Combined buckling (Columns: EC3 Eurocode)

The column must be restrained laterally in two directions, and torsionally at the top and bottom of the 'design length'. This equates to LTB restraint to faces A and C and restraint to major and minor axis compression buckling all being coincident. A design length is allowed to have intermediate restraint and if the restraint requirements are not met at a particular floor then the design length does not have to be between adjacent floors. Thus a stack can 'jump' floors or sheeting rails can be attached. It is assumed that the restraints for compression buckling are fully capable of forcing the buckled shape. Hence, the compression buckling resistance is based on the restrained lengths whilst the LTB resistance ignores the intermediate restraint and hence is based on the full design length.

Note: It is conservative to ignore the intermediate restraints in this latter case.

Loading within the design length is allowed.

Effective lengths

Effective lengths for flexural (i.e. strut major and strut minor) and lateral torsional buckling are as described in the appropriate section above.

Combined buckling resistance

The combined buckling resistance is checked in accordance with Equations 6.61 and 6.62. Both equations are evaluated at the ends of the design length and, except for simple columns, at the position of maximum moment, if that lies elsewhere.

Eccentricity moments due to beam end reactions are added to the “real” moments due to frame action:

in the first case the uniform moment factors are calculated from the real moments and applied to the real moments. Eccentricity moments are only added if they are more critical.
in the second case all moments are “combined” and all uniform moment factors are based on the combined moments and applied to them.

Warning: 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.