British Sections designed, in RAM Frame, in accordance with BS EN 1993-1-1:2005, can, depending on the initial section size assigned, be classified as Class 4 in accordance with section 5.5 of this standard. As a reminder, the role of cross section classification is to identify the extent to which the resistance and rotation capacity of cross-sections is limited by its local buckling resistance.
Four classes of cross section are defined in BS EN 1993-1-1:2005 as follows:
Class 1 - cross-sections are those which can form a plastic hinge with the rotational capacity required from plastic analysis without reduction of the resistance.
Class 2 - cross-sections are those which can develop their plastic moment resistance, but have limited rotational capacity because of local buckling.
Class 3 - cross-sections are those in which the stress in the extreme compression fiber of the steel member assuming an elastic distribution of stresses can reach the yield strength, but local buckling is liable to prevent development of the plastic moment resistance.
Class 4 - cross-sections are those in which local buckling will occur before the attainment of yield stress in one or more parts of the cross-section.
For a member subject to both bending and axial compression as shown below, the critical section checks may occur at the beam ends or at the point of contra flexure where the beam is subject to compression only:
The above continuous beam comprises two spans, one 6.0m and one 4.0m. The 6.0m span will be considered for this example.
It should be noted that all UB sections which are classified as Class 4 when subject to a compressive force only, that the section classification is controlled by the web dimensions. This is confirmed in pages B – 62 and B – 63 of SCI Publication P 363.
RAM Frame does not check the design capacity of Class 4 sections in accordance with the requirements of BS EN 1993-1-1:2005 and this article outlines how users can perform a hand calculation to verify the adequacy of a Class 4 section.
Assume the section being designed is a 457x191x67 UB, its properties for section classification are:
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