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How is damping specified for different parts of the structure?

Answer: The Additional Material Damping menu (Figure 1) allows users to specify different material-damping properties for each material present in the analytical model.

Figure 1 - Additional Material Damping menu

Can the software calculate the percentage of critical damping achieved by dampers added to a model?

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1. For response-spectrum analysis, the damping for each mode is reported in the Response Spectrum Modal Information table, which users may access through Display > Show Tables > Analysis Results > Structure Output > Modal Information > Table: Response Spectrum Modal Information. The linear, or effective, damping of these dampers contributes to this value, which may be used for estimation purposes.
2. When performing time-history analysis, a single degree-of-freedom approximation method is described in Dr. Edward L. Wilson’s text Static and Dynamic Analysis of Structures (Energy Dissipation in Real Structures, page 228). Here, a static displacement is applied to the structure, then released such that the structure vibrates with displacement decay. The attributes of this decay pattern may be used to calculate critical damping ratio.

Why do results differ between steady-state and modal analyses when damping is apparently consistent?

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Further discussion: damping is useful for simulating energy dissipation that is not explicitly modeled within a structural system. Nonlinear behavior such as friction is an example of such a mechanism. The plastic-hinge formulation will explicitly account for energy dissipation, within the hinge region, which is caused by yielding during dynamic analysis. 

References

• Wilson, E. L. (2004). Static and Dynamic Analysis of Structures (4th ed.). Berkeley, CA: Computers and Structures, Inc.
  Available for purchase on the CSI Products > Books page