4.21 MASS21 Structural Mass

4.21 MASS21 Structural Mass (UP19980821 ) 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 Section 14.21 in the ANSYS Theory Reference for more details about this element.

Another element, MATRIX27, with a full mass matrix capability (off-diagonal terms) is described in Section 4.27.

Figure 4.21-1 MASS21 Structural Mass



4.21.1 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)). See Section 2.3.2 for a discussion of elements that operate in the nodal coordinate system. 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 two-dimensional 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 4.21-1.

A summary of the element input is given in Table 4.21-1. Section 2.1 gives a general description of element input.

Table 4.21-1 MASS21 Input Summary

Element Name

MASS21

Nodes

I

Degrees of Freedom

UX, UY, UZ, ROTX, ROTY, ROTZ if KEYOPT (3) = 0
UX, UY, UZ if KEYOPT (3) = 2UX, 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 if KEYOPT (3) = 0
MASS if KEYOPT (3) = 2
MASS, IZZ if KEYOPT (3) = 3
MASS if KEYOPT (3) = 4
(Mass and moments of inertia directions are in the element coordinate system, see also KEYOPT (2) ).

Material Properties

None

Surface Loads

None

Body Loads

None

Special Features

Large deflection, Birth and death.

KEYOPT(2)

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 (see Section 2.3.2)

KEYOPT(3)

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
Note-All 2-D elements are assumed to be in the global Cartesian X-Y plane

4.21.2 Output Data

Nodal displacements are included in the overall displacement solution. There is no printed or post element data output for the mass element.

4.21.3 Assumptions and Restrictions

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 only on the x mass term if directional mass is input. In an inertial relief analysis, the full matrix is used. All terms are used during the analysis.

4.21.4 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/LinearPlus