Today’s subject, while a little technical, is nonetheless very relevant when it comes to outdoor lighting, and most particularly to the structural aspect of it. The concept in question is what we call Effective Projected Area (EPA) and its wind force on objects.
Firstly, let us precise that EPA is one of the basic principles for the structural engineering process in outdoor lighting and other mechanical engineering fields as well. EPA is in fact the measurement of a three dimension object on a two dimension area .
EPA stands for Effective Projected Area and is used in many applications, including solar lighting system design, construction and installation. The EPA is calculated to help determine the strength of the pole needed to provide support to the solar lighting system during wind events. This calculation takes into consideration the entire area that the solar power system and light fixture will take up at the top of a pole and helps manufacturers determine the size of pole, the type of anchors used, the embedment and foundations used at installation, and the types of brackets required to keep everything mounted during a high wind event.
EPA and AASHTO Standards are used when calculating the requirements for the pole used in any solar lighting application. These two factors are used to determine the size of pole required to ensure that the light will still be standing after a wind event up to a certain mile per hour.
The EPA of any system varies depending on angles, shapes, and size of the systems. Even the shape of the pole can change the EPA of a complete system as square poles have a larger EPA than a round pole. When designing a solar power system, the angle of the system affects both the EPA and the solar power production of the system. All these factors must be taken into consideration when designing a project as well as ensuring the installation will withstand for years to come.
So why does this all matter? When designing a system, the EPA of the complete system needs to be taken into consideration to ensure that the solar power assembly will not blow apart during a storm, the pole won’t be knocked over due to the large area at the top of the pole, etc. To find local AASHTO wind load ratings, you can look online or talk to your pole manufacturer, solar lighting specialist or local engineering firm.
Different locations, such as mountainous areas, coastal areas and areas around the Great Lakes have different wind speed requirements than other inland areas. Consulting a local authority is the best way to ensure you are purchasing equipment that can stand up to these windy areas. The best way to determine the requirements of the project is to:
- Determine the site location
- Determine to total weight and EPA of the equipment
- Determine the wind load requirement
- Talk to your manufacturer to ensure the pole can hold up
In other words, the EPA is the projected area combined with the appropriate drag coefficient. What needs to be mentioned here is that depending on the shape of the object, the drag coefficient will vary. The drag coefficient can be defined as the resistance created by the object or shape in a fluid environment, in our case (outdoor lighting) air. And the lower the drag coefficient, the least resistance the object will create. An example is often the best way to understand a technical concept such as this one. Let’s take two basic forms, a round surface object (drag coefficient of 0.5 according to the latest version of AASHTO LTS-5 table 3-6) and a rectangular flat shape object (drag coefficient of 1.2 according to the latest version of AASHTO LTS-5 table 3-6). For the very same area, the rectangular object will create around 60 % more resistance than the round object. This kind of information is needed in order to calculate the wind force acting on an object and also on the overall structure. It is only by knowing the EPA value for each object and their respective weight that the proper pole can be calculated and designed accordingly.
The wind force acting on an object is calculated by multiplying the EPA and the velocity pressure of the wind (this designed wind pressure is computed in accordance with a specific standard. In our case, we use the equation 3-1 in AASHTO Standard specifications for structural supports for highway signs, luminaires and traffic signals). These designed wind pressures are usually based on 50 years’ studies or other methods, depending on the standard used.
Another detail important to mention is that the use of the proper wind speed, or wind pressure, should not be taken lightly since in some cases, the force projected by the wind on an object can be many times the value of its weight. A perfect example to understand this is a banner. This kind of object is extremely light, about 25 to 30 lbs; but depending on its size and also its geographical location (wind pressure or wind speed), the object in question can create in some cases more than 400 lbs of force. In such cases, Philips Lumec strongly recommends that a qualified professional be consulted for the proper design and selection of poles and foundations.
If you are still unsure, additional engineering calculations can be performed. These typically are a small fee ranging from $500 – $1000 for signed and sealed calculations. These calculations are performed by a third party engineering firm.
In the end, talking to your manufacturer or local engineering firm will help you determine what size and type of pole will ensure your equipment will withstand local wind events. Learning about different options when it comes to poles will also educate you on the final decision when it comes to system design. Check out: My Pole or Yours? Why Solar Light Poles Differ
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