A unified solution for various boundary conditions for the coupled flexural-torsional instability of closed thin-walled beam-columns

Abstract

The problem of coupled flexural-torsional instability of closed thin-walled beam-columns under the combined action of axial loads and equal end moments is studied numerically. The characteristic equations for stability are obtained and the load interaction curves and mode shapes are plotted for various boundary conditions. From a study of the numerical results and the characteristic equations, equivalent load and geometry parameters are defined. These yield a single solution common for all boundary conditions for the ioad interaction curve and the mode shape. The effect of various boundary conditions and mode numbers is manifest only in the value of an eigen coefficient independent of the loads which is used in defining the equivalent parameters. The case of lateral instability within the elastic limit is also examined and it is shown that even when tensile axial loads are present elastic lateral instability may occur.