Combined buckling (Beams: EC3 Eurocode)
Combined buckling in Tekla Structural Designer is limited to doubly symmetric sections (I, H, CHS, SHS, RHS). In the context of combined buckling, beams are assumed to be dominated by moment with axial force.
Restraints are treated as described previously and summarized as follows:

Each span is assumed to be fully supported at its ends (i.e LTB, yy and zz restraint)  this cannot be changed.

Tension flange LTB restraints are ignored unless they are coincident (see next point).

Coincident top and bottom flange restraints are considered as 'torsional' restraints i.e. as good as the supports.

All intermediate LTB and strut restraints in a cantilever span are ignored.
For each span of the beam, the design process is driven from the standpoint of the individual LTB lengths i.e. the LTB lengths and the yy lengths that are associated with each LTB length and the zz lengths associated with the yy length. Thus a 'hierarchy' is formed  see the “Design Control” section below for details. Both Equation 6.61 and Equation 6.62 are evaluated recognizing that the combined buckling check is carried out for both the top flange and the bottom flange.
Effective lengths
In all cases Tekla Structural Designer sets the default effective length to 1.0L, it does not attempt to adjust the effective length in any way. Different values can apply in the major and minor axis. It is your responsibility to adjust the value from 1.0 where you believe it to be justified.
Combined buckling resistance
Equations 6.61 and 6.62 are used to determine the combined buckling resistance.
With regard to these equations the following should be noted:

The “k” factors used in these equations are determined from Annex B only, and reported as follows:

k_{yy} is reported as components k’_{yy} and Cmy where k’_{yy} is simply the Annex B term for k_{yy} with C_{my} excluded

k_{yz} is reported as k’_{yz}, a multiple of k’_{zz} (see below)

k_{zy} is reported as k’_{zy}, a multiple of k’_{yy} (see above), but only for members not susceptible to torsional deformations (i.e. SHS and CHS sections at all times, and I or H sections which have both flanges continuously restrained for LTB). For members which are susceptible to torsional deformations k_{zy} is reported per Table B.2 (i.e. with C_{mLT} included)

k_{zy,LT1} is a factor reported for columns only and is the Table B.2 term for k_{zy} with C_{mLT} set to 1.0

k_{zz} is reported as components k’_{zz} and C_{mz} where k’_{zz} is simply the Annex B term for k_{zz} with C_{mz} excluded.

The note to Table B.3 that C_{m} should be limited to 0.9 is not applied.