Damping may be specified for dynamic load cases. There are two distinct types of damping: Modal damping and proportional damping. These are described below.
Modal Damping
Modal damping is used for response-spectrum load cases and for modal time-history load cases. Modal damping is given as a fraction of critical damping for each mode in the structure. On the Modal Damping form, damping may be specified in one of the following ways:
Constant for all modes
Linearly interpolated by period or frequency. 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.
Mass and stiffness proportional. This mimics the proportional damping described below for direct-integration, except that the damping value is never allowed to exceed unity.
In addition, damping overrides may be specified. These are specific values of damping to be used for specific modes that replace the damping obtained by one of the methods above. The use of damping overrides is rarely necessary.
Proportional Damping
Proportional damping is used for direct-integration time-history load cases. 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 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.
Proportional damping is viscous (fluid-like), unlike hysteretic damping.