# Constraint

• Constraint is a set of two or more joints that are constrained such that their displacements relate. 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 certain 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. Multiple constraints should not be assigned to a single joint.
• Body constraint is applied to a set of joints that translate and rotate together as a rigid body. Rigid behavior is automatically applied to all DOF within the constraint, though only certain DOF may be specified. Body constraints (1) simulate rigid connections; (2) connect portions of a model that are defined by separate meshes; and (3) connect frames to shells.
• Welds are applied to a region of the model, generating multiple sets of body constraints among joints that are coincident, meaning located within a specified tolerance. Only joints within the weld are checked for coincidence. Welds are useful for connecting parts of a model that are defined by different meshes. Additional information on these constraint types, and others, is available in the CSI Analysis Reference Manual (Chapter V: Constraints and Welds).

• Restraints concern only the behavior of individual joints. Restraints differ from constraints in that constraints are associated with relationships among sets of joints. Restraints should not be applied to joints that are part of a constraint. Instead, restraints may be replaced with either stiff springs or large stiffness properties along those DOF. Restraints must be applied when joint displacement is known at specific DOF. Displacement may be zero, as with support points, or nonzero, as with support settlement. The reaction, generated during analysis, is the force necessary to produce a specified restraint displacement. Restraints should also be applied when stiffness is zero along specific DOF, 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 (Chapter IV: Joints and Degrees of Freedom, Restraint Supports)

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