Damping Ratio |
 
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Damping Ratio |
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Rather than explicitly defining a coefficient (or coefficients), stiffness proportional damping may alternatively be defined as a function of a damping ratio and a damping period in a time domain dynamic analysis. The damping matrix is assembled in exactly the same manner as described in the Damping (Eq.1), however in this case λ and μ are computed from user-specified values of ξ and T as follows:
(1)
where:
•ξ is the damping ratio
•T is the damping period
The damping ratio ξ must be between 0 and 1. If the excitation in your dynamic analysis is a single regular wave, then the specification of damping period T is optional. If omitted, T defaults to the regular wave period. If the excitation is of any other type, then the period input is required.
As with explicit values of λ and μ, it is also possible to assign different values of ξ and T to various element sets within the model. Similarly, individual entries of K and M are scaled by appropriate values of λ and μ, and C is assembled by a matrix addition of the relevant component terms. Again this option is only available in time domain dynamic analyses, and any values of ξ and T specified in a frequency domain analysis are applied to all elements in the model. Note also that the two specification options (i.e. damping ratio/period and stiffness/mass coefficients) are mutually exclusive for any given element set.
•*DAMPING RATIO is used to specify stiffness damping coefficients as a function of a damping ratio and a damping period.