Modal Damping Table
Description: Defines model damping as a tabular function of frequency.
Format:
Example:
| Field | Definition | Type | Default |
|---|---|---|---|
| TID | Table identification number. | Integer > 0 | Required |
| TYPE | Type of damping units, one of the following character variables: G, CRIT, or Q. | Character | G |
| fi | Frequency value in cycles per unit time. | Real ≥ 0.0 | Required |
| gi | Damping value. | Real | Required |
Remarks:
- Modal damping tables must be selected with the Case Control command SDAMPING = TID.
- The frequency values, fi must be in either ascending or descending order, but not both.
- Discontinuities may be specified between any two points. If g is evaluated at a discontinuity, then the average value of g is used. In Figure 1, the value of g at f = f3 is g = (g3 + g4)/2.
- At least one continuation entry must be specified.
- Any fi-gi pair may be ignored by placing SKIP in either of the two fields.
- The end of the table is indicated by the existence of ENDT in either of the two fields following the last entry. Any continuations that follow the entry containing the end-of-table flag ENDT will be ignored.
- TABDMP1 uses the algorithm
where f is input to the table and g is returned. The table look-up is performed using linear interpolation within the table and linear extrapolation outside the table using the two starting or endpoints, see Figure 1. No warning messages are given if table data is input incorrectly.
Figure 1. Example of Table Extrapolation and Discontinuity
- This form of damping is only used in modal formulations of complex eigenvalue, transient response, or frequency response analysis. The type of damping used depends on the solution sequence (structural damping is displacement-dependent and viscous damping is velocity-dependent).
- PARAM, KDAMP may be used in solution sequences that perform modal frequency and modal complex eigenvalue analysis to select the type of damping. (See Section 5, Parameters, for more information on KDAMP.)
- If TYPE is G or blank, the damping values gi, etc., are in units of equivalent viscous dampers, as follows:
If TYPE is CRIT, the damping values gi, etc., are in the units of fraction of critical damping C/C0. If TYPE is Q, the damping values gi are in the units of the amplification or quality factor, Q. These constants are related by the following equations:
