By Pete Manis, P.E., and Wes Jones, P.E.
The authors were recently involved in a project for which site work consisted of curb and gutter, sidewalks, parking lot paving, and light poles with foundations in the parking lot and along the roadway. Approximately five months prior to ribbon cutting, the client noticed that nearly all of the light poles were swaying considerably under wind velocities of approximately 17 to 28 mph, with gusts up to 46 mph. Figure 1 illustrates the observed light pole movement, which had a magnitude of approximately 8 to 12 inches.
The very next day, the client discovered one of the light poles on the ground, with what appeared to be fatigue cracking at the weld between the light pole base plate and the pole itself (Figure 2). The client took down the remaining poles to prevent further failures. Fortunately, there were no injuries associated with the light pole failure, since this event occurred during the night when the construction crew was not present.
Review of the light pole submittal revealed that the subcontractor had proposed a different size and type of pole than what had been originally selected – a 30 foot tall, 6 inch square aluminum pole. Instead, the subcontractor proposed a 30 foot tall, 4 inch square steel pole, which was approved since the 4 inch pole more than adequately met the performance specification according to the manufacturer's literature.
Consultations with the light pole supplier and manufacturer indicated that the failure of the light pole was "most likely" due to wind induced harmonic resonance of the light pole, and subsequent fatigue cracking of the weld between the base plate and the pole. The light pole manufacturer responded to a request for replacement light poles by saying that its standard one year warranty does not cover "naturally occurring harmonic vibration light pole failures". Additional calls to various light pole manufacturers revealed that none of them warrant failure due to harmonic vibration.
It is important to note that the failed light pole met all of the manufacturer's requirements, and had been properly selected and installed based on their criteria. Many light pole manufacturers publish wind speed maps and light pole selection criteria for their products.
The following is a common light pole selection procedure:
1) Select the light fixture, and obtain its effective projected area (EPA) and weight. The EPA is the area that is loaded by wind. This information is located on the fixture cut sheet.
2) Determine the number of light fixtures and any special mounting methods (arm or bracket) to be installed on the pole. Obtain the EPA and weight for any arms or brackets from the corresponding cut sheets.
3) Add up the EPA and the weights of all fixtures, arms, and brackets.
4) Select the design wind speed for the project location from the light pole manufacturer's wind map. Typically, this is a fastest mile wind speed, which is different from the current building code values for a 3 second gust. Tables exist for converting between the two.
5) Select a pole, and compare both the EPA and weights of the fixture with the allowable EPA and weights for that specific pole. If the actual EPA and weights of the fixtures are less than the allowable EPA and maximum weight listed on the pole cut sheet, then the pole meets the requirements.