In previous work an economical, yet rigorous, method was developed for analysing the complete collapse behaviour of steel plates subjected to in-plane compressive loading. The opportunity is now taken to present the results of applying this method to the analysis of 960 different simply-supported rectangular plates primarily under longitudinal compression. With regard to stiffened panels, each case analysed is representative of a rectangular plate element between longitudinal stiffeners. The results indicate the aspect ratio of buckling a/b to have an important effect, in addition to the effects of plate slenderness, initial out-of-flatness, and residual welding stress. Various types of plate panel are analysed depending on their positions within the stiffened panel as a whole. For certain types the boundary conditions give rise to secondary in-plane transverse loadings. There is a similarity of behaviour between the various types which supports the design simplification of treating them all in the same way. The results support the class P and Q plate strength design curves proposed previously for stiffened panels and thin-walled box columns.