Member buckling resistance, clause 4.8.3.3.2 (Beams: BS 5950)
This check is used for Class 1, 2 and 3 Plastic, Compact and Semi-compact rolled I- and H-sections with equal flanges (Class 4 Slender sections and Effective Class 2 sections are not included in this release).
Three formula are provided in clause 4.8.3.3.2 (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 4.8.3.3.2 (c) can also be used in such cases by setting the axial force to zero.
All three formula in clause 4.8.3.3.2 (c) are checked; the second 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.
Warning: Important Note.
Clause 4.8.3.3.4 defines the various equivalent uniform moment factors. The last
three paragraphs deal with modifications to these depending upon the method used
to allow for the effects of sway. This requires that for sway sensitive frames
the uniform moment factors, mx, my and mxy,
should be applied to the non-sway moments only. In this release there is no
mechanism to separate the sway and non-sway moments, Tekla Structural Designer adopts a conservative approach and sets these 'm' factors equal to 1.0 if the
frame is sway sensitive (in either direction). This is doubly conservative for
sway-sensitive unbraced frames since it is likely that all the loads in a design
combination and not just the lateral loads will be amplified. In such a case,
both the sway and non-sway moments are increased by kamp and neither
are reduced by the above “m” factors. The calculation of mLT is
unaffected by this approach, and thus if the second equation of clause 4.8.3.3.2
(c) governs, then the results are not affected.