Lateral torsional buckling (Beams: EC3)

Tekla Structural Designer
Modified: 27 Oct 2020
2024
Tekla Structural Designer

Lateral torsional buckling (Beams: EC3)

Lateral torsional buckling checks are required between supports, or LTB restraints on a flange which is in bending compression, for all lengths that are not continuously restrained.

Note: A warning message will be given in the lateral torsional 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.

Note that coincident LTB restraints (i.e. top & bottom flange) are the equivalent of a support and will define one end of a 'check length' for both flanges regardless of whether a particular flange is in compression or tension at the coincident restraint position. However, note also that in a cantilever all intermediate restraints are ignored.

Tekla Structural Designer allows you to 'switch off' LTB checks for either or both flanges by specifying that the entire length between start and end of the beam span is continuously restrained against lateral torsional buckling. If you use this option you must be able to provide justification that the beam is adequately restrained against lateral torsional buckling.

All intermediate LTB restraints in a cantilever span are ignored.

When the checks are required you can set the effective LTB length of each 'check length' by giving factors to apply to the physical length of the beam. Any individual 'check length' less than the full span length can be continuously restrained in which case no LTB check will be carried out for that 'check length' provided all segments of the 'check length' have been marked as Continuous. Each 'check length' which is not defined as being continuously restrained for its whole length is checked in accordance with clause 6.3.2.3.

The formula for elastic critical buckling moment, Mcr is taken from standard texts. The moment factor C1 that is part of the standard formula has been derived analytically.

LTB does not need to be checked for the following sections:

  • circular and square hollow sections,
  • equal and unequal flanged I/H sections loaded in the minor axis only.

Effective lengths

The value of effective length factor is entirely the choice of the engineer. The default value is 1.0. There is no specific factor for destabilizing loads - so you will have to adjust the 'normal' effective length factor to allow for such effects.

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