Use the Modal Damping form to specify or modify the modal damping. For convenience, specify a damping that applies to all modes and then, as required, overwrite the damping for any mode(s).
Constant Damping for All Modes option. When this option is selected, use the edit box to enter a constant damping value for all modes. This is a percent critical damping. A damping that is 5% of critical damping is entered as 0.05.
Note: Please refer to the Damping topic for an explanation of the damping described by the next two bullets.
Interpolated Damping by Period or Frequency option. Use this option to specify damping interpolated by period or by frequency. Enter the Period or the Frequency and then enter the damping value. Click the Add button. Highlight an entry and click the Modify button to make adjustments to existing damping. Highlight an entry and click the Delete button to delete interpolated damping.
Mass and Stiffness Proportional Damping by Coefficient option. Use this option to specify damping using mass and stiffness proportional coefficients. Directly specify the coefficients, or specify the coefficient using modal damping by period or by frequency.
Modal Damping Overwrites option. Use this option when modal damping for some modes is different from the damping specified for all modes.
To overwrite a modal damping value type, select the mode number in the Mode drop-down list and then type the damping value in the Damping edit box. (A damping that is 5% of critical damping is entered as 0.05.) Click the Add button.
To modify an existing modal damping value, highlight the existing damping value in the Mode/Damping display list. Note that the data associated with that load case appears in the Mode drop-down list and Damping edit box. Modify the mode number or damping as required. Click the Modify button.
To delete an existing modal damping value, highlight the existing damping value in the Mode/Damping display list. Click the Delete button.
Modal damping overwrites can be specified for any mode number. If the damping overwrite specified for a mode is larger than the number of modes specified for the analysis, that damping overwrite is simply ignored when the analysis is run.
Access the Modal Damping form as follows:
Click the Define menu > Load Cases command to display the Load Cases form.
Click the Add New Case button or highlight a previously defined load case and click the Add Copy of Case or the Modify/Show Case button to display the Load Case Data form.
Do one of the following:
Set the Load Case Type/Subtype to Time History/ Linear Modal to display the Load Case Data form for a Linear Modal history case
Set the Load Case Type/Subtype to Time History/Nonlinear Modal to display the Load Case Data form for a Nonlinear Modal history case.
Click the Modal Damping Modify/Show button.