What is EPA (Effective Projected Area)?
While a little technical, this is nonetheless very relevant when it comes to Solar outdoor lighting, Solar Lighting, or solar security camera mounting, and most particularly to its structural aspect. The concept in question is what we call the Effective Projected Area (EPA) and its wind force on objects, i.e., the wind forces on the solar panel and battery boxes, which then cause forces on the pole.
Effective Projected Area (EPA) is one of the basic principles of the structural engineering process in outdoor lighting, solar mounting, and other mechanical engineering fields. EPA measures a three-dimensional object on a two-dimensional area.
EPA is used in many applications, including solar lighting systems, solar security racking for security cameras, and solar racking design, construction, and installation. The EPA is calculated to help determine the pole’s strength needed to support the solar lighting system during wind events. This calculation considers the entire area that the solar power system and light fixture will take up at the top of a pole and helps structural engineers and manufacturers determine the size of the pole. The type of anchors used, the embedment and foundations used at installation, and the 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 or power system application. These two factors determine the pole’s size and thickness required to ensure that the light solar and battery will still stand up to a certain mile per hour after a wind event.
The EPA of any system varies depending on the angles, shapes, and sizes of the system components. Even the shape of the pole can change the EPA of a complete system, as square poles have a more significant EPA than round poles. When designing a solar power and lighting 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 and 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 must be considered 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, look online or talk to your pole manufacturer, solar lighting specialist, or local engineering firm. Weather is very regional, and a local structural engineer will have the most knowledge of the soil and the microclimates that can affect the calculations.
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 purchase equipment that can withstand these windy areas.
The best way to determine the requirements of the project is to:
- Determine the site location
- Determine the total weight and EPA of the equipment
- Determine the wind load requirement
- Talk to your manufacturer to ensure the pole can hold up
The EPA is the projected area combined with the appropriate drag coefficient. What needs to be mentioned here is that depending on the size and 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 solar lighting and power kits) air. The lower the drag coefficient, the less resistance the object will create. An example is often the best way to understand a technical concept such as this one. 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 shape object (drag coefficient of 1.2 according to the newest version of AASHTO LTS-5 table 3-6). The rectangular object will create around 60 % more resistance for the same area than the round object. This information is needed to calculate the wind force acting on an object and the overall structure. Only by knowing the EPA value for each object and their respective weight can the proper pole 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 by a specific standard). In our case, we use the equation 3-1 in the AASHTO Standard specifications for structural supports for highway signs, luminaires, and traffic signals. These designed wind pressures are usually based on 50 years of studies or other methods, depending on the standard used.
Another important detail is that using 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 of this is a banner. This kind of object is exceptionally light, about 25 to 30 lbs, but depending on its size and 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, Suninone strongly recommends that a qualified professional local engineer be consulted for adequately designing and selecting poles and foundations.
If you are still unsure, engineering calculations should be performed locally. These typically are a small fee ranging from $500 – $1000 for signed and sealed calculations. A third-party engineering firm performs these calculations.
In the end, talking to your manufacturer and 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 regarding poles will also educate you on the final decision regarding system design.