Mass Idealizations—Generative Design
A mass or point mass idealization is used to represent a concentrated mass without stiffness and no specific shape. The mass and shape of an object determines how the object resists translation and rotation. You can use a point mass idealization to study how the model behaves with that mass located at a particular location without specifying the entire geometry. The point is considered as the center of gravity of that mass.
The mass must be connected to the model by selected geometrical references. In a structural study, the mass only has an effect if gravity, acceleration, or centrifugal loads are also defined for the study.
To Define a Mass Idealization
1. Click 
Mass. The Point Mass Definition dialog box opens.
Mass. The Point Mass Definition dialog box opens.2. Specify a name for the mass, or use the default name. Optionally, click the color swatch adjacent to the Name box to change the color of the icon, the distribution, or the text displayed for the mass.
3. Select the reference where the point mass idealization is to be located. Your selection is shown in the Location collector. Valid reference is a single datum point.
4. Click the Connected references collector and select the references that are connected to the location references. Valid references are surfaces. You can select multiple references of the same type only.
5. Select one of the following behaviors:
◦ Deformable—Allow connected bodies to deform.
◦ Rigid—Do not allow connected bodies to deform.
 | A point mass idealization with deformable behavior on a large area may increase the optimization time. |
6. Specify a positive real number, a parameter, or an expression that evaluates to a positive real number as the value of mass.
7. (Optional) Expand Additional Information and specify the moment of inertia about the mass element's center of gravity with respect to the axes and principal planes of the WCS. Select units for moment of inertia or retain the principal units.
Depending on the situation, you may need to specify the off-diagonal terms when you specify moments of inertia.
8. The default coordinate system of the model is the reference coordinate system and is displayed in the Coordinate system collector. If the model does not have a default coordinate system, then the 
button is not available and the Coordinate system collector is empty. You can select any coordinate system, or create a UCS (user coordinate system) and designate it as the default coordinate system.
button is not available and the Coordinate system collector is empty. You can select any coordinate system, or create a UCS (user coordinate system) and designate it as the default coordinate system.You can also select an alternative reference coordinate system from the Model Tree or graphics window.
Click to return to using the default coordinate system of the model.
to return to using the default coordinate system of the model.9. Click OK to define the mass and to save it.
Specifying the Off-diagonal Moments of Inertia
The moment of inertia of a mass element depends on the coordinate system you use to define the property. It consists of six independent components: Ixx, Iyy, Izz, Ixy, Ixz, and Iyz, where the subscripts refer to the axes of the reference coordinate system.
In most cases, the moment of inertia definition requires only three diagonal terms—Ixx, Iyy, and Izz—with zero values for the off-diagonal terms. This depends on the principal coordinate system you use to define the moment of inertia and is always true if any two of the three coordinate system planes are planes of reflection symmetry for the mass element.