Steel Buildings in Europe

Part 4: Detailed Design of Portal Frames 4 - 21 5 CROSS-SECTION RESISTANCE 5.1 General EN 1993-1-1 requires that the resistance of cross-sections and the member buckling resistance are checked by separate calculations. Additional checks are required for the resistance of webs to shear buckling and buckling due to transverse loads. The calculated resistance depends on the classification of the cross-section. Cross-section resistance is treated in Section 6.2 of EN 1993-1-1. 5.2 Classification of cross-section In EN 1993-1-1, cross-sections are classified according to the relative thickness of the flanges and web, together with the magnitude of the bending moment and axial compression on the section. The classification according to the slenderness of flange or web elements is given in EN 1993-1-1 Table 5.2. EN 1993-1-1 covers sections under axial load alone, under pure bending and under combined axial load and bending moment. The class of a section is the highest class of either the flanges or the web. It is important to note that the classification depends on both the geometry of the cross-section and the ratio of the moments and axial force at the cross-section. For example, a typical I-beam might be Class 1 under pure moment but Class 2 or 3 under pure axial loading; under combined loading it might then be Class 1, 2, or 3, depending on the proportions of axial force and bending moment at the cross-section under consideration. The classes indicate the following structural behaviour: Class 1 can support a rotating plastic hinge without any loss of resistance from local buckling. Class 2 can develop full plastic moment but with limited rotation capacity before local buckling reduces resistance. Class 3 can develop yield in extreme fibres but local buckling prevents development of plastic moment. Class 4 has proportions such that local buckling will occur at stresses below first yield. 5.3 Member ductility for plastic design As specified in EN 1993-1-1:2005 § 5.6, all members formed from rolled sections (and therefore uniform apart from haunches) containing plastic hinges that rotate prior to reaching the ULS loading must have a Class 1 cross-section. Elsewhere, they may be Class 2.