D5.D.3.7 Clauses 6.3.2.2 and 6.3.2.3 – Lateral torsional buckling curves
Clause 6.3.2.2 – Lateral torsional buckling curves  general
The DNA states that the values for the imperfection factor, αLT, to be used in equation 6.56 of EC3 are to be obtained from sTable 6.3 of EC3. These are the values used by STAAD.Pro.
Clause 6.3.2.3 – Lateral torsional buckling curves for rolled sections or equivalent welded sections
The DNA states that:

The values for the:
 Imperfection factor αLT0 = 0.4 (used in equation 6.57 of EC3)
 Β = 0.75 (used in equation 6.57 of EC3)
These are the default values used by the program.

The buckling curves shall be selected as per Table 6.5.

The reduction factor, f, is given by
F = 1 – 0.5(1  kc)[1  2x (λLT 0.8)^{2}].
kc is a correction factor for moment distribution determined from Table 6.6. This value can be specified or calculated by the program using the KC parameter. Refer to D5.C.6 Design Parameters
The current implementation of STAAD.Pro conservatively uses a value of f = 1.0.