# In a model with surface supports, the calculation does not converge or it results in very large deformations and implausible stresses. Why?

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 Figure 01).

To avoid the numerical problems, it is better to work with springs. It is often sufficient to define very stiff springs (see Figure 02). The same applies to the foundation perpendicular to the surface. You can find more information in [1] and under the links below this FAQ.

#### Reference

 [1] Barth, C., & Rustler, W. (2013). Finite Elemente in der Baustatik-Praxis (2nd ed.). Berlin: Beuth.