The authors recently proposed a simplified method to estimate the ultimate strength of a continuous stiffened plate under combined thrust in stiffener direction and lateral pressure on plate side, based on a series of elastoplastic large-deflection FEA. This paper extends the method so as to consider the lateral pressure on stiffener side as well as on plate side. Since the lateral pressure on stiffener side cause compression at the stiffener top at the midspan, the so-called stiffener-induced failure takes place until it changes to the hinge-induced failure mode under a high lateral pressure. In addition, as a result of the stabilizing effect of a clamped-mode deflection caused by lateral pressure on the buckling strength of stiffeners, the ultimate compressive strength is attained after the yielding widely spreads over the stiffener's cross section at the mid-span. These FEA observations are taken into account in the formulation of the extended method. Through a comparison of the estimated ultimate strengths with the FEA results, the validity of the extended method is demonstrated.