This document, authored by Jon Ness, PE, of Matrix Engineering Consultants, and Gerald Robinson of Lawrence Berkeley National Laboratory, addresses a specific, underspecified area of solar PV joint design: the geometry of clearance holes and slots, and the washers that must work with them. It provides interim practical guidance to fill a gap, as most PV racking joints use fasteners smaller than ½ inch — a size range that falls outside the RCSC Specification for Structural Joints Using High-Strength Bolts, the primary standard governing bolted connections in structural steel.
The consequences of getting clearance holes and washers wrong are not theoretical. Oversized holes and slots are standard features in PV racking components, necessary to accommodate manufacturing tolerances and uneven terrain. But when paired with washers that are too thin, too soft, or too small to fully cover the hole, the result is compressive yield, preload relaxation, and eventual joint loosening — failure mechanisms that develop quietly and are rarely traced back to washer specification decisions made during design.
What the document covers:
- How this document fits within the larger standards framework — The ASCE Solar PV Structures Manual of Practice (MOP) categorizes bolted joints as either traditional structural joints, governed by AISC 360 and the RCSC Specification, or alternative structural joints, which cover the majority of PV-specific connections, including module through-bolted joints, top-down clamps, and torque tube connections. For traditional joints, AISC and RCSC requirements already define hole sizes and washer specifications. This document focuses on alternative structural joints, where the Engineer of Record or mounting system designer bears direct responsibility for rationally specifying hole geometry and washer properties — and where no equivalent standard currently exists.
- Clearance hole and slot standards for alternative joints — For alternative structural joints, ASME B18.2.8 and ISO 273 provide clearance hole size references in close, normal, and loose fit categories. The document notes that because most PV racking holes are thermally cut with laser or plasma rather than drilled, designers have latitude to deviate from these standards as needed, provided the deviation is intentional and documented.
- Compressive yield in soft washers — When high-strength bolts are tightened, the bearing stress at the nut and bolt head contact area is concentrated over a relatively small surface. Mild steel and 300-series stainless steel washers often cannot withstand this stress without yielding, leading to gradual preload relaxation after installation. The solution is straightforward: specify washers with hardness equal to or greater than the mating bolt or nut, and specify through-hardened washers rather than surface-hardened, since surface-hardening does not protect against subsurface compressive yield. The document provides a hardness reference table covering common bolt and screw materials used in PV applications, from low-strength ASTM A307 through ISO 898-1 Class 10.9 and stainless ISO 3506-1 A2-70.
- Washer bending on oversized holes and slots — When a washer does not fully cover an oversized hole or slot, it acts as a cantilevered beam under pretension load, bending at the unsupported edge. This bending concentrates stress at the slot edge of the clamped component — typically aluminum in PV applications — and can cause compressive yield of the base material even when the washer itself remains intact. Finite element analysis of a representative 3/8-16 UNC joint with a 0.04-inch washer on a 0.625-inch wide slot confirms that both the washer and the aluminum plate yield under the applied pretension. The solution requires washers large enough to completely cover the hole or slot in all tolerance positions, and thick enough to remain effectively flat under load. For long slots where a round washer cannot achieve full coverage, plate washers are required — with corner radii specified to avoid stress concentrations at plate corners.
- Minimum washer thickness guidelines — Recommended minimum washer thicknesses are provided in a reference table derived from iterative 3D finite element analysis across common PV fastener sizes and clearance scenarios. For fasteners with diameters from 1/4 to 7/16 inches, the minimum thickness is 0.063 inches for clearance up to 1/16 inch, and 0.125 inches for clearance between 1/16 and 1/4 inch. For fasteners 1/2 inch and larger, RCSC Section 2.5 Table 6.1 applies. These values assume the washer fully covers the hole, with a maximum washer clearance hole of d + 1/32 inch.
- Design checklist — The document closes with a practical verification checklist covering clearance hole and slot specification, washer coverage, washer thickness, washer hardness, and bearing pressure under the bolt head and nut. The checklist is intended as a minimum set of checks for the EOR or mounting system designer, not a substitute for engineering judgment or calculation.
These guidelines apply to any PV racking or module mounting design that uses alternative structural joints with slotted or oversized holes, which describes the vast majority of connections in both fixed and tracking systems currently deployed. The underlying mechanics are not unique to solar, but the combination of small, fastener sizes, soft aluminum base materials, and wide slots found in PV applications makes this a particularly consequential area where the absence of clear guidance has led to avoidable failures.
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.