Our world is more focused on renewable energy sources than ever. It is no surprise that more companies are powering their operations with roof solar panels. These projects need structural engineers to provide design services for installation because photovoltaic (PV) systems alter structural stability by increasing roof exposure, causing sliding loads, and creating obstructions with resulting snowdrifts.
Data required:
• Applicable building codes
• Design snow load
• Structural framing system
• Thermal properties
• Renovation history
When a PV array is mounted on low rise industrial and commercial buildings, it can change the patterns of wind flow and snow accumulation. There is particular concern regarding the structural adequacy of older buildings constructed or renovated before drifting snow was addressed in building codes. The additional weight of equipment on the rooftop increases the demand on the underlying structure and roof deck.
Problems arise in the form of structural performance and higher project costs when estimates on snow loads are inaccurate. The only way to avoid this is to increase knowledge in the structural design of green and renewable facilities, as well as expertise on recent guidelines for snow loads and solar panels.
In the US, the rate of installation for PV systems has outpaced code adoptions. We have to modify what we know about wind and roof loads in general and look to the newest ASCE 7-10 building code regulations. Solar panel design and installation must adequately perform for at least 25 years in different climates and various weather conditions.
Where to go for more information:
Reputable solar panel manufacturers provide reports detailing design, snow, and wind pressure information that is useful to structural engineers. Make sure suppliers have a snow and wind report available before starting a project that also covers the racking and ballast system layouts. Check that they can provide support during the whole design process.
PV System Statistics
Commercial PV module weight varies depending on the manufacturer. Most solar panels are certified to withstand winds of up 140 MPH. Unique solar panels with a more resistant glass cover and sturdier frames are made for regions with an extremely high snow load. The manufacturer’s maximum snow load means that the module and its frame can withstand the weight described only if it is mounted to the racking system properly. Typical ratings can range between 60 and 120 pounds per square foot (psf) and more.
Snow doesn’t always slide off solar PV panels, and flat roofs and wet snow are variables. In the US, the snow load is typically between 20 and 40 psf. Only four inches of wet snow weighs over eight psf. To calculate snow load, you must know the climate, roof pitch angle, and the altitude of your location. This data determines the solar PV panels and the PV mounting system design, in addition to the underlying roof and wind loads. (v)
IBC and IRC Requirements
The majority of US states refer to the 2015 and 2018 editions of the International Building Code (IBC) and International Residential Code (IRC) with local amendments. Both have specific sections dedicated to the design and construction of roofs with PV panels, including live load, dead load of roof-mount rack systems, wind resistance, and snowdrift loads created by the system.
The new or existing roof framing must be capable of supporting the PV panel loads in each of these conditions, which is why we must address the age, prior renovation, or retrofitting of the structure. Local amendments and codes should always be reviewed to verify any requirements that vary significantly from the general requirements.
ASCE 7-16 Requirements
The most significant change in the updated ASCE 7 regulations pertain to wind loading. Detailed pressure coefficients have been developed based on research and testing for rooftop-mounted solar panels. The requirements outlined for wind loading resulted from the research and development at the Structural Engineers Association of California (SEAOC).
In addition:
• the weight of solar panels and supports are considered dead loads,
• roof live loads do not need to be applied to areas covered by solar panels under a certain spacing or height,
• seismic design is based on already established principles in section 13.3 for non-structural component design.
The design values provided by manufacturers can now be approximated and checked to reduce misunderstandings or inadequate detailing leading to design errors, construction defects, contractor liability, or property damage claims. Engineering projects involving roof solar panels installation and snow loads can be much easier and safer than before. For more topics on greener engineering efforts, read our article on Engineering 2020: How to Create Infrastructure with Sustainability and Resilience in Mind.
REFERENCES:
1. "Risk Management Series: Snow Load Safety Guide." FEMA P-957, January, 2013. https://www.fema.gov/media-library-data/7d8c55d1c4f815edf3d7e7d1c120383f/FEMA957_Snowload_508.pdf
2. Brooks, Albert., Gamble, Scott., Dale, Jan., and Bond, Jill., "Combined Probabilities of Peak Wind and Snow Load Events." Resilient Infrastructure - Abstract, June, 2016. https://www.researchgate.net/publication/308631772_Combined_Probabilites_of_Peak_Wind_and_Snow_Load_Events
3. Canuck65., "Solar Panels and Snow Load." Engineering Tips Forum, Accessed February, 2020., https://www.eng-tips.com/viewthread.cfm?qid=414923
4. "Snow On Solar Panels: How To Protect Your Solar Panel System From The Snow Load?" Power from Sunlight website, July 19, 2017. https://www.powerfromsunlight.com/snow-on-solar-panels-how-to-protect-your-solar-panel-system-from-the-snow-load/
5. Harper, Alan., "Dealing With Structural Issues for Solar Permitting." City of Madison Building Inspection, Accessed February, 2020. https://www.growsolar.org/wp-content/uploads/2012/06/Solar-Permitting-2012-Harper.pdf
6. Ryan, Eric., "Roof-Mounted Solar PV Panels - Part One: Structural Code Requirements." Sections of referenced document 3.1.5 Solar Panels, 4.17 Solar Panels Loads of, and 13.6.12 Rooftop Solar Panels., VERTEX, February 27, 2019. https://vertexeng.com/blog/roof-mounted-solar-pv-panels-1-structural-requirements/
