# D5.D.11.5 Clause 6.3.2.3(2) – Modification factor, f, for LTB checks

The MS NA specifies the use of eqn. 6.58 of MS EN 1993-1-1:2005 to evaluate the modification factor, f, for the LTB reduction factor χ_{LT}. To evaluate the modification factor MS EN 1993-1-1:2005 uses a correction factor, kc, given by Table 6.6 in the code.

The program does not calculate the kc factor and conservatively uses a reduction factor equal to 1. The proposed implementation will allow for the reduction factor based on the MS NA.

These values are for an end restraint factor of k = 1 (i.e., `CMN` = 1.0). Hence for all other values of `CMN` (i.e., 0.7 or 0.5), the program uses the values of C1 from DD ENV 1993-1-1:1992 Annex F.

You can also manually specify a value for kc by setting the design parameter, `KC`, to the desired value. The user can also get the program to calculate the value of kc automatically by setting the value of the `KC` parameter in the design input to 0. 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 (Refer to
D5.D.11.2 Clause 6.3.2.2 –Elastic critical moment and imperfection factors for LTB checks).

Note that for the MS NA, the program will attempt to evaluate k_{c} by default using the equation in NA,

where C1 will be the value used for the M_{cr} calculations.

If k_{c} evaluates to be greater than 1.0, the program will then evaluate kc as per Table 6.6 of EN 1993-1-1:2005.