# Discussion Forum

## Effect of tension on eigenfrequency

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Jillian Kiser

January 30, 2013 9:55pm UTC

Effect of tension on eigenfrequency

Hi,
I'm trying to model the effect of tension on the eigenfrequency, and I think I'm misunderstanding the use of boundary loads. I have a long and narrow bridge (fixed at both ends, free along the length). I would expect that adding a boundary load on either end of the bridge, in opposing directions, would essentially be like tensioning a string, which will cause the resonance frequency to increase. But I'm not seeing any change in resonance. Why not?

I've also tried anything else I could think of that might change the resonance - body loads, strain at either end, etc. Nothing effects the first fundamental resonance. Any ideas would be hugely appreciated!

Regards,
Jillian

Josh Thomas
Certified Consultant
AltaSim Technologies, LLC

January 30, 2013 10:36pm UTC in response to Jillian Kiser

Re: Effect of tension on eigenfrequency

Jillian,

Well, your fixed constraint conditions are overriding your boundary load condition at the end points. So, even in a stationary solution you will get a zero stress solution. You will want to consider only fixing one end.

There are a couple of ways to simulate the effect of tension (stress) on the natural frequency of your system. COMSOL has a very nice model in the model library called "Vibrating String" that I would advise you work through. I think that will help you a lot.

In my opinion, the most intuitive way to set up the problem is by solving in 2 steps. First, a stationary step that solves for the pre-stressed state of your system. Then, an eigenfrequency step that solves for the natural frequencies of the tensioned member(s). This method is covered in the Vibrating String example I mentioned.

Best regards,
Josh Thomas
AltaSim Technologies

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