Select Modal decomposition and click Parameters in the Time history analysis dialog to define the parameters of the modal decomposition method.
The previous dialog also displays when you define damping (Damping - Varying for each mode) for the following analysis types:
- Harmonic
- Harmonic analysis in the frequency domain (FRF)
- Footfall analysis.
The method of modal decomposition takes advantage of the settings and results obtained for modal analysis that are included in the list of load cases.
The values of damping (greater than 0.0) for each eigenmode in modal analysis are the allowable parameters for the decomposition method. Damping is understood as relative damping (part of critical damping).
To define the damping value for an eigenmode, Enter the eigenmode number and the corresponding damping value and click Add.
You can also modify and delete active table rows by clicking the appropriate buttons. Click Delete all to delete the defined list of damping.
You can define a value of damping for each mode, or only for selected modes. A value of damping for intermediate values that are not defined on the list, is interpolated based on the specified values. Start and end values are constant. This is shown in the following image.
The following values of damping are used:
- for mode 3 damping is 0.03 (3%)
- for mode 5 damping is 0.05 (5%).
Following this principle:
- damping 0.03 is assigned to modes 1 and 2
- an interpolated damping value of 0.04 is assigned to mode 4
- a constant value 0.05 is assigned to the remaining modes - above mode 5.