Many of the structures most commonly designed by the plastic method contain members that are subjected to high bending moments and low axial forces. Pitched-roof portals used in industrial buildings contain both rafters and columns that are examples of members subjected to load effects of this type. One of the requirements of plastic design is that the moment magnification in the member due to axial force should not cause premature failure prior to the development of the collapse mechanism of the complete structure.
Currently available column deflection curves do not adequately cover the case of low axial forces. The analytical procedure for the determination of limiting slenderness in the plane of bending with various end conditions and axial loads is described in this paper. It is concluded that the limiting slenderness ratio for beam columns with an axial load of less than one half of yield load can be represented by a single empirical formula, which has been adopted in SABS 0162.