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D5.D.8.5 Clauses 6.3.2.2 and 6.3.2.3 — Calculation of LTB Reduction factor, χ_{LT} as per Finnish 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 PN 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 Finnish 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 Finnish National Annex gives equations to evaluate the imperfection factors to be used for various section types (Refer to
D5.D.7.3
Clause 6.3.2.2 –Elastic critical moment and imperfection factors for LTB checks
). Hence for all cases dealt with by the equations in the Finnish 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, M_{cr}, (used to evaluate the non dimensional slenderness) will be evaluated as previously given. 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.7.3
Clause 6.3.2.2 –Elastic critical moment and imperfection factors for LTB checks
) 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.

`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 BS EN 1993-1-1:2005. 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).