Radiation damping or absorbent boundary conditions

[oscar]

I am working on site response analysis and I want to use elastic base with outcropping motion. When we define the seismic input in terms of shear stresses (or forces for 2D) we define a zero length element with dashpot and spring. We can calculate the damping coefficient (c) of dashpot by using the shear wave velocity and mass density of the rock.

What about the spring coefficient "k" ? Should it be zero? If not how can it be calculated?

Thanks

[oscar]

Explanation:

By spring coeficient "k", I mean the "E", modulus of elasticity of the "uniaxialmaterial elastic" .

uniaxialMaterial Elastic $matTag $E <$eta>

I called $E as "k" and $eta as "c" in the previous message.