A body constraint is a set of two or more joints (aka points or nodes) that are constrained such that their displacements relate. Body constraints may be used to model (1) rigid-body behavior, in which joints translate and rotate together in a rigid connection; (2) equal-displacement behavior, in which displacement along selected degrees of freedom (DOF) is equal; and (3) symmetry / anti-symmetry conditions. Constraints enhance computational efficiency by reducing the number of equations necessary for solution, and avoid numerical instabilities which can be introduced with the use of rigid "dummy" members. Multiple constraints should not be assigned to a single joint, and each different pair or set of joints which are constrained together need to have their own separate body constraint defined (Body1, Body2, etc.).
A body constraint is assigned to a set of selected joints (Assign menu>Joint>Joint constraints) that translate and rotate together as a rigid body. Moment differentials due to the distance between joints is automatically accounted for by the body constraint.
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After assigning constraints, CSiPlant highlights the constrained joints as shown. Click 'Show undeformed shape' icon 
to clear this graphical display.
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