Hi everybody,
I have set up a simple 1 DOF system to to practice with frequency response loadcases.
My model has a 0,001ton (1kg) mass placed on a spring with stiffness (4
^2)*(10^-3)N/mm. Consequently, the natural frequency is 1Hz.
I also set up G (structural damping, found in the PARAM card) as sqrt(3), in order to have a damped resonant frequency of 0,5Hz (w_damped=w_natural * (1-Z^2); where Z stands for damping factor and G corresponds to 2*Z).
Finally, I imposed a sinusoidal load to the mass with magnitude equal to the spring stiffness (in order to have a unitary displacement at zero frequency).
I have currently two problems with my model:
- The displacement magnitude at zero frequency is affected by the structural damping (which shouldn't be) and has a magnitude of 0,5 instead of 1. Putting G (structural damping) to 0 results in a correct amplitude of 1 (see undamped_freq_resp.hm) .
- The resonant frequency is not affected by structural damping (whereas it should, according to the following law: w_damped=w_natural * (1-Z^2). In fact, resonant is found at 1Hz for both damped and undamped simulations.
Could anybody help me? Is my mistake theoretical or practical (modelling)?
Three models to be found attached: Damped frequency response, undamped frequency response and static case. To be found also the graph displacement z vs frequency for the damped frequency response and the displacement for the static case.
Thank you all.
Yuri
