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# How to avoid natural frequencies

## How to avoid natural frequencies

(OP)
I have a truss structure and I can't get rid of 3 modes of vibration. I tried different configuration, increase the thickness of the pipes, nothing seems to help. Is there a general rule to follow? I've been trying trial and errors so far, maybe I'm missing something.

Thanks!

### RE: How to avoid natural frequencies

So you are performing dynamic analysis (other than eigenfrequency extraction) and you want to avoid natural frequencies occuring in the range of excitation, right ? Or is it eigenfrequency simulation where you want to get different modes ?

Anyway, to modify natural frequencies you can follow the simple equation for eigenfrequency of 1 DOF system: f=√(k/m). This tells you that you have to change stiffness or mass (density) to get different natural frequencies. As you can see from the equation to increase f you have to increase stiffness or decrease mass. Another option is to use optimization to get desired frequencies.

### RE: How to avoid natural frequencies

(OP)
It's actually eigenfrequency problem. I have to stay away from three specific frequency and unfortunately the structure still has natural modes in these frequency ranges so I was wondering if there was a rule to apply in order to change the structure and avoid the modes. I case it depends on case by case.

### RE: How to avoid natural frequencies

If the first approach (changing stiffness and/or mass by modifying geometry) still doesn’t give you desired effect then I think it would be best to try with optimization. It can modify geometry in such way that selected eigenfrequencies will be avoided.

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