# Homework Help: How could the frequency of a guy wire be measured?

1. Mar 31, 2012

### Gurj

How to measure tension in guy wires?

Could somebody please help/guide me in the right direction as to how I would go about measuring the tension in guy wires for a college assignment?

I have been told that one method is to place some sort of rigid frame next to the cable, that touches the cable at two points a known distance apart. Then at the midpoint between the contact points use a spring scale to displace the cable a specific measurable distance and that the tension will be proportional to the amount of force required to displace the cable.
And also that I could measure the resonant frequency of the wire and go from there.

Would be really grateful if somebody could help and explain how these particular methods or any methods would actually work and also with the relevant maths/physics formulas that I would need etc.

Thank You.

2. Apr 1, 2012

### Gurj

I need to find the tension in guy wires so decided to go down the route of using the frequency to do so but don't know how i would go about doing so. Would anybody have any advice or help to get me going in the right direction?

Thanks

3. Apr 1, 2012

### scutterbob

There are several ways but most if not all require some calibration. If you are set on using frequency you need to be able to record a signal from an acceloramater or similar device (LVDT, strain gage, etc). You would want to know your boundry conditions are they do effect the best equation to pick (are the ends pinned or fixed). Now I know it would seem a no brainer is a cable hence pinned, but the stiffness of the wire itself actually is a factor if you want to know the value well. You will also need to know the material properties, density, section modulus, area. You will need to play around with the location of the sensor a bit, as you will excite many harmonics or inharmonics as the case may be. You normally need to create an impuse to excite the system (hit with hammer). You can measure it from natual background but you might end up with you cable simply driven by a entire structure natural freq, which is not what you are after.

After you record the signal, you'll do a FFT, big hint here, while you need to be sampling at at least 2x the highest freq. you want to caputre, you are probably dealing with a fundamental freq. in the single digit to 10s of Hz, so you need to sample for a long time to get good resolution of freq. Depending on damping of the system which is probably very small, you should get a good long ring from your wire, if you can sample for 30 sec. thats great, 11,025 samples/sec is more than enough, higher sample rates just make the data streams larger and more math, but no real improvement for these purposes (look up Nyquist function). You will need to identify the peaks in the fft as these are your natural freq and harmonics. As I aluded to before they can often be inharmonic, so if you fundatmental is 10 Hz, your next harmonic might be 22 Hz, not 20 and this nonlinear trend will continue. I use to fit at least 5 harmonics and when I was lucky maybe 8, one or two will only give you the roughest of estimates. Now, the really tricky part is since these are transedental functions your solution space will be filled with many, many, many, mixtures of stiffness, and tension that solve the equation. You have to have a reasonable guess at the loading, and stiffness or any solution is as good as any other which tells us nothing.

This is a reference I used on a checking tendon loading in a post-tensioned bridge project.

http://www.afn.org/~afn49304/youngnew.htm

As a far easier method there are direct tension devices for normal cable sizes, just look up "tension meters".