Wind Load Analysis for Commercial Roof-Mounted Arrays

PV modules mounted to racks or trackers should be designed to withstand wind forces as prescribed in the building codes, like any other important structure. However, there is a significant challenge in applying existing codes to commercial and industrial roof-mounted PV arrays.

Everyone is familiar with the potentially destructive forces that wind can exert on structures, whether from video footage or firsthand experience with structural damage from strong winds. The wind load on a structure depends on many factors, including the wind speed, the wind characteristics (turbulence and velocity profile), the geometry of the structure, the effect of surrounding objects and the height above the ground, to name just a few. A structural engineer must calculate wind forces or loads to ensure that a structure can resist wind conditions at a particular location.

The engineer should always determine wind loads on PV systems, even if local building departments do not require such an analysis. Wind-related failures of both roof- and ground-mounted systems have occurred—and more can be expected because wind loads are poorly understood. The spread of solar energy will depend on favorable public opinion, and a poor safety record is counterproductive. The longterm success of the solar industry depends on the deployment of systems that are structurally engineered to meet building codes. Unfortunately, this is not as easy as it sounds.

In this article, we discuss wind loads on sloped PV modules installed on standard open racks on a flat or low-slope roof. We also provide high-level guidance for other types of systems. We present some fundamentals of wind loading on rooftop PV systems, as well as challenges associated with applying existing building codes to this type of system. While building codes will eventually include improved guidelines for determining wind loads on PV systems, the process of developing these guidelines is likely to take years. Until then, structural engineers need strategies to reduce the guesswork in estimating wind loads, particularly for sloped PV modules on flat roofs, because building codes are least applicable to this type of system.

With support from its parent company, Det Norske Veritas (DNV), and its wind engineering partner, CPP, BEW Engineering has developed an analytical approach for determining wind loads on sloped PV systems mounted on flat and low-slope roofs. BEW analyzed the results of thousands of wind tunnel tests in the process of developing this method. While this approach is more complex than typical ACSE 7 methods, the partnership developed a free online design tool for designers and engineers to simplify the process and expedite the analysis. Here we discuss how this tool works and how to apply the results.

The Building Code and PV

While all US building codes have sections on wind loading, it is widely accepted that the most comprehensive guide for estimating wind loads on structures is Standard No. 7 of the American Society of Civil Engineers (ASCE), Minimum Design Loads for Building and Other Structures (ASCE 7). All other building codes allow for the use of this ASCE standard. The latest edition of this standard is ASCE 7-10, the 2010 edition. However, ASCE 7-10 has not been widely adopted, and the 2005 edition is still mainly in use. Therefore, we reference ASCE 7-05 in this article. By the time ASCE 7-10 is widely adopted, it is likely that a broader range of publicly available wind tunnel data will be available that will improve upon the methods described in this article. The wind loading content found in ASCE 7 was primarily developed to calculate wind loads on buildings, though a small subset of other structures, such as billboards and chimneys, is included. As a result, building codes do not provide clear guidance on how to calculate wind loads on PV arrays, unless these are shaped like buildings—for instance, PV carports.

With proper guidance— see “Wind Load Analysis Recommendations by PV System Type” (sidebar, below)—designers and engineers applying ASCE 7-05 methods can do a reasonable job of estimating wind loads on some flat-plate PV systems. For example, ground-mounted PV systems are very much like small open buildings— meaning buildings with no walls, such as carports—closely spaced together. In addition, wind load patterns on modules mounted parallel to and close to the roof—as is common in residential applications—may be estimated by calculating the loads expected on the exterior of the building cladding. In many but not all of these cases, building codes overestimate wind loads on these types of PV systems, and designers and engineers can use these conservative results with confidence.

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