Learning Forum

[oglawal2]

Hi good day,

 

I previously did impact simulation on a bridge and set the frequency to be 2.5 hz. Now I changed the frequency to be 10 hz but still seems like my response is 2.5 hz. How can I fix this?

[peteroznewman]

The natural frequency of the bridge is 2.5 Hz.

You set the damping ratio to be 1% at 2.5 Hz which affects the rate at which the envelope of the amplitude of vibration reduces over time. Try a much larger damping ratio like 10% to see the envelope reducing over time.

Then you changed the damping ratio to be 1% at 10 Hz, but that does not change the natural frequency of the bridge, it affects the rate at which the envelope of the amplitude of vibration reduces over time.

Do some research on Rayleigh Damping and you will find an equation that converts a Damping Ratio into a Stiffness Coefficient and an equation to plot Damping Ratio as a function of frequency. That equation will show you what the new damping ratio is at 2.5 Hz when you changed it to be 1% at 10 Hz.

If you want to change the natural frequency of the bridge, you need to reduce the mass and/or increase the stiffness of the structure.

[oglawal2]

 

Thanks for your reply, how can I make my bridge natural frequency to be at 10 Hz ? I need to increase the stiffness but I can't alter the dimensinal properties of the bridge because i'm trying to match a specific bridge.

 

[peteroznewman]

To increase the natural frequency of the bridge, reduce the mass and increase the stiffness of the structure.

There is a method to identify where on the bridge the mass most contributes to the low frequency and where on the bridge the low stiffness contributes the most to the low frequency.  Read this post: /forum/forums/topic/modal-energy-analysis-tutorial/

[oglawal2]

Thank you.