This check is used for Class 1, 2 and 3 Plastic, Compact and Semi-compact hollow sections (Class 4 Slender sections and Effective Class 2 sections are not included in this release).

Four formula are provided in clause 4.8.3.3.3 (c) to cover the combined effects of major and minor axis moment and axial force. These are used irrespective of whether all three forces / moments exist. Clause 4.9 deals with biaxial moment in the absence of axial force, clause 4.8.3.3.3 (c) can also be used in such cases by setting the axial force to zero.

The second and third formula are mutually exclusive – that is the second is used for CHS, SHS and for RHS when the limits contained in Table 15 are not exceeded. On the other hand the third formula is used for those RHS that exceed the limits given in Table 15. Thus only three formula are checked; the first, second and fourth or the first, third and fourth. Either the second or third (as appropriate) is calculated twice – once for each “flange”.

Only one value of F is used, the worst anywhere in the length being checked. If the axial load is tensile, then F is taken as zero.

See also the Important Note at the end of Member buckling resistance, clause 4.8.3.3.2.