Q: How can I add a damping of some percentage of critical?
REV 6.01

A: A problem here is the definition of critical damping. Critical damping is defined in terms of the natural period, but there is no "natural period". There is another FAQ on natural period. Thus the first part of the answer is to ask those asking for 6% Critical to define critical and then convert this into a real damping value.

The #DRAG command can then be used to add a diagonal damping matrix. Be careful to add it to the correct point to minimize coupling and you need to change the default to get it to be applied in the frequency domain.