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 λT,  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 Ncr,T,F and Ncr,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 Ncr,TF and Ncr,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:

where
 $i o 2$ = $i y 2 + i z 2 + y o 2 + z o 2$ iy and iz = the radius of gyration about the Y-Y (weak axis) and Z-Z (strong axis) respectively

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

$N c r , T F = i o 2 2 ( i y 2 + i z 2 ) [ N c r , y + N c r , T − ( N c r , y + N c r , T ) 2 − 4 N c r , y N c r , T i y 2 + i z 2 i o 2 ]$

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