In structural engineering, the local pressure factor (Kl) is crucial for assessing wind-induced pressures on structures, including both roof-mounted and ground-mounted solar panel systems. This factor accounts for localized wind effects that significantly impact the immediate supporting elements of these installations.
Kl Factors in Roof Design
In roof design, Kl factors address localized wind pressures affecting elements such as cladding, purlins, and fasteners. According to AS/NZS 1170.2:2021, Section 5.4, Kl factors are assigned based on specific roof zones:
These factors are essential for designing the immediate supports of the roofing system, ensuring resilience against localized wind pressures.
Kl Factors in Ground-Mounted Solar Panel Systems
The application of Kl factors extends to ground-mounted solar panel arrays. The AS/NZS 1170.2:2021 standard introduced specific provisions for these systems, informed by research conducted at James Cook University. In their 2019 study published in Australian Journal of Structural Engineering, “Wind loads for designing ground-mounted solar-panel arrays” researchers J.D. Ginger, G.G. Bodhinayake, and S. Ingham performed wind tunnel tests to determine wind pressures on various ground-mounted solar panel configurations. The findings from this study were instrumental in formulating the guidelines presented in Appendix B of the 2021 standard.
DesignDevise have clarified the use of Kl factor with Dr. John Ginger, supporting the application of Kl factors to ground-mounted solar panel structures, aligning with the approach used in roof design.
They observed significant net negative (upward) pressures on panels at leading edges when wind approached the bottom surface and substantial net positive (downward) pressures at the bottom leading edge when wind approached the top surface. The study provided aerodynamic shape factors suitable for inclusion in the Australian/New Zealand wind loading standard AS/NZS 1170.2. The research emphasized the importance of considering local pressure factors (Kl) in the design of ground-mounted solar panel structures to account for localized wind effects, aligning with the broader structural design practices outlined in AS/NZS 1170.2.
Quoting from the paper
Aerodynamic shape factors are given in a form appropriate for the wind loading standard AS/NZS 1170.2. The study found that the area reduction and local pressure factors given in Table 5.4 and Table D1 of AS/NZS 1170.2 (2011) can be applied with these values to obtain the net design pressures on each panel and the structural system supporting the array of panels for solar panel arrays inclined from α = 0° to 30°.
This confirms that Kl factors were considered in the experimental setup and later adjusted within the AS/NZS 1170.2 framework.
Crux of the Research Related to Peak Pressures and Kl Factor
This study found that peak wind pressures on ground-mounted solar panels are significantly higher at edges and corners, similar to roof-mounted structures. The net pressure coefficients (Cpn) obtained in wind tunnel tests already showed higher pressures at these locations, which led some to question whether a separate Kl factor was needed.
However, to maintain consistency with AS/NZS 1170.2, the study divided the raw wind tunnel Cp values by the respective Kl factors used in roof design clauses, ensuring that a consistent approach was followed for different structural elements. This means that rather than applying Kl factors on top of already high Cpn values, the shape factors (Cshp) provided in AS/NZS 1170.2 for ground mounts inherently include the adjustment for Kl.
Thus, Kl is not “double-counted” in design but rather integrated into the provided aerodynamic shape factors, maintaining a consistent treatment of localized wind effects across roof and ground-mounted systems.
Appendix B of AS/NZS 1170.2:2021 specifies that Kl factors are applicable to ground-mounted solar panel structures, mirroring the approach used in roof design. This inclusion underscores the necessity of accounting for localized wind pressures in the design of immediate supporting members, such as purlins and rails, in ground-mounted systems.
Evolution of Standards and Kl Factor Application
Prior to the 2021 revision, AS/NZS 1170.2:2011 lacked specific guidelines for ground-mounted solar panel arrays, leading engineers to adapt roof design coefficients for these structures. The 2021 update addressed this gap by incorporating dedicated provisions for ground-mounted systems, reflecting advancements in research and industry practices.
Notably, within Appendix B, the “3-zone method” for roof-mounted solar panels does not explicitly mention Kl factors. This omission has prompted discussions among professionals regarding the necessity and applicability of Kl in these scenarios. Some practitioners argue that local pressure factors primarily pertain to cladding elements, suggesting that solar panels, being distinct from traditional cladding, may not require the same considerations. However, given that solar panels and their immediate supports are subjected to localized wind pressures, it is prudent to evaluate the applicability of Kl factors in their design.
Practical Considerations in Applying Kl Factors
When implementing Kl factors, certain practical aspects should be considered:
Patch Size Limitations: The standard suggests that Kl factors apply to areas not exceeding a 1:4 aspect ratio. However, for design simplicity and to avoid excessive iterations, engineers often apply the worst-case Kl factor uniformly along the length of a purlin or rail.
Design Simplicity: Applying the highest Kl factor across an entire structural element simplifies the design process, ensuring that localized pressure peaks are adequately accounted for, enhancing the overall resilience of the structure.
International Perspectives on Kl Factors
International standards also address localized pressure factors in the context of wind loading on structures:
SEAOC PV2-2017: The Structural Engineers Association of California’s report provides guidelines on wind design for solar arrays, emphasizing the importance of considering localized wind effects in both roof-mounted and ground-mounted systems.
ASCE 7-16: The American Society of Civil Engineers’ standard includes provisions for determining wind loads on structures, including solar panels, and highlights the significance of accounting for localized pressure factors.
Eurocode 1: This European standard outlines actions on structures, including wind actions, and specifies factors similar to Kl to account for localized wind pressures on structural elements.
Conclusion
The Kl factor is a pivotal consideration in the structural design of both roof-mounted and ground-mounted solar panel systems. Its application ensures that immediate supporting members are adequately designed to withstand localized wind pressures. The evolution of standards, informed by comprehensive research, reflects the industry’s commitment to safety and structural integrity in the face of wind-induced forces.