A Controversy and Its Settlement in the Calculation of Buckling Moments of Thin walledBeams with Monosymmetrical I sections Under Distributed Loads

There exists two different formulae at present for calculation of lateral torsional buckling moments of thin walled beams with monosymmetric I sections, one being traditional formula on which the structural steel Codes GBJ17-88 and GBJ18-87 are based, the other is the newly proposed one. This paper compares the stability theories from which these formulae were derived. New questions are presented for the newer theory. In order to answer these questions, this paper presents a buckling analysis of I sectional beams based on the stability theory of shells, which is absolutely superior to the member theory because fewer assumptions are used to establish the shell buckling theory. A total of 36 I sectional beams have been analyzed using the universal FEM program ANSYS, among which 12 have doubly symmetrical sections, 12 are tensile flange strengthened and 12 are compressive flange strengthened. The element SHELL63 is used in the analysis. The shell buckling analysis verifies that the traditional formula, but not the new one, seems to be more rational. This paper presents a new formulation of the lateral torsional buckling theory of thin walled beams under transversely distributed loads. The proposed theory, which is in good agreement with the results of shell buckling analysis, verifies the traditional formula and makes the traditional theory more perfect.