D5.D.10.4 Clauses 220.127.116.11 and 18.104.22.168 — Calculation of LTB Reduction factor, χLT as per Belgium NA
Clauses 22.214.171.124 and 126.96.36.199 (EN 1993-1-1:2005) both give equations to evaluate the LTB reduction factor χLT to be used in eqn. 6.55 of NBN-EN 1993-1-1:2018.
Cl. 188.8.131.52 uses tables 6.3 and 6.4 to choose the buckling curve and the imperfection factors to be used for calculating χLT. Table 6.4 specifies the choice of buckling curves for "Rolled I Sections", "Welded I Sections" and "Any other sections". Cl 184.108.40.206 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. 220.127.116.11 states "Unless otherwise specified, see 18.104.22.168, for bending members of constant cross section the value of χLT 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. 22.214.171.124 to evaluate χLT. For any case that is not dealt with by Cl. 126.96.36.199, the program will consider Cl. 188.8.131.52 to evaluate χLT.
Cl. 184.108.40.206 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 220.127.116.11(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 18.104.22.168 to evaluate χLT.
For any other type of cross section that is not dealt with by the National Annex or Cl.22.214.171.124, the program will use Cl. 126.96.36.199 to evaluate χLT .
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 188.8.131.52(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).