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Harmonic Vibration: What’s shaking my light poles?

What is Harmonic Vibration?

In the simplest of terms, Harmonic Vibration is when a mechanical object vibrates at harmonic frequencies. There’s two different kinds of vibration; free and forced. Free vibrations are those of which you cause and let happen. They are usually desirable, like that of a tuning fork. Forced vibrations happen when a time-variance disturbance occurs to a mechanical system like a shaking a washing machine produces. Either of these vibrations can be harmonic or non-harmonic. For the sake of our discussion we will continue to use the term harmonic vibration.

What types of vibration do you encounter with light poles?

You can encounter one of two movements with light poles. First mode or second mode vibrations. First mode occurs at the top part of the pole with low frequencies that cause little harm to the pole. Second mode occurs near the middle of the pole with high frequencies and is usually caused by a wind vortex. This disturbance can cause harm to the structure of the pole and luminaire.

Credit: United Lighting Standards, www.unitedlightingstandards.com

How do I know if a light pole is experiencing second mode vibration?

・    Premature Lamp Failure

・    Missing or Loose Screws and Accessories

・    ‘Ticking/Clicking’ Sounds

・    Audible Humming Harmonic Frequencies

All the following are possible indicators of harmonic vibration happening in your light poles. To further confirm any vibrations, look for hairline cracks near your pole weld and any other areas. If you suspect anything, consult a licensed structural engineer or your lighting manufacturer.

How do I prevent harmonic vibration?

There’s no way to completely prevent harmonic vibration in light poles. Proper installation and use of proper parts is step one. Vibration dampers can also be used to minimize the destructive force of second mode vibration. These are usually installed inside the pole structure, but can also be in the form of something that mounts to the outside as well.

Predicting locations that can cause harmful harmonic vibration is also near impossible. Though the most susceptible environments are wide open areas with no wind breakers such as buildings; I.E. bridges, large parking lots, airports.

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