Why there is a difference between zeta factor used in LTB 5 and factors from literature references, or LTB 3?
The zeta factor = pressure force distribution is a correction factor allowing for determination of elastic critical moment Mcr for certain boundary conditions and loads.
In LTB 5, the zeta correction factor always refers to the greatest bending moment of the equivalent member. Contradictory information in literature concerning this matter specify the zeta factor referring to the middle of the equivalent member (see LTB manual, Chapter 2.4.2, page 19ff).
However, LTB 5 also provides an option to define the zeta factor manually. For this, select in chapter DETERMINATION TYPE M-cr -> Assign LC/LG separately.