Proceedings home preface contents authors keywords copyright reference ©2012 Civil-Comp Ltd				Paper 167 Effects of Approximations on the Lateral-Torsional Buckling and Postbuckling Analysis Y.-L. Pi and M.A. Bradford Centre for infrastructural Engineering and Safety, School of Civil and Environmental Engineering, The University of New South Wales, Sydney, Australia Keywords: approximations, lateral-torsional buckling, postbuckling, rotation, second order. full paper (pdf) - reference In the development of a finite beam element for the lateral-torsional buckling analysis of structures, a rotation matrix is usually used to derive the nonlinear strain-displacement relationship. Because of the coupling between displacements, twist rotations and their derivatives, the components of the rotation matrix are both lengthy and complicated. To facilitate the formulation, approximations have been used to simplify the rotation matrix. A simplified small rotation matrix is often used in the formulation for the lateral-torsional buckling analysis of structures, while a second order rotation matrix is used in the formulation for the postbuckling analysis. Because approximations are made in these rotation matrices, these rotations do not satisfy the orthogonality and unimodular conditions. As a result, the approximations may lead to a loss of some significant terms in the strain-displacement relationship. Without these terms, the rigid-space body motion cannot be separated from the real deformations. The superimposed rigid body motions may lead to the development of self-straining, which may in turn affect significantly the prediction of the lateral-torsional buckling loads and the postbuckling behaviour [1,2]. This paper investigates the effects of approximations on the elastic lateral-torsional buckling analysis and on the postbuckling analysis. It is found that the finite beam element based on the small rotation matrix cannot predict correctly lateral-torsional buckling loads of thin-walled beams in many cases, and that the finite beam element based on the second order rotation matrix will predict over-stiff postbuckling responses. References 1 Y.-L. Pi, M.A. Bradford, B. Uy, "A spatially curved-beam element with warping and Wagner effects", International Journal for Numerical Methods in Engineering, 63, 1342-1369, 2005. 2 Y.-L. Pi, M.A. Bradford, "A rotation matrix for 3D nonlinear analysis", Australian Journal of Mechanical Engineering, 3(1), 31-38, 2006.