This check is applicable to I- and H-sections with equal or unequal^[1] flanges.

The definition of this check is the out-of-plane buckling resistance of a member or segment that has a laterally unrestrained compression flange and the other flange has intermediate lateral restraints at intervals. It is used normally to check the members in portal frames in which only major axis moment and axial load exist. Although not stated explicitly in BS 5950-1: 2000, it is taken that the lateral torsional buckling moment of resistance, M_b, from the Annex G check can be used in the interaction equations of clause 4.8.3.3 (combined buckling).

Since this is not explicit within BS 5950-1: 2000 a slight conservatism is introduced. In a straightforward Annex G check the axial load is combined with major axis moment. In this case both the slenderness for lateral torsional buckling and the slenderness for compression buckling are modified to allow for the improvement provided by the tension flange restraints (λ_LT replaced by λ_TB and λ replaced by λ_TC). When performing a combined buckling check in accordance with 4.8.3.3 the improvement is taken into account in determining the buckling resistance moment but not in determining the compression resistance. If the incoming members truly only restrain the tension flange, then you should switch off the minor axis strut restraint at these positions.

The original source research work for the codified approach in Annex G used test specimens in which the tension flange was continuously restrained. When a segment is not continuously restrained but is restrained at reasonably frequent intervals it can be clearly argued that the approach holds true. With only one or two restraints present then this is less clear.BS 5950-1: 2000 is clear that there should be “at least one intermediate lateral restraint” (See Annex G.1.1). Nevertheless, you are ultimately responsible for accepting the adequacy of this approach.

For this check Tekla Structural Designer sets mt to 1.0 and calculates n_t. The calculated value of n_t is based on M_max being taken as the maximum of M₁ to M₅, and not the true maximum moment value where this occurs elsewhere in the length. The effect of this approach is likely to be small. If at any of points 1 - 5, R >1 ^[2], then Tekla Structural Designer sets the status of the check to Beyond Scope.

Reference restraint axis distance, a

The reference restraint axis distance is measured between some reference axis on the restrained member - usually the centroid - to the axis of restraint - usually the centroid of the restraining member. The measurement is shown diagramatically in Figure G.1 of BS 5950-1: 2000.

Tekla Structural Designer does not attempt to determine this value automatically, since such an approach is fraught with difficulty and requires information from you which is only used for this check. Instead, by default, Tekla Structural Designer uses half the depth of the restrained section, and you can specify a value to be added to, or subtracted from, this at each restraint point. You are responsible for specifying the appropriate values for each restraint position. The default value of 0mm may be neither correct nor safe.

^[1]Unequal flanged sections are not currently included.

^[2] Which could happen since R is based on Z and not S.