Designing a structurally sound solar system is one of the biggest challenges for solar contractors today. Solar arrays can be exposed to the worst weather conditions; including high wind, 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 or even not so extreme windy conditions as it was the case in recent incident in Yakima, WA @ Firman Pollen Co building.
“Recent high wind (gusty wind up to 50mph) has blown away Solar panels and bricks from buildings near North First and West Lincoln Avenue”. It was quite shocking to read this news in Yakima Herald, because Solar Array usually don’t fall off and should not fly away with 50mph gutsy wind as long system is properly design and install, even if it’s designed with minimum load requirement regardless of local AHJ permitting and installation criteria.
Roof-top solar arrays are increasingly being deployed on buildings across Central Washington in past decade, as the basic economics of PV generated electricity improves. Assuring that the building structures can withstand the additional loads imposed by PV arrays on roofs has become a key issue in the deployment of these systems, and therefore a key question for structural engineers and building officials responsible for reviewing and approving such systems.
In Central Washington, the basic wind load requirement for roof mount solar is 110 mph exposure C with system distributed weight less 3psf. The system is question seems to have “Ballast Roof Mount system” meaning it wasn’t physically attached to roof structure and held down by Ballast/ concrete block. This type of design mainly used on commercial flat or low pitch roof and perfectly within scope of design. However, these types of system generally add more load on the roof, sometime more than 5psf which is in most cases much more than building structural design load and require structural engineering analysis and/or retrofitting to support the additional load.
In many jurisdictions in the United States, 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. 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, with all this confusion, PV system can be installed with sub optimal design either purposely or due to lack of understanding.
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.
ZONES:
Roof zones determine the amount of wind load that is subjected to the system based on where the system is located on the roof.
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 or the racking manufacturer may have software that can assist with the zone calculations. For example, we here at Solora Solar t have been using our manufacturer design tool IronRidge for roof mount and SnapNrack design tool for ground mount solar. We provide these calculations not only to AHJ during permitting and inspection process but as well as to our customer for their record.
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. 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. For attached systems, adding more anchors than necessary can lead to water leaks and more costs down the road.
HEIGHT & EXPOSURE:
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, (as it may have happened for the system in questions in Yakima), 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.
It is necessary to avoid these ahead of time and plan your systems using the appropriate codes for your specific jurisdiction. It is important to evaluate equipment and attachment methods to ensure that PV equipment will remain attached to structures during windstorm events, and that additional loads or load concentrations do not exceed the structural capacity of the building. It isimportant for design professionals to stay current with existing codes and standards, because we expect the body of information about designing PV systems to withstand local wind loading to grow rapidly in the near future.