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The Overview Of Light Pole Vibration

  • source:yaolongpole
  • Date:11/27/2019

The light pole is a vertical cantilever structure that vibrates in some cases. Although extremely rare, severe extreme vibrations can be dangerous

Thestainless steel light pole/ aluminum light pole can vibrate in different modes and at different frequencies. Some external forces, such as natural winds, can “excite” the light poles and cause vibration.Aluminum light poles mounted on bridges may also be affected by traffic vibrations caused by the deck "bounce" . Another reason is the windblasts from passing through the trailer trucks. Once the excitation force is removed, the vibration "attenuates" and the aluminum light pole stops vibrating.

The geometry of the pole affects the rate of vibration attenuation. Tall and slender light poles tend to vibrate more easily and decay more slowly. This vibration attenuation rate is called a damping coefficient or a damping ratio. When certain conditions are present, the poles will vibrate and may remain vibrating for long periods of time. This is because the magnetic pole has poor damping performance. This may be due to a number of factors, including the geometry of the pole, the prevailing wind, the terrain of the site, and the type and weight of the luminaire.

light-pole-vibration-sway-stress.jpg

Due to the complex combination and interaction of these variables, it is difficult to predict when, where and which light pole will vibrate. However, experience has shown that under certain conditions, the magnetic poles are more susceptible to vibration. In the case of severe vibrations, the pole shaft and / or luminaire may malfunction. The following may cause the pole and / or luminaire failure:

  • Open terrain with little or no trees or buildings to break up the wind
  • Steady prevailing winds in the 8 to 25 mph range, which often occur in the Midwest and prairie states
  • Mountain passes
  • Poles that are 20 feet high or more
  • Poles with very lightweight luminaires or no luminaires

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Effects of vibration

The vibration and the resulting lateral displacement will result in a stress to the pole shaft. The maximum stress occurs at the base of the pole shaft. The greater the motion or displacement, the greater the stress will be. When these stresses are repeated, they are known as cyclic stress or fatigue stress.

Magnetic pole vibration stress levels are usually not severe enough to cause cracks or malfunctions

However, if the strength is large enough and applied over time, Pole vibration may also lead to premature failure of lamps and components. Pole Stresses,called stress concentration points or stress risers, are amplified at the base plate junction and the hand hole. Due to the high stress concentration at the corners, the square light poles are more prone to fatigue stress cracking. Following the initiation of a stress crack or fissure, the crack will continue to grow until the pole can no longer withstand even a modest wind event.

lighting-pole.png

How to detect vibration

In the first mode of vibration, the poles only sway and the vibration can be easily observed. This is very common and will not damage the pole. However, under strong winds and gusts, violent pole top displacement and whipping  may occur and may be dangerous.

The second mode vibration caused by vortex shedding may be more difficult to detect. The amplitude of motion, near the center of the pole may be small and difficult to observe. A knowledgeable investigator with a well-trained assessment must be at the job site to observe the condition when winds are blowing in the 8 to 25 mph range. In addition to observing the movement, the researcher should be able to "feel" the vibration or place the hand on the pole to detect the vibration. In addition, there may be noise, such as the conductor tapping inside of the pole. More sophisticated detection can be accomplished with accelerometers and chart recorders.

All pole vibrations will not cause damage, but when the light pole vibration is detected, the pole should be periodically monitored for cracking. If poles are significantly and/or continually vibrating, vibration dampers should be installed.

In the next week's article, we will explain what types of light pole vibrations are, and suggestions for avoiding vibrations. For details, please watch the news section next week.

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