This data category is used to input the external constraints on the motions of one or more structures. These constraints fall into two distinct categories; elimination of freedoms at the center of gravity, and articulations between structures. The latter are known as constraints.

Elimination of Freedoms at the Center of Gravity

This facility is used to eliminate one or more of the 6 Degrees of Freedom at the Center of Gravity; for example, if the analysis is 1- or 2-dimensional then the user may eliminate the appropriate 5 or 3 Degrees of Freedom respectively.

This facility is extremely useful when data for a full 3-dimensional analysis is unavailable, or when a simple model is being analyzed. This is achieved by use of the DACF (DeACtivate Freedom) data record (see The DACF Data Record - Deactivate Freedom at the Center of Gravity).

Constraints - Articulations Between Structures

This facility is used to physically connect one structure to another structure by means of an articulated joint. These positions are referred to in the documentation as constraints.

Constraints are defined by specifying the number of the first structure and the number of the second structure, together with their respective node numbers corresponding to the position and orientation of the constraint on that structure.

The position of a constraint at any stage of the analysis is UNIQUE. Only rotational freedoms exist at the articulated joint. The 'sliding' of one structure with respect to another (unconstrained translational freedoms) at a joint is therefore not permitted, i.e. there is no relative translational motion, but the constraint position may move with respect to the FRA.

In addition the term 'structure' is used in this context to mean either a floating structure or a fixed position in the fixed Reference Axis System (for example, an anchored plinth on the sea bed).