Structural Mass

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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 mass and rotary inertia may be assigned to each coordinate direction. 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 Geometry

MASS21 Input Data

The mass element is defined by a single node, concentrated mass components (Force*Time2/Length) in the element coordinate directions, and rotary inertias (Force*Length*Time2) about the element coordinate axes. The element coordinate system may be initially parallel to the global Cartesian coordinate system or to the nodal coordinate system (KEYOPT(2)). The element coordinate system rotates with the nodal coordinate rotations during a large deflection analysis. Options are available to exclude the rotary inertia effects and to reduce the element to a 2-D 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.

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

MASS21 Input Summary



Degrees of Freedom
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
MASS, if KEYOPT (3) = 2
MASS, IZZ, if KEYOPT (3) = 3
MASS, if KEYOPT (3) = 4
(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. See also KEYOPT(2)).
Material Properties

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

Surface Loads


Body Loads


Special Features
Birth and death
Coriolis effect
Large deflection
Linear perturbation

Real constant interpretation (mass/volume or rotary inertia/density):

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)


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


Rotary inertia options:

0 -- 

3-D mass with rotary inertia

2 -- 

3-D mass without rotary inertia

3 -- 

2-D mass with rotary inertia

4 -- 

2-D 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

  • 2-D 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, MASSY, and MASSZ if (KEYOPT(3) = 0). Only the precise mass summary includes the directional masses.

  • 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.

ANSYS Mechanical Pro 

  • Birth and death is not available.

  • Linear perturbation is not available.

ANSYS Mechanical Premium 

  • Birth and death is not available.

Release 18.2 - © ANSYS, Inc. All rights reserved.