D5.D.13.5 Clauses 188.8.131.52 and 184.108.40.206 — Calculation of LTB Reduction factor, χLT as per Swedish NA
Clauses 220.127.116.11 and 18.104.22.168 (EN 1993-1-1:2005), both give equations to evaluate the LTB reduction factor χLT to be used in eqn. 6.55 of BFS EN 1993-1-1:2005.
Cl. 22.214.171.124 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 126.96.36.199 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. 188.8.131.52 states "Unless otherwise specified, see 184.108.40.206, for bending members of constant cross section the value of χLT should be determined from…". Hence in the implementation of EC3 (and the Swedish Annex) in STAAD.Pro: by default the program will consider clause Cl. 220.127.116.11 to evaluate χLT. For any case that is not dealt with by Cl. 18.104.22.168, the program will consider Cl. 22.214.171.124 to evaluate χLT.
In any case, the elastic critical moment, Mcr, (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.13.3 Clause 126.96.36.199 –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.