1. The Modal analysis provides the first natural frequency of the structure. You use that information to decide the time step to use in Transient Structural. For example, if the first natural frequency was 500 Hz, you would not waste your time with a time step of 0.01 seconds (100 Hz). At the other end, if the first natural frequency was 5 Hz, you would not waste your time with a time step of 0.001 second (1000 Hz). If you have no idea what the first natural frequency is, how do you choose a time step for Transient Structural?
2. Yes, you can have any one of the four types and they are all additive. Damping applied to the material in Engineering Data is additive with damping applied under the Analysis Settings in the Damping Controls. You can have zero damping for any of those types applied from the material or the analysis.
3. Each type of damping is independent and additive.
4. If you can't obtain experimental data, do a sensitivity study to see how much the response changes for a range of possible damping values.
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