# D5.D.3.6 Clause 6.3.1.4 – Slenderness for torsional and torsional-flexural buckling

The D-NA requires the implementation of clauses 12.1.2 and 12.1.3 of NEN 6770

## Clause 12.1.2 (NEN 6770): Torsional stability

IPE, HEA, HEB & HEM sections and pipe sections do not need to be checked for torsional instability.

If torsional checks need to be performed, they should be done according to 12.1.2 of NEN 6771.

## Clause 12.1.2 (NEN 6771)

This clause gives the condition to check for torsion instability. The condition being:

$\frac{{N}_{c;s;d}}{{\omega}_{\theta}{N}_{c;u;d}}\le 1$

Where:

- N
_{c;s;d}= the applied axial load - N
_{C;u;d}= the axial capacity = A · f_{y}.

${\omega}_{\theta}=\frac{{\sigma}_{\theta ;d}}{{f}_{u;d}}$

## Clause 12.1.3 (NEN 6770): Torsional flexural stability

Doubly symmetric sections need not be checked for torsional flexural instability. However, for I sections that have rigid supports that is not along the axis of the section and any other sections will need to be checked as per clause 12.1.3 of NEN 6771.