A structure must be sufficiently restrained for analysis to be carried out.  A minimum requirement is that the structure be restrained against movement in each of the x, y and z directions and that it should not have any mechanisms caused by collapsible pinned structures.

The restraints on the structure are specified by first selecting the joint or joints to be restrained and then selecting the appropriate type of restraint.  The restraint may take the form of a zero displacement restraint or a spring.  A joint may have either a restraint, or a spring for each degree of freedom but not combinations of these restraints for any degree of freedom.  Thus a joint could have a vertical restraint and a horizontal spring but not a horizontal restraint and a horizontal spring or a vertical restraint and a vertical spring.

All restraints and springs that are applied to a joint are specified in the local coordinate system at each joint. The local coordinate system is typically the same as the global coordinate system but can be modified using the Joint Orientation feature described later in this chapter.

See the Prescribed Displacement  description in the Loads section for information on applying non-zero displacements.

You can modify the restraint or spring on a joint by double clicking on the restraint or spring icon at the joint (and not on the joint itself).