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.

Note: A warning message will be given in the combined buckling check results whenever (major or minor axis) high shear is present in a load combination. “HIgh shear is assumed not to affect buckling design. This assumption should be verified by the Engineer.”

Restraints are treated as described previously and summarized as follows:

Each span is assumed to be fully supported at its ends (i.e LTB, y-y and z-z 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 y-y lengths that are associated with each LTB length and the z-z lengths associated with the y-y 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.

Note: It is assumed that you will make a rational and “correct” choice for the effective lengths between restraints. The default value for the effective length factor of 1.0L may be neither correct nor safe.

Tip: As an alternative to using each beam's Properties dialog box to set restraints and effective lengths, you can review and set more effectively via Review View > Show/ Alter State. For more details see: Review and modify restraints and Effective Lengths and Factors - review and copy.

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.

Warning: Danger. Equations 6.61 and 6.62 are limited to doubly symmetric sections and do not consider torsional or torsional flexural buckling. Should either of these buckling modes govern the compression buckling check, you should consider very carefully whether the calculations provided by Tekla Structural Designer for combined buckling can be considered valid.