Clauses 6.3.2.2 and 6.3.2.3 (EN 1993-1-1:2005) both give equations to evaluate the LTB reduction factor χ_LT to be used in eqn. 6.55 of SS EN 1993-1-1:2005.

Cl. 6.3.2.2 uses tables 6.3 and 6.4 to choose the buckling curve and the imperfection factors to be used for calculating χ_LT. Table 6.4 specifies the choice of buckling curves for “Rolled I Sections”, “Welded I Sections” and “Any other sections”. Cl 6.3.2.3 on the other hand uses tables 6.5 and 6.3 to choose the buckling curves and imperfection factors. Table 6.5 however only deals with “Rolled I Sections” and “Welded I Sections”.

Cl. 6.3.2.2 states “Unless otherwise specified, see 6.3.2.3, for bending members of constant cross section the value of χ_LT should be determined from...”. Hence in the implementation of EC3 (and the Singaporean Annex) in STAAD.Pro: by default the program will consider clause Cl. 6.3.2.3 to evaluate χ_LT. For any case that is not dealt with by Cl. 6.3.2.3, the program will consider Cl. 6.3.2.2 to evaluate χ_LT.

Cl. 6.3.2.3 in the Singaporean National Annex states that Table 6.5 in SS EN 1993-1-1:2005 should be replaced with the table given in the NA (See section 4.3 of this document). Hence for all cases dealt with by the table in the Singaporean NA, this implementation will choose the buckling curves from the Singaporean National Annex. For any case that is not dealt with by the table in the Singaporean NA, the program will use the method given in Cl. 6.3.2.2 of SS EN 1993-1-1:2005.

For the following cross sections, the program will use the Table in the Singaporean NA for choosing a buckling curve for LTB checks (when the SS EN has been specified):

• Rolled doubly symmetric I & H Sections
• Rolled doubly symmetric hollow sections (SHS, RHS, CHS)
• Angle Sections
• Any other rolled section
• Welded doubly symmetric sections with h/b < 3.1

For the following cross sections, the program will use Cl. 6.3.2.3 of SS EN 1993-1-1:2005 to evaluate χ_LT

• Welded I & H Sections with h/b ≥ 3.1.

For any other type of cross section that is not dealt with by the National Annex or Cl.6.3.2.3, the program will use Cl. 6.3.2.2 to evaluate χ_LT .

In any case, the elastic critical moment, M_cr, (used to evaluate the non dimensional slenderness) will be evaluated as given above. Since this implementation uses the NCCIs mentioned in the sections above, only end restraint conditions corresponding to the CMN parameter=1.0 (See section above) will be considered. For all other cases of the CMN parameter values, this implementation will use the method specified in Annex F of DD ENV 1993-1-1:1992.