Rayleigh Damping and Viscous Damper Element

[tguha]

Hello,
I wanted to know the difference between the way to use Rayleigh damping in a structural analysis vs the way to use viscous damper elements. More specifically, if I know the viscous properties (storage stiffness, damping coefficient and alpha) of the visco-elastic damper installed in the structure, should I use the viscous damper element to model the force displacement response of the damper (for a non-linear time history analysis) and then additionally use Rayleigh damping (in the analysis case) to quantify the inherent structural damping?? Or should I just use Rayleigh damping with an estimated damping ratio obtained from field pluck test, without having to model the damper response?? I am aware that there is an example of a 1 story Moment frame with viscous dampers that uses both a damper element model and Rayleigh damping based on the first mode frequency. I want to know the reason for needing to use both.
Thank you for your anticipated help.

Regards
Tushar Guha

[fmk]

if you have a damper, model it explicitly and use rayleigh or similar to account for the typical structure/contents damping.