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Wind-Induced Vibration And The Effects On Light Pole

  • source:yaolong
  • Date:12/09/2019

First mode vibration

 

The first mode of vibration (sway) starts at a moderate wind speed. Its frequency is very low, about one cycle per second. The maximum deflection occurs at the top of the light pole, with few problems. However, under high gust conditions, it may cause more severe oscillations. When gusts occur at extremely high wind speeds (50 to 70 mph), severe "whipping" and "pulsation" may occur, resulting in radical movements and high stresses at the pole base.

light pole vibration sway stress

 

Second mode vibration


Second mode vibration is more disconcerting than first mode vibration.
This event is caused by a phenomenon called vortex shedding, which is a phenomenon in which small eddies alternately spinning off the side of a light pole. Since there is a pressure collapse when a vortex is generated, the poles are driven in the direction of the vortex. When the vortex spins off into a wind stream, another vortex is formed on the opposite side, so that the aluminum light pole is pushed to the side. This continues alternately, and the aluminum pole is forced back and forth 90 degrees in relative to the air flow.


Vortex shedding frequency increases with wind speed. When the vortex shedding frequency approaches the natural second mode frequency of the aluminum poles, they become "locked" and the aluminum poles vibrate. This resonant condition occurs between 8 and 25 mph wind speeds,with frequencies of 3 to 8 cycles per second. Unlike the first mode vibration, the position of the maximum displacement in the second mode occurs at or near the aluminum pole.
Although these stresses are low, over time, stress cycles can build rapidly into the thousands and millions. If the combination of stress level and number of cycles is sufficient, the metal’s fatigue stress tolerance limit  may be exceeded.


Areas of concern include:
►Base plate weld
►Areas in the heat-affected-zones (HAZ) of welds
►Corners of square poles, hand holes, etc.


Fatigue cracks can develop and lead to aluminum pole failure over time. Nearby trees, buildings, and wind speeds above 25 mph create turbulence and disrupt laminar airflow patterns, resulting in vortex shedding vibrations.

light pole harmonic vibration

Suggestions for avoiding vibration

Before ordering a pole, check the job site. Poles in flat, open terrain or exposed locations (such as bridge decks and parking lots where prevailing wind speeds of 8 to 25 mph) may experience a second mode of vibration. If field conditions indicate that the poles may be subject to prolonged vibration, then round poles will be better than square poles. Round, tapered poles are preferred over straight poles, and steel poles are preferred over aluminum.
Larger diameter electrodes are recommended and have higher EPA capacity than required. These rods are stiffer and have better damping characteristics. Fixtures and poles should be installed at the same time.
When the unipolar damping characteristics are poor, a shock absorber may be required. Wind energy is the driving force for vibration. This energy must be dissipated by adding a damper. There are several methods and damping devices that can be used to reduce eddy shedding vibrations, including:


►Mass-tuned vibration dampers
►Inertia dampers (Stockbridge)
►Viscous dampers
►Impact dampers

light pole for street


Although the size and length of the chain needs to be determined, the internal chain suspended from the top of the pole may reduce vibration. All dampers are used as energy absorbers, eliminating the movement of magnetic poles, thereby reducing or eliminating vibrations.
Dampers may be installed at the factory for poles made to order. If required, there are other types of dampers suitable for field installation. Some types of field-mounted dampers are mounted on the outside of the pole and may affect its appearance. Base pads and washers are used for bridges and structural supports.

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