The non-linear analysis of stiffened plate structure is of growing importance in structural engineering, where design methods are frequently based on a knowledge of ultimate strength behaviour. This thesis describes a finite element method for analysing nonlinear response of three dimensional stiffened plate assemblages. Both material and geometric non-linearities are considered in the formulation. Plasticity in the plate is assumed to be governed by a full section yield criterion, while a multilayer approach is adopted for the stiffeners. Procedures which trace the history of thermal loading curing plated fabrication are incorporated to account for the effect of welding stresses and geometric imperfections. Elastic beam element with multipoint contraint nodes is formulated to represent portions of the structure which are assumed to behave in a linear elastic manner. The modified Newton-Raphson nonlinear technique is improved by adopting a robust automatic incremental procedure. Further introduction of Riks' fixing length approach improves the overall efficiency, especially in the post-buckling range of the analysis. Published numerical examples and experimental results are used to verify the accuracy and to illustrate the capability of the computer prof gram. The proposed method is applied to an unstiffened box column to study the effect of geometric imperfections with the presence of residual welding stresses. A study is made on the strength of box girders, focusing on web panels and their interaction with other components under combined moment and shear force loading. Results are used to compare and justify the safe loads calculated using the recently published British Standard of Designing Steel Bridges (BS5400). Finally results from analysing a complete stiffened box girder are presented and compared to experimental data.