Wind Induced Vibration

Wind-Induced Vibration
There are two common types of wind-induced vibration observed in poles…First Mode Vibration and Second Mode Vibration.

First Mode Vibration
In first mode vibration, sometimes referred to as sway, the maximum deflection occurs at the top of the pole. First mode oscillation typically occurs at a low frequency of approximately one cycle per second. Normal deflection of this shape usually is not harmful to the pole or luminaire, but first mode oscillating vibration will cause damage and failure.

Second Mode Vibration
Second mode vibration can be the most damaging form of vibration. It occurs approximately at the midpoint of the pole with the deflection off center equal from side-to-side. It occurs at a higher frequency, typically three to six cycles per second. Second mode vibration occurs when the wind synchronizes with the pole’s natural frequency of vibration. This is known as resonance. As the steady low level wind moves past the pole, vortices are shed alternately from either side of the structural shaft causing displacement oscillations in a direction perpendicular to that of the wind.

Vortices are a swirling motion or pattern of the wind. The most serious situation arises when the vortex-shedding frequency synchronizes with the natural period of vibration in the pole, which can ultimately fatigue the pole to structural failure.
Vortex-street-animation
Vortex-Shedding

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First Mode – Second Mode

Wind induced vibration can be caused by steady, relatively low-speed wind (10-30 mph), by topography and by the structure to which the pole is mounted or nearby structures. Destructive vibration is not an indication of substandard material, workmanship or design of pole.

Light Pole Design
Pole design requires consideration of field conditions such as wind speed (sustained/gusts), pole height, appendages and local conditions. Wind induced vibration is a local, site-specific condition that may be overlooked by those selecting a pole because it is difficult to predict accurately. Poles which perform satisfactory in many installations all across the country may experience destructive vibration at a select location for no apparent reason. Typically, poles are designed or selected based on AASHTO (American Association of State Highway and Transportation Officials) criteria. The standards and codes take into account direct wind pressures on the pole and luminaire, the associated bending, shear, axial and torsional stresses on the pole, secondary moment effects (the pole and fixture being off center of the pole base when the wind deflects the pole) and the effect of heat on the base material in the area adjacent to the weld.

Hapco is the industry-recognized pioneer on the research of wind induced vibration in aluminum poles. Our decades of research and testing has led to several patents in this category, giving Hapco Engineers the knowledge and background to assure our customers the safest, longest lasting pole designs.

We have learned from experience that poles supporting certain types of post-top mounted fixtures are more susceptible to first and second mode vibration. The rectangular or “shoebox” shaped luminaires, for example, seem to encourage second mode vibration, while more aerodynamic lightweight fixtures encourage first mode vibration. Any oscillation should be dealt with as soon as it becomes apparent to prevent damage to the pole or luminaire.

Hapco Damper Solutions
To minimize the effects of first or second mode vibration, Hapco has developed dampers which very effectively quell wind induced vibration of lighting poles.

Free Vibration Test Curves
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Free Vibration of Test Pole with No Added Damping
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Free Vibration of Test Pole with Impact Damper

First Mode Damper
Hapco’s patented First Mode Vibration Damper is field installed at the top of square poles. This damper very effectively reduces first mode vibration of square lighting poles. Due to potential vibration fatigue issues that can be associated with moderate wind conditions, First Mode Dampers are recommended for square poles with mounting heights greater than 25’ and for square poles with light loading fixtures less than 2.0 EPA. Fatigue related pole failures are not an indication of substandard material, workmanship or design of the pole and is not covered under Hapco’s standard warranty.

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*Contact factory for First Mode Dampers for use on round poles.

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First Mode
U.S. Patent No. 7871186

Second Mode Damper
The Hapco Second Mode Damper was awarded the industry’s first patent in the vibration damping category in 1972. It is attached to the pole at approximately the midpoint of the pole, and can be factory bolted inside the pole or field mounted on the inside or outside of an existing pole.

Based on past experience, some Hapco poles include factory-installed Second Mode Vibration Dampers as a standard component. If the poles you intend to order are to be bridge mounted, used as camera poles, or may be subject to unusual site conditions, wide open terrain, steady or high energy prevailing winds, or other site conditions as mentioned, please make Hapco aware of these conditions. Or, if your installed poles are exhibiting oscillation, contact Hapco to pursue a damping solution that may include installing a Hapco Second Mode Damper on the poles.

Locations with high average wind speeds are more prone to pole vibration problems than locations with high maximum wind speeds. Areas of high average wind speeds can be identified on the U.S. Department of Energy Residential Scale 30 Meter Wind Map which can be accessed at the U.S. Department of Energy website.

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53185-388x623
Catalog No. 53185
Banded to Shaft
(Field Mounted)
Second Mode

53373-383x623
Catalog No. 53373
Bolded Inside Shaft
(Factory Bolted)
Second Mode

53884-389x623
Catalog No. 53884
Bolded Inside Shaft
(Field Mounted)
Second Mode

Each job site has different variables that may contribute to structural fatigue vibration. These pole variables should be taken into consideration, along with environmental and structural factors, to determine if the potential for vibration exists.

This is not a complete list.
Other factors can influence the effects of wind.

Total Load (EPA) and Shaft Length – Light loading, less than 2.0 EPA, and shaft lengths at or above 25 feet can significantly increase the probability of destructive vibration.

Shape – Straight Square Poles historically experience more effects of destructive first mode vibration. Round poles historically experience more effects of destructive second mode vibration. However, no shape is exempt.

Installation Procedures – Poles are designed to carry a load, and a pole cannot be installed before the luminaire is mounted. Never install a pole without the intended luminaire in place.

Parking Deck Installation – Influences from surrounding structures and transferred vibration generated by moving vehicles.

Near or at Airports – Little or no objects to break the wind currents and the presence of turbulence created by aircraft.

Bridge Installation – Little or no objects to break the wind currents and the transfer of vibration generated by
moving vehicles.

Mountain Foothill Areas – Air currents traveling from the higher elevations can create steady damaging winds.

Large Expanse of Flat Ground or Water – In tandem with little or no structures, the wind currents will not be disrupted which sets up the possibility for low steady winds and destructive vibration.

Steady Low Level Winds – The upper Midwest and Plains States have shown this trend.