

The buckling modes of cold-formed thin-walled sections with edge stiffeners are generally in the forms of the short half-wavelength local buckle, the intermediate half-wavelength distortional buckle and the long half-wavelength flexural/flexural-torsional buckle. These buckling modes usually occur at distinct lengths. However the possibility of the interaction of buckling modes may be present at certain lengths. This may be due to the distortional mode interacting with the local buckling mode, both of which may be in the post-buckling range. Local buckling has been well researched and accounted for in design standards with an effective width model developed by Von Karman et al (1932) to produce a simple model of the post-buckling reserve strength. Distortional buckling research has made much headway in recent years at the University of Sydney by Hancock (1985), Lau and Hancock (1990), Kwon and Hancock (1992) and Yang and Hancock (2004), at Johns Hopkins University by Schafer (2002) and the Technical University of Lisbon, Portugal by Silvestre and Camotim (2004). What has been known for some time is that the distortional mode has a post-buckling reserve strength which is generally less than that of local buckling. However, the nature of this post-buckling reserve is not clearly understood, particularly what precipitates failure. Some research has been done to understand the buckling mode interaction of local and distortional. This is usually carried out by analytically separating the combined modes into its composition of individual modes. Generalised Beam Theory (GBT) uses explicit analytical expressions to analyse the influence of local and distortional buckling modes. Another method currently being developed is modal identification and decomposition, where a numerical method is employed to calculate the critical loads for the pure buckling modes. The paper discusses the analysis of post-buckling in the distortional mode of a thin-walled section with edge stiffeners and the effect of interaction of buckling modes on failure loads. The analysis is based on the longitudinal stress development and redistribution using the finite element package ABAQUS. This methodology whereby the longitudinal stress redistribution is studied is similar to the work of Von Karman et al, in predicting the post-local buckling behaviour