# D5.D.11.7 Clause 6.3.1.4 - Slenderness for torsional and torsional-flexural buckling

Equations 6.52 and 6.53 of MS-EN 1993-1-1:2005 are to be used to calculate the non-dimensional slenderness parameter, λT,  to be used for torsional and torsional-flexural buckling checks. The MS-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 SS EN 1993-1-1:2005). Therefore, the NCCI document SN001a-EN-EU: Critical axial load for torsional and flexural torsional buckling modes provides methods to calculate the Ncr,T,F and Ncr,T factors and hence will to be included in this implementation of the MS NA.

The program will only consider Channel Sections and Tee- sections when evaluating the critical torsional and Flexural Torsional buckling loads as per Cl 6.3.1.4.

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.