The paper presents a means of determining the non-linear sti!ness matrices from expressions for the "rst and second variation of the Total Potential of a thin-walled open section "nite element that lead to non-linear sti!ness equations. These non-linear equations can be solved for moderate to large displacements. The variations of the Total Potential have been developed elsewhere by the authors, and their contribution to the various non-linear matrices is stated herein. It is shown that the method of solution of the non-linear sti!ness matrices is problem dependent. The "nite element procedure is used to study non-linear torsion that illustrates torsional hardening, and the Newton}Raphson method is deployed for this study. However, it is shown that this solution strategy is unsuitable for the second example, namely that of the post-buckling response of a cantilever, and a direct iteration method is described. The good agreement for both of these problems with the work of independent researchers validates the non-linear "nite element method of analysis. Figure 8. Flow chart deploying the direct iteration method of the value of to be used. After convergence on and , the results are recorded and a similar process starts again for the next factor.It can be seen that the eigenvector of a stability analysis is needed in order to build the metric matrix [G]. The most accurate eigenvector would be that which resulted from a quadratic stability analysis [2], in which the e!ects of the initial bending curvature are taken into account. The self-weight of the structure may also need to be incorporated into the stability analysis, and 548