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For each *joint* of a structural model, *displacement* may either be unknown and solved for, or it may be known and input. Known [joint|kb:Joint] displacement is either zero or nonzero. Zero displacement is specified for fixed DOF at *restraint* support locations. Nonzero joint displacement is specified for such a condition as given support settlement, or application of displacement time history to specific supports, which is described in the [Displacement time-history record|kb:Displacement time-history record] article.

When joint displacement is known for a given DOF (whether zero or nonzero), a [restraint|kb:Constraint] must be assigned along that DOF. A nonzero displacement value may then be assigned using the Assign > Joint Loads > Displacements menu.

Numerical solution proceeds by solving the matrix equation *K u = F*, where

* *K* represents the stiffness matrix

* *u* represents joint displacements

* *F* represents joint forces

{new-tab-link:http://www.csiberkeley.com/}CSI{new-tab-link} Software transfers nonzero joint displacements to the right side of this equation such that equivalent applied loads represent their effect. Unknown displacements on the left side are then solved.


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*Related Incident:*
* {incident:no=27923|comment=1/20/2011 memo: When displacement load is applied to joints that are restrained and connected to links with all DOF as fixed, these are equivalent to constraints. Displacement loading cannot be applied through restraints to constraints. Either a stiff spring should be used at the joint in place of restraints, or stiff properties used in the link rather than fixed DOF. }
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h1. See Also

* *Ground motion at specific supports* -- [Manual multi-support excitation|kb:Manual multi-support excitation]