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D5.D.10.4 Clauses 6.3.2.2 and 6.3.2.3 — Calculation of LTB Reduction factor, χ_{LT} as per Belgium NA

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
NBN-EN 1993-1-1:2018.

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 Belgian 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 Belgian National Annex gives equations to evaluate the
imperfection factors to be used for various section types. (Refer to
D5.D.10.3
Clause 6.3.2.3(1) – LTB for rolled sections or equivalent welded section
). Hence for all cases dealt with by the equations in the NBN-NA, this
implementation will use Cl 6.3.2.3 to evaluate χ_{LT}.

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 given above. Since this
implementation uses the NCCIs mentioned in the sections above, only end restraint
conditions corresponding to the `CMN` parameter=1.0 (Refer to
D5.D.10.3
Clause 6.3.2.3(1) – LTB for rolled sections or equivalent welded section
) 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.

You can override the default behavior and specify the clause that is to be used for LTB checks. This can be specified using the `MTH` design parameter (Refer to
D5.C.6
Design Parameters).

`NA`parameter in the design input = 0), the program will use Cl. 6.3.2.3 only in the case of Rolled or welded I & H Sections. For all other cases, the program will use Cl. 6.3.2.2 of NBN-EN 1993-1-1:2018. Also, I sections with plates will be treated as built-up sections only if the section has been explicitly specified as a built-up section (i.e.,

`SBLT`parameter = 1.0 in design input).