Answer:
In RFEM, a surface foundation is described by five parameters:
On one hand, there are three moduli of subgrade reactions in the directions related to the local surface axes (Cu,x, Cu,y, and Cu,z). Cu,z is technically equal to the Winkler modulus of foundation. It can be obtained from a soil report. This foundation can also be assigned to a nonlinearity, such as failure under tension; see Image 01. The parameters Cu,x and Cu,y describe the resistance of the foundation against horizontal displacement in the respective directions.
On the other hand, the parameters for considering the shear capacity of the foundation (Cv,xz and Cv,yz) are of importance.
The modulus of subgrade reaction for bedding the foundation, for example, should thus be entered in the Cu,z text box, as the foundation acts perpendicular to the surface.