COMBI250 Element Description

COMBI250 represents an element for which precise geometry is undefined, but elastic kinematic response can be specified by stiffness, viscous damping, or structural damping in diagonal matrix form in the element coordinate system. The element has two nodes, each with six degrees of freedom per node: translations in the nodal x, y, and z directions, and rotations about the nodal x, y, and z axes. See COMBI250 in the Mechanical APDL Theory Reference for more details about this element.

Other similar elements are the spring-damper element (COMBIN14) and the bearing element (COMBI214).

Figure 250.1: COMBI250 Schematic (2 nodes with KEYOPT(1) = 1)

Figure 250.2: COMBI250 Schematic (2 nodes with KEYOPT(1) = 2)

Figure 250.3: COMBI250 Schematic (3 nodes)

K located in xe-ye plane

COMBI250 Input Data

The locations of nodes I, J, and K, the element center C, and the element coordinate system (x_e, y_e, z_e) are shown in Figure 250.1: COMBI250 Schematic (2 nodes with KEYOPT(1) = 1), Figure 250.2: COMBI250 Schematic (2 nodes with KEYOPT(1) = 2), and Figure 250.3: COMBI250 Schematic (3 nodes).

The degrees of freedom are ordered as UX, UY, UZ, ROTX, ROTY, ROTZ for node I followed by the same for node J. Node K is optional and only used for the orientation of the element when the element coordinate system is not specified (KECN).

A mass (MASS) can be added at the element center. The masses at nodes I and J are calculated based on the distances between the element nodes, and the element center C.

An offset can be defined for the spring-damper location. Rigid-body motion is enforced between the nodes and this location. By default (KEYOPT(1) = 0), it located at element mid-length. Depending on KEYOPT(1), either the ratio of length XC, or the element center coordinates (XC, YC, and ZC) in the center coordinate system (identified with KCCN) are specified. By default, when KCCN is not input, coordinates are in the global coordinate system. This coordinate system must be Cartesian or cylindrical.

The element coordinate system (x_e, y_e, z_e) can be defined with the coordinate system identification number KECN or third node K. It is the coordinate system in which the element matrix is diagonal. This coordinate system KECN must be Cartesian or cylindrical. By default (no node K and no KECN value), the element coordinate system is the global Cartesian. KECN specification overrides the orientation given by node K if both are present.

If the element coordinate system is cylindrical, both nodes of the element must lie in the X-Y plane of the global coordinate system (plane of rotation). All three points: coordinate system origin, node I, and node J must not be coincident. If the coordinate system origin is not coincident with node I, the element x-axis is along the line passing through both the coordinate system origin and node I. When coincident, the element x-axis is along the element from node I to node J.

The element coordinate system, if specified, is the first rotation applied. Any specified nodal rotations follow (for example, the NROTAT command). Note that such rotations modify the element matrix.

The element characteristics are defined using real constants. K1 to K6 are stiffness values. CV1 to CV6 are viscous damping values. KIMAG1 to KIMAG6 are structural damping values also considered as the imaginary part of the stiffness values. In a full harmonic analysis, these constants can be defined as table parameters using the frequency as the primary variable (Var1 = FREQ on *DIM). In a full transient analysis, real constants K1 to K6 and CV1 to CV6 can be defined as table parameters using time as the primary variable (Var1 = TIME on *DIM). In a static analysis, real constants K1 to K6 can be defined as table parameters using time as the primary variable (Var1 = TIME on *DIM).

COMBI250 Output Data

The solution output associated with the element is in two forms:

The Element Output Definitions table uses the following notation:

A colon (:) in the Name column indicates the item can be accessed by the Component Name method (ETABLE, ESOL). The O column indicates the availability of the items in the file Jobname.out. The R column indicates the availability of the items in the results file.

In either the O or R columns, Y indicates that the item is always available, a number refers to a table footnote that describes when the item is conditionally available, and a - indicates that the item is not available.

Table 250.3: COMBI250 Item and Sequence Numbers lists output available via the ETABLE command using the Sequence Number method. See The General Postprocessor (POST1) in the Basic Analysis Guide and The Item and Sequence Number Table in this document for more information. The following notation is used in Table 250.3: COMBI250 Item and Sequence Numbers:

COMBI250 Assumptions and Restrictions

COMBI250 Product Restrictions

There are no product-specific restrictions for this element.