TABLE OF CONTENTS It is convenient to discuss the behavior of beam-columns under the three separate headings of in- plane behavior, flexural- torsional buckling, and biaxial bending. When a beam-column is bent about its weaker principal axis, or when it is prevented from deflecting laterally while being bent about its stronger principal axis [as shown in Fig. 3.3(a)], then its action is confined to the plane of bending. This in-plane behavior is discussed in Volume 1. When a beam-column which is bent about its stronger principal axis [as shown in Fig. 3.3(b)] is not restrained laterally, then it may buckle out of the plane of bending by deflecting laterally and twisting. This flexural-torsional buckling is discussed in this and the following sections. More generally, a beam-column may be bent about both principal axes and twisted, as shown in Fig. 3.3(c). This biaxial bending is discussed in the other chapters of this volume.
A simply supported beam-column which is bent about its major axis by end moments M x and ?M x and compressed by an axial force P is shown in Fig. 3.3(b). The beam-column supports prevent both lateral deflection and twist, but the flange ends are free to warp. It is assumed that the beam-column is elastic and of uniform doubly symmetric I-section, and the particular case is considered for which the end moments are equal and opposite ( ? = 1).
When the applied...
