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* *Constraint* is a set of two or more [joints|kb:Joint] which are _constrained_ such that their displacements relate. Constraints may be used to model 1). rigid-body behavior, where joints translate and rotate together in a rigid connection; 2). equal-displacement behavior, where displacement along certain DOF are equal; and 3). symmetry / anti-symmetry conditions. Constraints enhance computational efficiency by reducing the number of equations necessary for solution.

* *Body constraint* is applied to a set of joints such that they translate and rotate together as a rigid body. By default, [rigid behavior|kb:Rigid behavior] is applied to all degrees of freedom (DOF), though users may specify body constraints for only certain DOF. Body constraints 1). simulate rigid connections; 2). connect portions of a model defined by separate meshes; and 3). connect [frames|kb:Frame] to [shells|kb:Shell].

* *Weld* is useful for connecting parts of a model defined by different meshes in that it generates multiple sets of body constrains between joints which are coincident, meaning located within a certain distance of one another. This range is known as the tolerance. Only joints within the weld are checked for coincidence.
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The [_CSI Analysis Reference Manual_|doc:Analysis Reference Manual] (Chapter 5 > Page 49) provides additional information on these constraint types, along withand the many othersother, constraint types available.

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While a constraint is associated with the relationship among a set of joints, a *restraint* concerns only the behavior of an individual joint. Restraints must be applied to certain DOF when joint displacement is known. This displacement may be zero, such as with support points, or non-zero, such as with support settlement. The force necessary to produce a specified restraint displacement is known as the _reaction_, and is generated during analysis. Restraints should also be applied when stiffness is zero along certain DOF, such as with the out-of-plane translation and in-plane rotation of a planar frame. Additional information is available in the [_CSI Analysis Reference Manual_|doc:Analysis Reference Manual] (Chapter 4 > Page 34)

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