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 MS 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 MS NA) 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 MS NA states that Table 6.5 in MS EN 1993-1-1:2005 should be replaced with the table given in the NA (Refer to D5.D.11.3 Clause 6.3.2.3(1) – LTB for rolled sections or equivalent welded section). Hence for all cases dealt with by the table in the MS NA, this implementation will choose the buckling curves from the MS NA. For any case that is not dealt with by the table in the MS NA, the program will use the method given in Cl. 6.3.2.2 of MS EN 1993-1-1:2005.

Hence for the following cross sections the program will use the Table in the MS NA for choosing a buckling curve for LTB checks (when the MS NA 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 MS 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 “Mcr” (used to evaluate the non dimensional slenderness) will be evaluated as described in "Clause 6.3.2.2 –Elastic critical moment and imperfection factors for LTB checks". Since the MS NA uses the NCCI documents mentioned in the sections above, this implementation will only consider end restraint conditions corresponding to the CMN parameter=1.0. 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.