D5.D.12.5 Clause 18.104.22.168(2) – Modification factor, f, for LTB checks
The DE-NA specifies the use of Equation 6.58 of DIN EN 1993-1-1:2005 to evaluate the modification factor "f" for the LTB reduction factor χLT. To evaluate the modification factor SS EN 1993-1-1:2005 uses a correction factor "kc" given by Table 6.6 in the code.
The DE-NA however, specifies that the correction factor "kc" is to be obtained as below:
- C1 is to be obtained from the NCCI documents as previously described (Refer to D5.D.12.2 Clause 22.214.171.124 –Elastic critical moment and imperfection factors for LTB checks). The NCCI document SN003a-EN-EU specifies the values of C1 to be used in table 3.1 as shown below. The current implementation does not account for the Kc factor and conservatively uses a reduction factor equal to 1. The program allows for the reduction factor based on the DE-NA.
These values are for an end restraint factor of k = 1 (i.e., design parameter CMN = 1.0). Hence for all other values of CMN (i.e., 0.7 or 0.5) this implementation will use the values of C1 from DD ENV 1993-1-1:1992 Annex F.
The program will use a default value of 1.0 for Kc. However, you can also input a custom value of Kc by setting the design parameter KC to the desired value. If the KC parameter in the design input is set to 0, then the program will automatically calculate its value. This will cause the program to evaluate a value of C1 corresponding to the end conditions and the Bending moment of the member and in turn calculate Kc as given in the NA. To evaluate C1, the program will use the NCCI documents as previously described.