Joint Body Constraint

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. 

Joint body constraints can be used to account for offset from centerline connections such as a trunnion attachment point.

Here is a simple example of how the Joint body constraint works. We have a 10 ft. long cantilevered pipe with a 100 lb. gravity-direction (-Z) load assigned to an orphan joint which is 2 ft. away from the tip of the cantilever. That is, the point load is 12 ft. from the anchor. Select the end tip joint and the orphan joint, then Assign>Joint>Joint constraints and Define/Add a Body constraint using the defaults to connect them both in all 6 degrees of freedom, then click Apply button.  

After assigning constraints, CSiPlant highlights the constrained joints as shown. Click 'Show undeformed shape' icon to clear this graphical display. 

We define a load case called Point load to display only reactions from the 100 lb. point load without selfweight or other loads, run the analysis and display the 1200 ft-lb moment reaction at the anchor as expected (100 lbs. X 12 ft.).