From what you write you need to figure out what that natural frequency is for your particular case. Right now you only know your excitation frequency.

In a sinusoidal model of the form y = a⋅sin(b(x−c))+d, the period is found by taking 2⋅π|b|. Frequency is the reciprocal of the period.

Example: y = 2⋅sin(3x) would have a period of 2π/3, which is one-third the length of the "normal" period of 2π. Another way to describe this change from the Parent function would be to say that the graph would cycle through 3 times by the time it reaches 2π. The frequency looks like it is directly affected by the value of "b" in this equation, which is 3.

The response Spectrum of output motion will be helpful to find predominant frequency.

Hi Ahmed Moussa

I have not worked on Plaxis, but based on your question I can understand that the software wants you to select two target frequencies for defining the rayleigh damping (it should also be asking you the damping ratios at these frequencies).

Basically we need to define the unknown rayleigh damping coefficients "alpha" and "beta" so that we can estimate the damping in the model as C=alpha*M + beta*K. Now, these coefficients depend on the target frequencies (f1,f1) and the damping ratio (e1, e2) that we pick. Typically one picks the damping ratio and natural frequencies in such a way that you obtain the desired damping (e(w) = 0.5*((alpha/w) + (beta*w))) in your frequency range of interest.

Hope this helps, you should be able to find more on this in any dynamics textbook.