MASS21

Structural Mass

MASS21 Element Description

MASS21 is a point element having up to six degrees of freedom: translations in the nodal x, y, and z directions and rotations about the nodal x, y, and z axes. A different concentrated mass component and rotary inertia may be assigned to each coordinate direction. The full mass matrix can also be directly defined, with only the upper triangular portion of the mass matrix required. See MASS21 in the Mechanical APDL Theory Reference for more details about this element.

Another element with a full mass matrix capability (off-diagonal terms) is MATRIX27.

Figure 21.1: MASS21 Geometry

MASS21 Input Data

The mass element is defined by a single node. Its mass matrix can be defined with one of the following two sets of input data:

Concentrated mass components (Force*Time²/Length) in the element coordinate directions, and rotary inertias (Force*Length*Time²) about the element coordinate axes, activated using KEYOPT(1) = 0 or 1. For 3D mass with inertia option (KEYOPT(3) = 0), a distance from the concentrated mass location to the element node can also be input as three offset components in the element coordinate directions (see Figure 21.1: MASS21 Geometry).
Matrix constants in the upper triangular portion of the element mass matrix under the element coordinate system (available only to 3D mass with inertia option), activated using KEYOPT(1) = 2.

The element coordinate system may be initially parallel to the global Cartesian coordinate system, the nodal coordinate system, or a user-defined element coordinate system (KEYOPT(2)). The element coordinate system rotates with the nodal coordinate rotations during a large deflection analysis.

If a distance is defined between the mass location and the element node via the real constants (OFFX, OFFY, and OFFZ), the orientation pointing from the element node to the mass location rotates with the element coordinate system.

Options are available to exclude the rotary inertia effects and to reduce the element to a 2D capability (KEYOPT(3)). If the element requires only one mass input, it is assumed to act in all appropriate coordinate directions. The coordinate system for this element is shown in Figure 21.1: MASS21 Geometry.

KEYOPT(1) = 1 defines the mass in volume*density form, which allows plotting of the mass using /ESHAPE, as well as the use of a temperature-dependent density.

If a distance is defined between the mass location and the element node, EPLOT can be used with /ESHAPE,1 to visualize the mass at its actual location for both /GRAPHICS,POWER and /GRAPHICS,FULL. Additionally, the deformed shape with offsets can be plotted in postprocessing using PLDISP and /ESHAPE,1 but only with /GRAPHICS,FULL.

A summary of the element input is given in "MASS21 Input Summary". Element Input gives a general description of element input.

MASS21 Input Summary

Nodes

Degrees of Freedom

UX, UY, UZ, ROTX, ROTY, ROTZ if KEYOPT(3) = 0

UX, UY, UZ if KEYOPT(3) = 2

UX, UY, ROTZ if KEYOPT(3) = 3

UX, UY if KEYOPT(3) = 4

(degrees of freedom are in the nodal coordinate system)

Real Constants

MASSX, MASSY, MASSZ, IXX, IYY, IZZ, KECN, OFFX, OFFY, and OFFZ if KEYOPT(3) = 0 and KEYOPT(1) = 0 or 1

KECN, C1, C2, …, C21 if KEYOPT (3) = 0 and KEYOPT(1) = 2

MASS if KEYOPT(3) = 2

MASS, IZZ, if KEYOPT(3) = 3

MASS if KEYOPT(3) = 4

Note: MASSX, MASSY, and MASSZ are concentrated mass components in the element coordinate directions.

IXX, IYY, and IZZ are rotary inertias about the element coordinate axes. KECN is a coordinate system reference number. See also KEYOPT(2).

OFFX, OFFY, and OFFZ are components in element coordinate directions to define the distance from the offset mass location to the element node.

C1, C2, …, and C21 are matrix constants in the upper triangular portion of the element mass matrix. See the matrix constant distribution below.

Material Properties

MP command: DENS (if KEYOPT(1) = 1), ALPD

Surface Loads

None

Body Loads

None

Special Features

KEYOPT(1)

Real constant interpretation:

0 --: Interpret real constants as masses and rotary inertias
1 --: Interpret real constants as volumes and rotary inertias/density (Density must be input as a material property)
2 --: Interpret real constants as mass matrix constants (applicable to KEYOPT(3) = 0 only)

KEYOPT(2)

Initial element coordinate system:

0 --: Element coordinate system is initially parallel to the global Cartesian coordinate system
1 --: Element coordinate system is initially parallel to the nodal coordinate system
4 --: Element coordinate system is initially parallel to the user-defined element coordinate system (real constant KECN). Only supported for 3D mass with rotary inertia option (KEYOPT(3) = 0).

KEYOPT(3)

Rotary inertia options:

0 --: 3D mass with rotary inertia
2 --: 3D mass without rotary inertia
3 --: 2D mass with rotary inertia
4 --: 2D mass without rotary inertia

MASS21 Output Data

Nodal displacements are included in the overall displacement solution. There is no element solution output associated with the element unless element reaction forces and/or energies are requested.

MASS21 Assumptions and Restrictions

2D elements are assumed to be in a global Cartesian Z = constant plane.
The mass element has no effect on the static analysis solution unless acceleration or rotation is present, or inertial relief is selected (IRLF).
The standard mass summary printout is based on the average of MASSX (or C1), MASSY (or C7), and MASSZ (or C12) if (KEYOPT(3) = 0). Only the precise mass summary includes the directional masses.
The distance between the mass location and the element node remains constant.
In an inertial relief analysis, the full matrix is used. All terms are used during the analysis.

MASS21 Product Restrictions

When used in the product(s) listed below, the stated product-specific restrictions apply to this element in addition to the general assumptions and restrictions given in the previous section.

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