To describe the design rules in Eurocode 3 [1-3] for members with Class 4 sections.
For members with Class 4 sections  the effect of local plate buckling on the overall member behaviour has to be taken into account. The buckling is allowed for by using effective cross-sections which assume parts of the gross cross-section are inactive. The rules for the determination of the effective cross-section and the design checks required are given.
For members with Class 4 sections, the effect of local buckling on global behaviour at the ultimate limit state is such that the elastic resistance, calculated on the assumption of yielding of the extreme fibres of the gross section (criteria for Class 3 sections), cannot be achieved.
Figure 1 gives the moment deflection curve for a point loaded beam (Class 4). The reason for the reduction in strength is that local buckling occurs at an early stage in parts of the compression elements of the member. The stiffness of these parts in compression is thereby reduced, and the stresses are distributed to the stiffer edges (see Figure 2).
To allow for the reduction in strength, the actual non linear distribution of stress is taken into account by a linear distribution of stress acting on a reduced 'effective plate width' leaving an 'effective hole' where the buckle occurs (see Figure 2).
By applying this model, an 'effective cross-section' is defined for which the resistance is then calculated as for Class 3 sections (by limiting the stresses in the extreme fibres to the yield strength).Read more