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Designing your solar array for wind loads

Wind Design

Learn about solar regulations and how to apply best practices to your project

Designing a structurally sound solar array is one of the biggest challenges for solar contractors today. Solar arrays can be exposed to the worst weather conditions; including hurricanes, snow and hail. These systems need to be able to withstand the wind loads in their specific locations if they are to remain in service for up to 25 years. Solar panels are extremely durable and can withstand severe and harsh weather conditions when mounted correctly. Although you may wonder, how do we design mounting systems to be resilient to these extreme wind forces? 

 

In many jurisdictions in the United States and around the world, there can be little regulation for ensuring structural stability of the PV racking system. This causes a great deal of confusion for installers, as they are responsible for knowing their own jurisdiction’s regulations and they can’t always rely on computer designed bill of materials or production programs to correctly design systems according to those regulations. 

 

Until recently, the American Society for Civil Engineers (ASCE) provided codes and regulations for buildings, but not specifically for solar arrays.  In ASCE 7-5 and 7-10, a section called components and cladding has been used in many jurisdictions as a reference on how to design and permit solar arrays on buildings. While useful, this section of the code really does not account for the ways in which environmental loads impact solar panels and associated hardware.  For this reason, design requirements for permits vary greatly from one town to the next. Unfortunately, this means that contractors need to do their research to ensure their installs are safe and able to handle the wind load at their specific location.

 

Due to the lack of a national standard, installs can be held up in permitting.  Additionally, projects can be greatly under designed if the regulating body does not verify any load requirements. This can result in expensive redesigns which adds costs to the system that were not budgeted for. Townships may require structural changes to the building that can make projects financially unattractive or invasive. An example of this could be installing positive attachments where ballasts were originally designed. This drastically increases tertiary costs like roof warranties 

 

There are a multitude of factors that go into designing a system that can survive harsh weather events. It is important to understand what some of these major factors are to mitigate risks associated with wind-related failures. The good news is that the newest ASCE 7-16 codes include a section specifically referencing the design of solar arrays on structures. This new manual has not yet been adopted by many AHJ’s as of yet.

 

Roof Zones

#1 Roof Zones

Roof zones determine the amount of wind load that is subjected to the system based on where the system is located on the roofPer ASCE 7-10, there are three major zones that need to be considered.

 

Zone one has the lowest load and consists of the interior space on the roof. Zone two represents the perimeter of the roof and is a higher risk zone.

Zone three is located on the corners of the roof and is the highest risk area. Most system failures occur on the edge or corner of the roof. Installing modules on the edge or corner of the roof can be dangerous and risky. In many cases, installations in corner or edge zones require more attachment points or ballast. Even residential installations in low-risk areas, like California, may require shorter spans with increased attachment points in corner zones. Calculating roof zones can be challenging depending on the code used in a specific jurisdiction. It is best to consult the appropriate code and consult the engineer of record for the project. If it is a residential installation, the racking manufacturer may have software that can assist with the zone calculations. For example, we here at Everest have BaseOn and our CrossRail Design Tool softwares to help you design your project with ease. The softwares are simple and easy to use, but should you have any questions our employees are always available to help you.

Building Risk Category

#2 Building Risk Category

More precautions need to be made based on the importance factor of the building. Building codes classify buildings by risk of human life, health and welfare. You can refer to ASCE 7 provisions to determine appropriate classifications. For example, a building like a barn would likely represent risk category one due to the low risk of life lost in the event of a failure.

A hospital would be categorized as a level four risk due to it being a building necessary to human life. A functioning PV system could be critical for a hospital located in a remote area. Most PV systems are installed on risk category two structures. The structures included are typically houses, business warehouses, restaurants and hotels. PV systems on risk category three and four buildings are expensive. This is due to the required additional ballast or attachments to mitigate risk of failure. In many cases, these systems cannot be installed in roof zones two and three on risk category three and four buildings. These additional requirements can limit the amount of PV that can fit on the roof. 

 

Many contractors do not bid projects on these types of structures to avoid the headaches associated with permitting the project. While this is valid, there can be many ways to lessen these risks by understanding the additional requirements before the design process begins.  

 

For attached systems, adding more anchors than necessary can lead to water leaks and more costs down the road. In 2017, Hurricane Maria ravaged through San Juan, Puerto Rico. The San Juan VA Hospital had a 645 KW rooftop array that was able to withstand 180mph winds! Had that system not been designed correctly, the damage and loss of life could have been catastrophic.

Roof Height and Exposure Category

#3 Roof Height and Exposure Category

The wind loads on a PV system increase as the building gets taller. Any residential project that exceeds 30 feet typically requires custom engineering. It is important to understand the risks associated with installing PV on roofs that exceed 30 feet. This is especially true for instances on buildings such as hotels, that get up to as high as 100 feet tall. In many of these cases, the engineer of record will require anchors instead of a ballast to mitigate risks of failure.

Racking and anchoring systems are key to determining wind resiliency. If too little ballasts are used, the array can flip or move when faced with strong winds. Tilted racking systems are typically more susceptible to higher wind loads than flush-mounted systems. In the case of ballasted systems, it might seem wise to simply increase the ballasts to reduce risks. However, there are risks in over-designing the system as well. Too many ballasts can cause structural issues with the building; especially those that experience seismic or heavy snow loads.  

 

Every jurisdiction has its own specific design criteria. Since there is a lack of a national standard, and it is impractical to perform wind tunnel studies on each system designed, contractors need to carefully design the systems to ensure function and safety. The Structural Engineers Association of California (SEAOC) has been working to set an industry standard for installing solar systems. Until that is done, installers need to do their research and understand their own jurisdiction regulations. Due to a lack of a national guidelines for solar design for commercial roofs, every AHJ has to establish their own interpretation of the appropriate design parameters. There are three main problems regarding this: 

 

  1.  Over-engineered systems can sometimes cause projects to become too costly, too heavy or too intrusive on the existing roof.  
  2.  Major delays in permitting can upset end customers and cause distrust in the company or loss of a sale.  
  3.  AHJ’s design changes that drastically alter the design from what was initially proposed, which also upsets end customers. 


It is necessary to avoid these 
ahead of time and plan your systems using the appropriate codes for your specific jurisdiction. 

Do you have any questions? Do you need help planning your project? Our team of solar racking experts are here to help you!

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