A simplified method to calculate the ultimate strength of a continuous stiffened plate under uniaxial thrust is presented. The proposed method is characterised as follows : (1) A continuous stiffened plate is idealised as a double-span beam-column model having an effective width of plate to consider the interaction between adjacent spans. (2) For flat-bar stiffeners, the reduction in axial stiffness due to the interactive local buckling between plate and stiffener is taken into account by introducing a concept of effective thickness. (3) Both effective width of a plate and effective thickness of a flat-bar stiffener are expressed as a function of a thrust load to evaluate the effective cross-section of a stiffened plate at ultimate strength accurately. (4) The influence of torsional stiffness of a stiffener on the local buckling strength of a plate and the resistance of a plate against the rotation of a stiffener are considered as plate/stiffener interactions. (5) The ultimate strength of a stiffened plate is examined based on the initial yield criterion considering both bending and warping stresses. A series of ultimate-strength estimates has been performed for the continuous stiffened plates with flat-bar, angle-bar and tee-bar stiffeners, and the validity and usefulness of the proposed method have been demonstrated through the comparison with the results of elastoplastic large-deflection FEM analyses.