INTRODUCTION Compression members with thin-walled open sections, because of their low torsional rigidities and because their cross-sectional configuration is such that the centroid and the shear center often do not coincide, can buckle at loads well below the Euler load by interaction of torsion and bending. Fast expanding applications of such sections in civil and architectural engineering call for an extensive investigation of torsional-flexural buckling. Basic theory of torsional-flexural buckling is treated by a number of authors, I,2 and simplifications of the basic theory in the elastic range for the design purposes together with some test results were reported previously. 3 Some investigation of inelastic torsional-flexural buckling of as-rolled, thick-walled I-beams by application of bi-axial moments has been reported. 6,7,8 However, there has not been sufficient investigation of axially loaded columns with thin-walled open sections in the inelastic range. All inelastic buckling problems are inherently nonlinear and non-static. In view of the even more complex nature of torsional-flexural buckling, simplifications of the basic theory for practical purposes are necessary. While the simplification may reduce the design procedure to an easier form, the computed values must also be reasonably accurate when compared to the actual buckling loads. This investigation, primarily semi-empirical, was conducted for the following objectives: 1. To develop and verify an approximate inelastic torsional-flexural buckling theory based on certain simplifying assumptions. 2. To recommend a practical design procedure and a formulation of code provisions against torsional-flexural buckling covering both the elastic and inelastic ranges. A series of tests on specimens of various cross-sectional shapes and dimensions was made to verify the basic theory. The test results agree with the predicted values within reasonable limits.


Civil, Architectural and Environmental Engineering

Research Center/Laboratory(s)

Wei-Wen Yu Center for Cold-Formed Steel Structures


American Iron and Steel Institute



Publication Date


Document Version

Final Version

Document Type

Report - Technical

File Type




Technical Report Number

Report No. 320A