Modal Damping Form

Access the Modal Damping form as follows:

Click the Define menu > Analysis Cases command to display the Analysis Cases form.
Click the Add New Case button, or highlight a previously defined analysis case and click the Add Copy of Case or the Modify/Show Case button, to display the Analysis Case Data form.
Select Response Spectrum, Time History, Steady State, or Power Spectral Density from the Analysis Case Type drop-down list and in the case of a Time History, choose the Nonlinear or Linear option for the Analysis Type, to display the
Analysis Case Data - Response Spectrum,
Analysis Case Data - Nonlinear Modal History (FNA), or
Analysis Case Data - Linear Modal History forms.
Click the Modal Damping Modify/Show button to display the Modal Damping form.

Three options for modal damping are available. The input required in the form depends on which of the options is selected.

Constant Damping for all Modes option. As the name suggests, the damping is constant for all modes. When this option is selected, the form displays a Constant Damping for all Modes edit box. As an example, to apply 5% damping, enter 0.05 in this edit box. The form will also display a Modal Damping Overrides area that can be used to add, modify, or delete damping overrides.

Interpolated Damping by Period or Frequency option. Specify the damping value at a series of frequency or period points. Between specified points, the damping is linearly interpolated. Outside the specified range, the damping value is constant at the value given for the closest specified point. When this option is selected, the form displays the following further options:

Interpolated by Period option. If this option is selected, the Period and Damping edit and display areas become enabled. Use the appropriate Add, Modify or Delete button to add, modify, or delete the damping per period in these areas.
Interpolated by Frequency option. If this option is selected, the Frequency and Damping edit and display areas become enabled. Use the appropriate Add, Modify or Delete button to add, modify, or delete the damping per frequency.

Mass and Stiffness Proportional Damping by Coefficient option. The damping matrix is calculated as a linear combination of the stiffness matrix scaled by a user-specified coefficient, and the mass matrix scaled by a second user-specified coefficient. The damping value is never allowed to exceed unity. The two coefficients may be specified directly, or they may be computed by specifying equivalent fractions of critical modal damping at two different periods or frequencies. Stiffness proportional damping is linearly proportional to frequency; mass proportional damping is linearly proportional to period. When this option is selected, the form displays further options for the following:

Direct Specification option. When this option is selected, directly enter values for the Mass Proportional Coefficient and the Stiffness Proportional Coefficient.
Specify Modal Damping by Period option. When this option is selected, enter values in the First and Second Periods and associated Damping edit boxes. Note that as values are entered in those edit boxes, SAP2000 calculates and displays the Mass Proportional Coefficient and the Stiffness Proportional Coefficient in their respective display boxes. If after entering values in the Damping edit boxes, the Mass Proportional Coefficient and the Stiffness Proportional Coefficient do not change, click the Recalculate Coefficients button.
Specify Modal Damping by Frequency option. When this option is selected, enter values in the Frequency and Damping edit boxes. Note that as values are entered in those edit boxes, SAP2000 calculates and displays the Mass Proportional Coefficient and the Stiffness Proportional Coefficient in their respective display boxes. If after entering values in the Damping edit boxes, the Mass Proportional Coefficient and the Stiffness Proportional Coefficient do not change, click the Recalculate Coefficients button.

Modal Damping Overrides area. Use the buttons in this area to add, modify, or delete damping overrides.

Add button. Use the Mode scroll arrows to select the Mode. Then enter a value in the Damping edit box and click the Add button to add an override.
Modify button. Highlight the mode to be modified in the Mode display area. Change the damping value in the damping edit box. Click the Modify button.
Delete button. Highlight the mode to be deleted in the Mode display area and click the Delete button.