Symmetry About a Plane

For a structure that is symmetric about a plane, symmetric loading causes symmetric displacements as follows:

Forces and displacements parallel to the plane of symmetry are equal.
Forces and displacements normal to the plane of symmetry are opposite.
Moments and rotations parallel to the plane of symmetry are opposite.
Moments and rotations normal to the plane of symmetry are equal.

Consider the example of a structure that is symmetric with respect to a plane normal to the X axis and subjected to symmetric loading. One Equal Constraint and one Opposite Constraint must be defined for each pair of joints that is symmetrically located with respect to the plane. The degrees of freedom that would be specified for the Equal Constraints are UY, UZ, and RX, and for the Opposite Constraint, degrees of freedom UX, RY, and RZ. The constraint types would be switched to model the symmetric case.

Any joints on the plane of symmetry should not be constrained, but instead have their UX, RY, and RZ degrees of freedom restrained.