# D5.D.12.5 Clause 6.3.2.3(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 χ_{LT}. 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:

Where:

- C
_{1}is to be obtained from the NCCI documents as previously described (Refer to D5.D.12.2 Clause 6.3.2.2 –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 K_{c}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 C_{1} from DD ENV 1993-1-1:1992 Annex F.

The program will use a default value of 1.0 for K_{c}. However, you can also input a custom value of K_{c} 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 C_{1} corresponding to the end conditions and the Bending moment of the member and in turn calculate K_{c} as given in the NA. To evaluate C_{1}, the program will use the NCCI documents as previously described.