Current HLOI0022 Pole Wind Induced Flyer Instructions
- June 9, 2024
- current
Table of Contents
Current HLOI0022 Pole Wind Induced Flyer
As an industry leader in light pole design and manufacturing, Current is committed to providing information to help designers avoid wind induced pole vibration. This paper presents a practical guide for recognizing and minimizing the potential damaging eff ects of wind on light poles.
Defi ning Wind-Induced Vibration
Wind induced pole vibration is partially defi ned, or characterized, by the type of movement observed in a pole system.
Two common types of pole vibration are
FIRST MODE (Harmonic) VIBRATION SECOND MODE (Resonance) VIBRATION
FIRST MODE VIBRATION is characterized by a side to side movement that has a maximum displacement at the top of the pole. This behavior may be referred to as “sway”. The frequency of movement is about 1 cycle per second or less. Because the vibration frequency is comparatively small, the onset of material fatigue will take longer than poles exhibiting higher modes of vibration.
SECOND MODE VIBRATION is characterized by a symmetric oscillation at or near the midpoint of the pole shaft. The oscillation frequency typically ranges from 3-6 cycles per second. Second mode vibration occurs when air moving across a pole reaches “lock-in” velocity. At “lock-in” velocity, Vortex Shedding produces alternating transverse forces that oscillate the pole at one of its natural frequencies. The onset of fatigue cracking or structural failure may occur quickly under such conditions.
VORTEX SHEDDING is the phenomenon caused by a steady, constant velocity wind moving across a slender, fl exible object, like a pole. Vortices forming on the sides of the pole create localized, pockets of low pressure. These pockets alternate, creating alternating forces that act on the pole perpendicular to the wind direction.
EFFECTS OF WIND INDUCED VIBRATION
The eff ects of wind induced vibration will vary as they are infl uenced by many factors; including site features, pole design and shape, and wind conditions. Wind induced vibration can lead to reduced performance in the pole systems and may result in structural failure.
Environmental Conditions that Increase Risk
Environmental variables like wind conditions, terrain, landscape, and site structures greatly infl uence how wind will aff ect pole systems. This partial list includes application attributes that may facilitate wind induced pole vibration.
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Regional Geography – Expansive, fl at areas with little natural or man-made wind breaks as common in the upper mid-west, plains states, and some coastal regions.
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Flat, Open Terrain – Localized fl at areas with little or no wind breaks (i.e. large parking lots, large fi elds, top of parking structures, causeways)
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Mountains, Foothills, Cityscapes – Areas with steady air currents or areas with natural or man-made features that alter wind patterns
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Airports – Turbulent air from aircraft and localized fl at areas absent of objects to break the wind currents
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Bridges and Parking Decks – Flat, elevated structures in open areas can also transfer vibration generated by moving vehicles or move on their own as a result for vortex shedding about its structural supports
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Local Conditions – Site specifi c terrain and conditions
Design Factors To Consider
Pole selection can also contribute to the potential for wind induced vibration. Relevant design attributes are:
Loading – Luminaire size (EPA), luminaire weight, luminaire quantity, and
luminaire orientation will infl uence the behavior of the pole and its
susceptibility to vibration. Loads that exceed the design limits of the pole
may result in structural failure. Also problematic, very small loads and zero
loaded poles (lightly damped) may be prone to vibration.
Height – Pole height governs the amount of defl ection a pole will move
and the size of the moment generated at the pole base. An increase in height
will increase both values, making it more susceptible to wind induced
vibration.
Wall Thickness – Pole wall thickness aff ects the rigidity of the pole.
Increasing the wall thickness will reduce its fl exibility and reduce the eff
ects wind has on the system.
Material – Common pole materials include steel, aluminum, concrete, fi
berglass, cast iron and wood. Material properties vary between material types.
Steel and aluminum are very common materials used in light pole manufacturing.
Of the two types, steel has higher strength of material properties than
aluminum.
Shape – Pole shape (cross section geometry) also infl uences pole
performance because aerodynamic characteristics change by shape. When
comparing square poles to round poles, square poles have higher drag coeffi
cients. Square poles may also have more complex, asymmetric fl ow patterns
around the pole. This is especially true for winds approaching off angle from
the either the fl at surfaces or corners.
Taper – Taper can also change the aerodynamic characteristics of the pole, shifting the critical wind velocity that causes Second Mode vibration.
