For a single massive foundation block, the majority of the damping would be due to radial or geometric damping provided by the surrounding soil. The effect of material damping within the foundation block itself would be likely much smaller than the effect of geometric damping.

When the soil-structure interaction is modeled by a single link, the damping input for such a link on the "Linear Link/Support Directional Properties" form is the actual damping in units of Force-Sec/Length (i.e. Force/Velocity). The damping assigned to the link should represent the actual radial damping. This approach is described in Wilson 2004.

The modal damping specified on the "Analysis Case Data - Linear Modal History" form would represent the material or structural damping provided by the foundation system itself.

References

• Wilson, Dr. Edward L. Static & Dynamic analysis of Structures. 4th ed. Berkeley: Computers and Structures, Inc., 2004. Print.
  (Chapter 16.8: Use of Springs at the Base of a Structure, page 197)
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