There is a difference between the following eccentricities:
In RFEM and RSTAB, a member length corresponds to the distance between two nodes, which is defined by the line of a member. In the case of cross-section connections or downstand beams, the reality is only modelled approximately. By using member eccentricities, it is possible to connect members eccentrically by specific member end sections. For example, you can thus reduce design moments at beams for the frames with large column sections. The member eccentricities are taken into account by a transformation of the degrees of freedom in the local element stiffness matrix.
A plane in the neutral axis of a surface represents the reference surface of thickness. It is applied in equal proportions on both sides of this 'central plane'. By specifying the eccentricity e<sub>z</sub>, it is possible to arrange an offset of height for the surface. In this way, you can achieve the adjacent surfaces with different thickness to have a uniform top or bottom edge.