Influence of the plate effective width on cross section capacity.
Whenever the effective width of a panel becomes smaller than the width between stiffeners, there is an influence on the maximum capacity of the cross section. For compression in the local panel this is taken care of by the code. However, attention should be given to the capacity of bending moment.
Reference is made to the Eurocode 3: EN 1993-1-5. Figure 4.2 explaines the influence on the elastic neutral axis. Below, another example is given of a cross section.
Skin panels between stiffeners loaded in compression will buckle far before the ultimate strength is reached of the complete stiffened panel. Therefore capacity checks of stiffened panels loaded in compression are usually done based on an effective width. When the effective width becomes smaller than the distance between the stiffeners, this has an effect of the maximum capacity of the panel.
Let's assume that in the figure above, the cross sectional moment loads the bottom panel in compression. Due to the effective width of the skin, the bottom panel is not fully effective. The contribution to the moment capacity of the cross section is not 100%.
The results of an elastic Finite Element Model do not include the shift of the neutral axis. When the maximum moment capacity of the cross section must be considered, be aware to base this on the effective material in the cross section only.