# D5.D.5.7 Clause 6.3.1.4 - Slenderness for torsional and torsional-flexural buckling

Equations 6.52 and 6.53 of BS EN 1993-1-1:2005 are to be used to calculate the non-dimensional slenderness λ_{T}, to be used for torsional and torsional-flexural buckling checks. BS EN 1993-1-1:2005 does not provide equations to calculate the elastic critical loads N_{cr,T,F} and N_{cr,T} (refer 6.3.14 of BS EN 1993-1-1:2005).

The NCCI document SN001a-EN-EU: Critical axial load for torsional and flexural torsional buckling modes provides methods to calculate the N_{cr,TF} and N_{cr,T} factors and therefore these methods are used to evaluate the elastic critical loads for the UK NA.

The critical axial load for torsional buckling is evaluated as:

= | ||

_{y} and
i_{z} | = |

The critical axial load for torsional-flexural buckling is evaluated as:

For details on these equations, refer to the NCCI document SN001a-EN-EU.