Recommendations
Evaluate risk – Inspect site conditions and evaluate the risk of
problematic wind conditions.
Mitigate risk through pole selection
- Avoid poor damping conditions caused by under loading poles. Poles should not be installed without the luminaires they are designed to support.
- Strengthen or stiff en the pole via material selection (i.e. choose steel over aluminum), increase wall thickness, increase the cross sectional size, and/or minimize height.
- Consider round tapered poles.
Increase Damping – The uses of factory or fi eld installable damping devices off er an economical option for minimizing destructive vibration. Most dampers reduce the amplitude of mechanical vibrations by adding mass that moves at a diff erent rate than the shaft of the pole. Some dampers also change the natural frequency of the pole by moving the excitation frequency, thereby changing the critical velocity and the forces generated by vortex shedding.
First Mode Dampers
First Mode vibration dampers are designed to reduce the amplitude of pole top
defl ection caused by First Mode vibration. Current off ers a First Mode
vibration damper (Q41) that is factory assembled, painted to match, and ready
for fi eld install. It is designed for 4”, 5”, and 6” square aluminum and
steel lighting poles.
Second Mode Dampers
Second Mode vibration dampers are designed to alter the natural frequency at which the pole will resonate, reducing the pole movement and the onset of material fatigue caused by wind. Both fi eld installable and factory installed Second Mode vibration dampers are available.
Current off ers two types of Second Mode vibration dampers
Serpentine Tube (special order) vibration dampers are fi eld installed through hand holes. These devices wrap around the inner surface of the pole shaft, are free standing, and reduce motion.
Interior Canister (Q42) vibration dampers feature an internal cylinder that houses a cushioned, weighted rod pendulum. This device moves in the opposite direction of the shaft, disrupting vibrations. The impact of the pendulum against the cylindrical housing may be audible under certain conditions. This type of damper is factory installed.
Perform and Record Pole Maintenance
An important tool to reduce risk and the eff ects of wind induced vibration is
a properly implemented pole maintenance plan. Inspections should be conducted
on a routine basis, starting at the time of installation, week 1, month 1,
month 6, and then annually thereafter. It is also advisable to check
installations for changing pole movements, damage, or signs of stress after a
major wind event or changes to the site that may aff ect wind patterns. Site
changes may include new building structures, new landscape plantings, or
growth of existing landscaping. Inspecting for the eff ects of vibration is a
very important part of any proper maintenance plan. If any eff ect exists, the
pole can be compromised in a short period of time. The results can be as
severe as structural failure. Signs of vibration can be observed following
this basic guideline, special training or certifi cation is usually not
required.
- Observe pole movement during windy conditions. Excessive defl ection, rapid oscillation at the shaft midpoint, or audible thumping of electrical conductors in the pole may be indications of pole vibration.
- Low torque values on anchor bolts.
- Loose or missing parts at the luminaire and/or pole top are an early indication that the pole may be vibrating in application. Missing pole caps, loose HID sockets, and prematurely failing HID lamps are a few examples. Correct or replace as necessary.
- Look for signs of rust and corrosion above the pole base plate weld and about the hand hole. Corrosion may indicate an area of fatigue. If present, remove the corrosion and wipe clean. Hairline cracks just above the pole base plate weld may indicate shaft material fatigue. An inexpensive die may be used to aid in detection.
- For square poles, inspect the corners near the base plate in a similar manner.
- Review the entire pole shaft for signs of fi nish deterioration.
If any indications of fatigue exist, a qualifi ed and experienced structural engineer should be consulted, the aff ected pole should be taken down immediately, and an inspection should be performed for the entire site. If vibration has or is occurring, but the pole is deemed to be structurally sound, mitigation steps should be taken immediately by adding dampers and monitoring the pole behavior. If vibration continues after dampers are added, an alternate pole shape or construction should be considered.
Pole Warranty
Wind induced vibration is a naturally occurring phenomena that may lead to structural failure of poles. Such a failure is not an indication of inferior design, material, or craftsmanship. As such, Currents’s warranty does not apply to and Current shall have no liability for any failure of the products or damage due to fatigue failure or similar phenomena resulting from induced vibrations, harmonic oscillation or resonance associated with movement of air currents around the product.
currentlighting.com © 2022 HLI Solutions, Inc. All rights reserved. Information and specifications subject to change without notice. All values are design or typical values when measured under laboratory conditions.