TABLE OF CONTENTS The solutions obtained in Sect. 3.8 for the flexural-torsional buckling of beam-columns are only valid while they remain elastic. When the in-plane loading on a beam-column causes prior yielding, the effective rigidities of some sections of the member are reduced, and buckling may occur at a load which is significantly less than the elastic buckling load, as indicated in Fig. 3.36. An example of this is given in Fig. 3.37 (Galambos and Fukumolo, 1966), in which are shown the variations with the major axis slenderness L/ r x of the elastic critical moment (the dotted line) and the plastic critical moment (the full line). Also shown is the variation of the maximum in-plane moment (the dashed line).
A detailed study of the flexural-torsional buckling of plastic beam-columns was made by Lim and Lu (1970). They first reviewed previous research and then conducted a systematic theoretical investigation of the plastic buckling of a range of restrained isolated beam-columns under various combinations of axial load and unequal end moments. They concluded that the effects of the prebuckling deformations and of minor axis flexural restraints were important for long beam-columns, and that the effects of warping restraints were only important in short beam-columns.
Lim and Lu's theoretical work was based on lower bound tangent modulus solutions obtained by assuming that the modulus of elasticity is equal...
