Intermediate lateral torsional buckling restraints on columns designed to EC3

For column design using the Eurocode, the LTB design length is assumed to be between those positions where the column is restrained in both the major and minor compression buckling  directions and at flanges A and C. This assumes the ‘standard conditions of end restraint’ and thus an effective length factor less than 1.0 would be inappropriate. Further, since flanges A and C can be given an effective length, the worst of L_eA, L_eC and the system length should be adopted for the LTB design length. This is because the interaction equations given in the Eurocode only cover design between points of full restraint in both axis.  No facility is provided for intermediate restraints. The Eurocode drafting body is aware of this and may provide a solution in due course, but we are unable to give any timescales for this.

The SCI has published the design guide for portal frames, which provides a solution for portal columns with uniaxial bending in the major axis that are restrained between the end positions of the columns, effectively treating the column as a "beam".  However, this does not cover the general case of biaxial moments and potentially very large axial loads.  Thus, for 'true' column designs, where axial load dominates, for the time being this remains a limitation of the Eurocodes. Thus for columns any intermediate restraints are ignored.

In Tekla Structural Designer it is possible to define vertical 'beams' to design circumstances, such as gable post, where this issue is most likely to arise.  It is a matter of engineering judgement when to apply this approach.  Following are additional comments from our Chief Engineer that may assist with comprehending the issue and the rationale for the current approach.

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EC3 Clause 6.3.3

This clause deals with combined bending and compression in uniform members. The note to 6.3.3 (2) indicates the following,

“Note 1. The interaction formulae are based on the modelling of simply supported single span members with end fork conditions and with or without continuous lateral restraints which are subjected to compression forces, end moments and/or transverse loads.”

Notably there is no mention of intermediate restraints. Clause 6.3.3 (3) goes on to say,

“For members of structural systems the resistance check may be carried out on the basis of the individual single span members regarded as cut out of the system.” This assumes that the individual member has end restraint equivalent to that in ‘Note 1’.

The European body for standards, CEN, in which the UK is represented by the British Standards Institution is currently considering amendments and revisions to EC3. A comprehensive list of items that require further work and funding is being compiled by the relevant committees.  A major one of those items relates to Clause 6.3.3, Uniform member in bending and axial compression. It identifies that work is required to establish how this clause relates to members with intermediate lateral restraint.

Consequently, on the basis that columns are dominated by axial force but have significant moment (possibly in two directions), at the present time any intermediate LTB restraints are ignored by Tekla Structural Designer. On the other hand taking beams to be dominated by moments but with significant axial force, the majority of lateral restraints are taken into account by Tekla Structural Designer.  In the latter case the efficacy of those restraints should be carefully considered and switched off if necessary.

In all cases the final choices and accuracy of the design is the responsibility of the designer.