When you are simulating the modal vibration of a part that is supporting another part—for example, a bracket—you must take into account the mass of the supported part. If the supported part is much stiffer than the bracket, you can assume it will vibrate as a rigid body, and you need not consider its stiffness.

In such a case, you can substitute a mass element for the supported part. Mass elements must be connected to the rest of the part with elements, usually beams. Because these beams should behave in a rigid manner, you should increase the stiffness or the dimension of the section and give them a zero material density. Review the modal shape results to ensure that the beam elements are not bending. In a similar way, you can substitute spring elements for flexible components.