Classification (Beams: BS 5950)
General
The classification of the cross section is in accordance with BS 5950-1: 2000.
Steel beams can be classified as:
- Plastic Class = 1
- Compact Class = 2
- Semi-compact Class = 3
- Slender Class = 4
Class 4 sections are only acceptable for angle and tee sections.
Sections with a Class 3 web can be taken as Class 2 sections (Effective Class 2) providing the cross section is equilibrated to that described in Clause 3.5.6 where the section is given an “effective” plastic section modulus, Seff. For rolled I and H sections in the UK, this gives no advantage in pure bending since the web d/t is too small. Hence for beams there is likely to be little advantage in using this approach since the axial loads are generally small, this classification is therefore not implemented.
All unacceptable classifications are either failed in check mode or rejected in design mode.
Hollow sections
The classification rules for SHS and RHS relate to “hot-finished hollow sections” only (“cold-formed hollow sections” are not included in this release).
Warning: Important Note.
The classification used to determine Mb is based on the maximum axial
compressive load in the relevant segment length. Furthermore, the Code clearly
states that this classification should (only) be used to determine the moment
capacity and lateral torsional buckling resistance to Clause 4.2 and 4.3 for use
in the interaction equations. Thus, when carrying out the strength checks, the
program determines the classification at the point at which strength is being
checked.