Answer:
The definition of surface supports should be as close to reality as possible. Experience shows that the equation solver works most efficiently in this way. For simplification reasons, degrees of freedom are often defined as "fixed". However, this may have a great impact on the overall stiffness matrix and cause numerical problems (see Image 01).
To avoid numerical problems, it is better to work with springs. It is often sufficient to define very stiff springs (see Image 02). The same applies to the foundation perpendicular to the surface. You can find more information in [1] and by clicking the links below this FAQ.
