The moment resistance of steel cross sections is limited due to the local buckling of the parts (flange and web) under axial loads. Cross sections can be used as elastic or plastic, depending on the fragility of the parts under compression. The purpose of cross section classification is to determine to what extent the resistance and rotation capacity of the cross sections are limited by local buckling.

The classification of a cross-section depends on the width to thickness ratio of the parts subject to
compression. The classification is determined according to EN 1993-1-1 Table 5.2.


Cross section capacity

1 and 2





Elastic (taking into account local buckling)

Class 1

Cross sections in the class can generate plastic hinge with the rotation capacity required by the capacity design without any loss in strength.

Class 2

Cross sections of this class can develop plastic hinge with having limited rotation capacity due to local buckling.

Class 3

Cross sections of this class can reach the yield strength, but they cannot develop plastic moment resistance due to local buckling.

Class 4

In cross sections of this class, local buckling occurs in one or more parts of the cross section without reaching yield strength.