Use Soft Spring to Stabilize Model

Check this option to instruct the program to add soft springs to prevent instability. If you apply loads to an unstable design, it will translate and/or rotate as a rigid body. You should apply adequate restraints to prevent rigid body motion.

To add soft springs, select Use soft spring to stabilize model in the Static dialog box

If you cannot stabilize the design by applying adequate restraints, select this option to add soft springs and rerun the analysis. Animating the resulting deformed shape will show you excessive motion in one or more directions. You can then apply appropriate restraints to prevent rigid body motion.

A rigid (free) body mode of a structure represents motion without internal deformations. In structural analysis, rigid body modes arise due to either insufficient or nonexistent constraints imposed on the structure. However, there are analysis problems where the imposition of constraints is not desirable, yet the deformable behavior of the structure under loads is of interest.

For example, you may come across a stress analysis problem where an equilibrium state is maintained by the applied loads, thus requiring no constraints. Ordinarily, for such problems the program will execute unsuccessfully, indicating a message about singular stiffness matrix. However, if you specify the option for soft spring addition, the program will automatically add a small stiffness to the diagonal terms of the structural stiffness matrix as shown below, enabling numerical stability during solution:

where k_ss denotes the stiffness added due to the soft spring option.

Singular stiffness matrices are also encountered due to ill-posed problems, for example models with very low or highly disproportionate elastic moduli and section properties.