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Constraints and Modal Analysis
In modal analysis, your model can be constrained or unconstrained.
When it is constrained, its geometry features are fixed in place or in particular directions.
When it is unconstrained, you can consider it to be vibrating while floating in space, without any tethering features.
If your model is very flexible compared to its mountings, use an unconstrained analysis. If it is a stiffer structure—for example, attached to ground—use constraints to simulate the fixtures and supports.
Constrained modal analyses are required for all base excitation dynamic problems. You can use unconstrained modal analysis for dynamic time, frequency, and random analyses, provided a load set is specified.