When calculating my model with a floor slab, I get very large shear forces displayed in that slab and also very large, unrealistic support forces displayed on the support ends, which makes the slab non-designable. What is the reason?
The support forces are large because singularities occur at the ends of the line supports due to the rigid restraint.
It is possible to avoid this problem by a realistic modelling of the structural system. Floor slabs resting on walls are not really fully fixed in direction of the self-weight. With regard to the stiffness of the wall it is more realistic to model the support as a spring. The spring stiffness of a line support intended for a simulation of a wall is calculated by k = (E*d) / h, where E = modulus of elasticity of wall, d = thickness of wall, and h = height of wall.
By modeling line supports with springs you can avoid singularities as you can see in the picture showing the line support reactions.
Furthermore, modeling supports as springs is also useful for nodal supports. RFEM provides the "column" function for the nodal support generation where the spring parameter is automatically calculated and applied from the column geometry.