November 22, 2021 at 9:27 pm

peteroznewman

Subscriber

You say that the first modal frequency of your structure is around 8 Hz, but in the opening of this discussion, you say the first mode is 1.66 Hz with 2.4% damping. Is the 1.66 Hz an experimental measurement from the lab? If so, your structural model is a long way from the experimental data.

I am familiar with the elcentro data. In fact, you are using the NS direction of the data. There is also an EW direction data set as well as an UP direction.

I use a set of scripts called vibrationdata in matlab to help manipulate data. One of the things it can do is subject a SDOF mass/spring/damper to a base input and calculate the response.

I convert the 2.4% damping you reported in the opening to a Q value of 20.83 for use in the calculation. I can use that value of damping with an 8 Hz natural frequency SDOF system and obtain the following relative displacement between the base and the mass:

But look at the response of the mass when it has a natural frequency of 1.66 Hz. It is huge!

The above plots are just for illustration since you have a multiple DOF structure and not a SDOF system.

I see you applied Damping by entering Frequency and Damping Ratio, and let ANSYS compute Beta, but left Alpha at 0. This means the damping ratio will increase linearly with frequency. So while you get 2.4% Damping Ratio at 1.66 Hz, by the time you get to 10.2 Hz, the damping is up to 14.7% which is way too high!

If you use both Alpha and Beta, you can keep the Damping relatively constant over the range of 1.66 Hz to 10.2 Hz.

I guessed at Alpha and Beta, but you can solve for values that put two frequencies on the same value of Damping Ratio.

I am familiar with the elcentro data. In fact, you are using the NS direction of the data. There is also an EW direction data set as well as an UP direction.

I use a set of scripts called vibrationdata in matlab to help manipulate data. One of the things it can do is subject a SDOF mass/spring/damper to a base input and calculate the response.

I convert the 2.4% damping you reported in the opening to a Q value of 20.83 for use in the calculation. I can use that value of damping with an 8 Hz natural frequency SDOF system and obtain the following relative displacement between the base and the mass:

But look at the response of the mass when it has a natural frequency of 1.66 Hz. It is huge!

The above plots are just for illustration since you have a multiple DOF structure and not a SDOF system.

I see you applied Damping by entering Frequency and Damping Ratio, and let ANSYS compute Beta, but left Alpha at 0. This means the damping ratio will increase linearly with frequency. So while you get 2.4% Damping Ratio at 1.66 Hz, by the time you get to 10.2 Hz, the damping is up to 14.7% which is way too high!

If you use both Alpha and Beta, you can keep the Damping relatively constant over the range of 1.66 Hz to 10.2 Hz.

I guessed at Alpha and Beta, but you can solve for values that put two frequencies on the same value of Damping Ratio.