Nonlinear Stability Analysis of Simple Steel Frames with Load Imperfections

A simplified nonlinear stability analysis for moderately large rotations and small strains is performed on a rectangular two-bar frame subjected to a concentrated eccentrically applied joint load. Such a simplification consisting of adopting linear kinematic relations leads to very reliable results for the initial postbuckling path in the vicinity of the critical point of the above imperfect frame. The existence of an asymmetric bifurcation point is thoroughly discussed and a direct evaluation of the bifurcational load is readily obtained. Using this technique the effect of imperfection sensitivity is also addressed. A qualitative analysis associated with the physical phenomenon yields a substantial reduction of the computational work. The efficiency and reliability of this approximate nonlinear stability analysis proposed herein is illustrated by means of several examples for which a lot of numerical results based on a more accurate nonlinear analysis are available.