August 15, 2018
It’s not rare to come across uprooted trees, telephone poles, light poles and power lines especially during the storm season. It leaves the roads in a mess and could also endanger lives. Although no one can predict the damages that a severe storm can cause, every inhabited town has certain safety guidelines for outdoor installations, that must adhered to.
Severe storm or not, all outdoor poles including metallic light poles are affected by wind induced vibration. Light poles and assemblies must hold up without developing fatigue cracks, which eventually result in the light pole to fail.
Depending on what part of the country you live in there are standards and codes that light poles need to adhere to. The standards are based on historical values, analysis and research for every region. The wind maps take into account the various bending and shearing stresses, wind loads on the light pole and the luminaire. Building codes also affect light pole selection. For instance, mountainous and coastal areas with vast open spaces are likely to have wind velocities that are considerably higher, than the plains. There could also be variance in wind speeds from one end of a city to the other. Keeping these and the local building regulations in mind you could appropriately calculate the EPA on a light pole.
EPA or effective projected area helps to calculate the resistive force of a luminaire. This essentially translates to determining what kind of force a pole will be required to handle, which is exerted by a certain retrofit outdoor lighting fixture or luminaire at a given wind velocity.
So how is EPA calculated?
EPA is calculated by multiplying FPA( Frontal Project Area) by the Drag Coefficient. FPA of a luminaire is its maximum cross-sectional area and it’s mounting. And the Drag Coefficient is a number that determines the factor of drag a certain sized object would produce. As we don’t always know or predict how the wind will flow it's almost always consider the worst case drag coefficients while calculating.
EPA = FPA( Frontal Project Area) ft2 X Drag Coefficient
Different Vibrations The Lighting Pole Has to Endure:
The maximum deflection at the top of a light pole occurs when there are sudden gusts of high velocity winds. This results in the swaying of the pole, also known as first mode vibrations which aren’t that damaging to the pole or the luminaire.
The more damaging kind of vibrations is the "second mode vibrations" or Aeolian. These are caused by steady winds with a lower velocity between 5-35 mph. Due to these kinds of winds, the poles vibrate at a frequency of up to 20 Hz, and the air flows in a motion similar to whirlpool around the pole. It creates multiple high frequency vibrations at various parts of the pole which creates tremendous stress on the structure.
Steady slow winds also cause light pole resonance. Dampers can be installed to change the natural frequency of light pole vibrations so that it doesn’t coincide with the specific wind speed range. A few tips to reduce the chances of wind induced resonance:
And it’s not only street lights, if you are planning to illuminate a large outdoor area for example a tennis court, a stadium or a commercial driveway you will require an installation of light poles. For each of these installations while you zero in on a light pole you’ll need to keep the EPA of the light fixture in mind.
How to pick the right pole and luminaire? Here are the things you should consider:
If you need help with calculating Wind Loads or choosing the right poles and bases for your outdoor project, get it touch with us today.