This guide covers light pole vibration including effects, detection and how to avoid it.
Overview of light pole vibration
Light poles are vertical cantilevered structures and will vibrate under certain conditions. Although rare, severe pole vibration can be hazardous.
Light poles can vibrate in different modes and at different frequencies. Some outside forces, such as natural wind, may “excite” the pole and instigate vibration. Poles mounted on bridges may also be subject to traffic-induced vibrations from deck “bounce.” Windblasts from passing trailer trucks are another cause. Once the excitation force is removed, vibration “decays,” and the pole stops vibrating.
Pole geometry affects the rate of vibration decay. Tall, slender poles tend to vibrate more easily and decay more slowly. This rate of vibration decay is known as the dampening coefficient, or damping ratio. When certain conditions exist, poles will vibrate and may sustain the vibration for long periods of time. This is because the poles have poor damping properties. This can be due to many factors including pole geometry, prevailing winds, site terrain and luminaire type and weight.
Because of the complex combinations and interactions of these variables, it is difficult to predict when, where and which poles will vibrate. However, experience has shown that under certain conditions, poles are more prone to vibration. In the event of severe vibration, the pole and/or luminaires can fail. The following can contribute to pole and/or luminaire failure:
Effects of vibration
Vibration and its resulting lateral displacement will result in a stress to the pole. Maximum stress occurs at the base of the pole. The greater the movement or displacement, the greater the stress will be. When these stresses are continually repeated, they are called cyclic or fatigue stresses.
Pole vibration stress levels are usually not severe enough to cause cracks or failures.
However, if sufficient in magnitude and applied over time, vibration may cause stress cracks in the pole. 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 connection and hand holes. Square poles are more susceptible to fatigue stress cracking due to the high stress concentrations at the corners. Following the initiation of a stress crack or fissure, the crack will continue to grow until the pole is no longer capable of withstanding even a modest wind event.
How to detect vibration
With first mode vibration, the pole merely sways, and vibration can be easily observed. This is common and not damaging to the pole. However, under high wind and gusting conditions, violent pole top displacement and whipping may occur and can be dangerous.
Second mode vibration caused by vortex shedding may be harder to detect. The amplitude of motion, located near the center of the pole, may be small and difficult to observe. A knowledgeable investigator trained to assess the situation must be at the job site to witness the condition when winds are blowing in the 8 to 25 mph range. In addition to seeing the motion, an investigator should be able to “feel” the vibration or detect it by placing a hand on the pole. In addition, there may be noise such as conductors slapping the inside of the pole. More sophisticated detection can be accomplished with accelerometers and chart recorders.
All pole vibration is not destructive, but when detected, poles should be monitored on a regular basis for cracks. If poles are significantly and/or continually vibrating, vibration dampers should be installed.
First mode vibration
First mode vibration (sway) starts at moderate wind speeds. Its frequency is low, about one cycle per second. Maximum deflection occurs at the top of the pole and is rarely a problem. However, under high gusting conditions, more severe oscillation may result. When gusts occur at very high wind speeds (50 to 70 mph), violent “whipping” and “pulsing” may occur, producing aggressive motion and resulting in high stresses at the pole base. Gale-force winds and cold weather fronts with high wind velocities may be accompanied by heavy, wet snow. This type of “perfect storm” can be destructive.
In rare incidences, large populations of poles have failed during a single storm event. Fortunately, these localized weather conditions do not occur frequently and are usually short-lived.
Second mode vibration
Second mode vibration is more disconcerting than first mode vibration.
This event is caused by a phenomenon known as vortex shedding, referring to the small eddies alternately spinning off the sides of the pole; a canoe paddle creates the vortex at the sides of the blade. Because there is a pressure collapse when a vortex is created, the pole is driven in the direction of the vortex. When that vortex spins off into the wind stream, another vortex forms on the opposite side, causing the pole to be driven toward that side. This continues alternately, and the pole is forced back and forth, 90 degrees in relation to the wind stream.
Vortex shedding frequency increases with wind velocity. When the vortex shedding frequency approaches the poles’ natural second mode frequency, they become “locked-in,” and the pole vibrates. This resonant condition occurs at wind speeds between 8 and 25 mph, with frequencies of 3 to 8 cycles per second. Unlike first mode vibration, the location of second mode maximum displacement occurs at or near the middle of the pole.
Although these stresses are low, stress cycles can build rapidly into the thousands and millions over time. If the combination of stress levels and number of cycles is sufficient, the metal’s fatigue stress endurance limit may be exceeded.
Areas of concern include:
Fatigue cracks may develop and, over time, cause pole failure. Nearby trees, buildings and wind velocities over 25 mph create turbulence and disrupt the laminar wind flow patterns that cause vortex shedding vibrations.
Suggestions for avoiding vibration
Check the job site prior to ordering a pole. Poles located in flat, open terrain or exposed locations, such as bridge decks and parking garages where there are prevailing winds in the 8 to 25 mph range, may experience second mode vibration. Where site conditions indicate the poles may be subject to prolonged periods of vibration, round poles would be a better choice than square poles. Round, tapered poles are preferred over straight poles, and steel poles are preferred over aluminum.
Larger diameter poles with higher EPA capacities than required are recommended. These poles are stiffer and have better dampening characteristics. Luminaires and poles should be installed at the same time.
When a single pole exhibits poor damping characteristics, a vibration damper may be required. Wind energy is the driving force of vibration. This energy must be dissipated by the addition of a damper. There are a variety of methods and damping devices available to reduce vortex shedding vibration including:
Internal chains suspended from the top of the pole may reduce vibration, though the size and length of the chain will need to be determined. All dampers function as energy absorbers, canceling the motion of the pole and thus reducing or eliminating vibration.
Dampers may be factory-installed for built-to-order poles. There are other types of dampers suited for field installation when required. Some types of field-installed dampers are mounted on the exterior of the pole and may detract from its appearance. Fabreeka base pads and washers are used on bridge and structure mounts.
Examples of structural vibration
Instances of structural vibration can be found in two well-known cases.
Washington state’s famous Tacoma Narrows Bridge of Washington, also known as “Galloping Gertie,” was built in 1940. Four months after opening, the bridge collapsed in spectacular fashion. The subsequent investigation led to the study of wind-induced vibration, and the findings helped engineers gain a greater understanding of fluid dynamics. As a result, a similarly designed bridge in Maine was spared by the addition of structural elements and fairings.
When windows of Boston’s John Hancock Tower popped out and crashed to the street below during initial construction in the late 1960s, wind-induced vibration was determined to have caused the 800-foot building to torque and flex. The solution involved the installation of two 300-ton, mass-tuned vibration dampers on the 58th floor. As a result, recently constructed, slender high-rise buildings in New York City have large damper systems.