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Testing for Harmonic Resonance in pole structures

  1. Mar 12, 2017 #1
    Not sure if this is the best place to ask this question but forums on this seem to be limited, can anybody explain how one would discover if a steel pole lighting structure is being effected by harmonic resonance?

    I do not know what tools and methods would be used, if anyone can provide some detail on that. But what I struggle to understand is if a pole structure is being effected by this, it would likely only be at certain times with the right wind speeds and possibly wind direction, so if its a case of using test instruments onsite would one have to continuously visit and test in different conditions in an attempt to strike the right one?

    Also I believe you can use computer software to model and discover the natural frequency of a structure, but to know if it's environment will make it resonant at that exact frequency seems difficult, does anyone know the logic behind designing against such a problem?



    Thanks
    David.
     
  2. jcsd
  3. Mar 12, 2017 #2
    The designer should know the fundamental resonance from the stiffness and mass. Then you might check the frequency of vortex shedding (von Karman resonances) up to the max designed windspeed. This only requires knowledge of diameter and wind speed. I should have thought that the designer would have done all this as part of his responsibility.
    If the pole has asymmetrical loads on it, like a dish, at least check that the static deflection, twist and loadings are OK.
    A good test would be to go up on a cherry picker and give it a shake and try to get it going in various modes, including twist.
     
  4. Mar 12, 2017 #3

    Nidum

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    Permanently fitted accelerometers . Time log . Met data .
     
  5. Mar 12, 2017 #4
    Thanks that's very helpful.

    Would I be correct in saying that having a confirmation that the structure is designed for wind speeds up to X value is not the same as saying those wind speeds would not cause a natural resonance/vortex shredding problem?

    I would interpret "Designed for wind speeds up to X" as being able to withstand the loading applied by those winds, but not the same as my above statement.

    Tech99 I would expect the design engineer should have done these calculations but my impression so far is that these have been considered a "standard" design, and put in to use where ever a client requests this pole (Around the country). But also having different outreach arms/luminaires, but still considered a "Standard" pre-designed pole. My current impression at least.
     
  6. Mar 13, 2017 #5
    I hope if either of you are still reading this you don't mind the continued questions. But i appreciate your expert advise.

    I have been doing some more reading, found some interesting research papers. Now if you hadn't guessed my questions are based on a real world problem, of which we are having investigated by an expert. However while this cause was originally suggested to be very likely it seems to have gone on the back burner.

    One thing i was told was its not so much the sway of the pole but the vibration, which you can feel by touching the pole. Would any body know is it normal to feel a vibration in a pole? Or is that a clear sign of resonance?

    What I'm reading now comments that "In addition to seeing the motion, one should be able to ‘feel’ the vibration. By placing a hand on the pole, one may be able to detect the vibration"

    On a reasonably windy day I did just that (Strong steady wind), and would describe what i felt as a shuttering in the pole, would almost go so far to say a violent shuttering. The poles would also sway and now and then whip more strongly.


    Thanks
    David
     
  7. Mar 13, 2017 #6

    Baluncore

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    Swaying and whipping are probably not a major problem, but the shuddering induced by the airflow may become a real problem as the fatigue cycle limit might be reached earlier than expected. Shuddering probably involves reflection of energy from the base of the pole where the failure will probably later occur. Tapered poles have less problems due to the tapered acoustic impedance mismatch towards the base.

    High voltage transmission lines have dampers attached to the wires either side of the insulators to prevent high amplitude oscillation fracturing the wires. If you can identify the mode of the shuddering you may be able to dampen it. That might involve a fixed lump of mass near an antinode in the pole. Sometimes a small amount of dry flowing sand in a tube will dampen tube vibration.

    Have there been any failures? Do the poles also support wires?
     
  8. Mar 13, 2017 #7
    Thanks Baluncore. There has been one complete failure where the pole fell across the road. Since which our testing has identified a number of others which are cracking to various degrees around the HEZ of the base weld. (Of which we have removed the poles)

    There are other factors being looked at, a previous contractor appears to have installed the bases in a way which may have caused damage (High impact driving). However that was 9 years ago, I would expect damaging the bases from day one would have shown problems sooner. But based on my reading and the comments here it sounds more and more like a harmonics problem, also realized luminaires falling off is a sign of this, which we find 3 or 4 a year hanging off by the cable.

    The poles do not support any wires, they are between 11 - 14.5m high, and in a very windy environment. The poles are rounded, with a thinker size pipe about 1/3rd the way up then drops down to a smaller round pipe to the top, but not tapered, with a almost sail shaped outreach and large luminaire.
     
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