Answer:
Length LT represents the buckling length for torsional buckling and lateral-torsional buckling under systematic centric pressure. Length LT works similarly to Column D and Column G: a critical and buckling load are calculated for the design of a pure compressive force according to EN 1993‑1‑1, Section 6.3.1.4.
The length LW, by contrast, is not an effective length, but the length to which the moment and shear force distribution with the boundary conditions kz and kw (Columns J and K) as well as the lateral intermediate supports are related. Generally, this length should not be changed.
