In this article, Jon Ness, PE of Matrix Engineering Consultants examines a failure mechanism that is easy to overlook in PV racking design: the gradual loss of fastener preload after installation. The article focuses specifically on the aluminum structures and bonded joint interfaces common to PV racking systems, where the combination of soft materials, short grip lengths, and electrical bonding requirements creates conditions that are particularly hostile to maintaining adequate clamp load over time.
Fasteners in PV racking systems carry both static loads — dead, live, and snow — and dynamic wind loads. They also serve a second function in many systems: providing the low-resistance electrical conduction path required for equipment grounding. Both functions depend entirely on the joint maintaining sufficient preload in service. When preload relaxes below the threshold needed to keep the joint closed and resist shear, fastener failure becomes a matter of time.
What the article covers:
- How embedment causes preload loss — Joint surfaces that appear smooth are microscopically rough, consisting of asperities that plastically flatten under bearing load when the fastener is tightened. This embedment accounts for the majority of initial preload loss, and it doesn’t stop at assembly. Embedment continues during the initial service loading cycles, and since most racking joints are never re-tightened after installation, this ongoing relaxation is permanent. For small fasteners with short grip lengths made from relatively soft materials like stainless steel, total elastic stretch at installation can be less than 0.001″, meaning even modest embedment represents a significant fraction of the total preload.
- How grip length governs sensitivity to embedment — The article includes an embedding loss chart showing preload loss as a percentage of initial preload versus the length-to-diameter (L/D) ratio of the fastener. Shorter grip lengths, which are common in PV racking, produce dramatically higher percentage losses for the same absolute amount of surface deformation. Under moderate surface stress conditions, initial embedment alone can account for up to 10% preload loss within seconds of tightening, with further losses of up to 40% possible under dynamic field loading.
- The additional complication of bonding devices — Aluminum racking components are typically anodized for corrosion protection, but that anodized layer is electrically insulating. To maintain a continuous grounding path, star washers or similar piercing devices are used at joint interfaces to cut through the anodized surface and connect the conductive aluminum core beneath. This piercing introduces an additional source of plastic deformation at the joint interface — one that behaves similarly to embedment but is harder to quantify using conventional design methods and remains an area where further research is needed.
- Mitigation strategies — The most straightforward approach is periodic re-tightening after initial assembly and during early service loading cycles, which allows preload relaxation to stabilize. Where re-tightening is impractical — as it often is in installed racking systems — the alternative is to reduce fastener stiffness by increasing grip length, making the joint less sensitive to a given amount of surface deformation. Regardless of approach, safety factors in PV racking fastener design must be set high enough to account for the inherent uncertainty in residual preload.
These considerations apply to any PV racking installation using aluminum structure and bonded grounding connections, which describes the majority of utility and commercial systems in service today. The article provides the mechanical foundation needed to understand why fastener preload in these systems is more difficult to maintain than in conventional structural steel applications — and what can be done about it at the design and installation stage.
Jon Ness
PE, PMP, NPDPJon is a Managing Governor and Principal Engineer at Matrix Engineering and has over 34 years of experience in business and engineering team leadership. His career has been focused on the development of off-highway equipment and powertrains. He has unique technical expertise in designing and validating dynamically loaded bolted joints. In his consulting role, Jon has led numerous joint failure investigations, including re-design efforts to mitigate risks to the system owners. He actively participates in ongoing research projects and has taught many classes related to Failure Modes and Effects Analysis and Bolted Joint Design and Validation. He received a Bachelor of Science in mechanical engineering from South Dakota State University. A licensed engineer in Minnesota, Jon is an active member of the UL2703 Standards Technical Panel, a contributor to the ASCE Manual of Practice for Solar PV Structures, and a Certified Fastener Specialist through the Fastener Training Institute.
Looking for a collaborative partner?
Whether you’re innovating from the ground up or enhancing an existing product, Matrix Engineering is here to guide you. Our team excels in providing dynamic solutions for new developments, improvements, and complex challenges across various industries.
