Improving Bolted Joint Reliability in Utility-Scale PV: Gaps and Improvements in Tightening Methods, Tools, and Fasteners

This report, authored by Jon Ness, PE, of Matrix Engineering Consultants and Mary Beth Wigginton of RFA Engineering for Lawrence Berkeley National Laboratory, examines a systemic problem in utility-scale solar PV construction: the gap between how bolted joints are tightened in the field and what is required to achieve reliable, predictable pretension. It identifies specific shortcomings in current tightening methods, installation tooling, and fastener specification practices — and maps a path forward through near-term standards improvements, better education, and longer-term innovations in tool and fastener technology.

The stakes are structural and electrical. A typical utility-scale PV site contains millions of bolted joints, many in the structural load path transferring dead, snow, and wind loads from modules to foundations, and some serving as part of the grounding system. Every one of those joints depends on pretension — the clamping force generated when a fastener is stretched during tightening. When pretension is poorly controlled, joints slip, open, fracture, or lose electrical bond. Yet across the industry, torque remains the default control variable, despite decades of evidence from structural bolting that torque is an unreliable proxy for clamp load when friction, tool performance, and field conditions are not tightly managed.

What the report covers:

• Why torque control tightening produces unpredictable pretension — When a fastener is tightened using torque control, roughly half the applied torque is lost to friction under the turning element, about forty percent is consumed in the threads, and only the remaining ten percent actually stretches the bolt to create clamp force. Because of this distribution, even small changes in surface condition, coatings, lubrication, contamination, or corrosion can produce large variations in the pretension achieved at a given torque setting. Conventional structural bolting in buildings and bridges moved away from torque-based methods decades ago for exactly this reason, shifting to twist-off tension-control bolts and direct tension indicator (DTI) washers that provide visual confirmation of minimum pretension. The solar PV industry has not yet made that shift.
  
• The tightening tool landscape and its limitations — The report surveys the full range of torque tools currently used on PV sites, from impact drivers and clicker-style torque wrenches through shut-off impact wrenches, slip-clutch nut runners, and controlled nut runners. These are organized into a “Solar Tool Pyramid” that illustrates the trade-off between accuracy, cost, and field durability. Impact drivers offer speed and ruggedness but provide no meaningful torque control (±20–40% repeatability). Clicker wrenches achieve roughly ±10% in practice but are subject to operator fatigue and misuse. Precision tools like controlled nut runners can reach ±5% absolute accuracy, but they were designed for clean factory environments and generally do not hold up under the dust, heat, drops, and long duty cycles of utility-scale PV construction. Installation manuals rarely specify tool type or model, leaving selection to contractors and creating significant variability across projects.
  
• Fastener specification gaps that undermine joint capacity — Many PV module and mounting system installation manuals specify fasteners by size and a general material description only — for example, “5/16-inch stainless steel bolt” — without referencing a consensus standard that defines yield strength, tensile strength, hardness, thread geometry, or coating characteristics. Stainless steel fasteners marked “304” or “316” identify material composition but not mechanical properties. When fastener properties are undefined, joint capacity is uncertain regardless of how carefully torque is applied. The report also identifies a parallel gap in field control: fasteners are routinely exposed to dust, moisture, and rough handling before installation, and manuals rarely address storage, cleanliness, or handling requirements — all of which alter friction and directly affect the pretension achieved.
  
• The June 2025 revision to ANSI/UL 2703 introduces mandatory and optional requirements that begin to close several gaps identified in the report. Manufacturers seeking a UL 2703 listing must now identify critical structural and bonding fasteners, fully specify their properties through recognized consensus standards or a documented quality management system, and provide installation instructions that go beyond a torque value to include the assembly details needed to achieve controlled pretension. Optional provisions allow manufacturers to establish a statistically derived minimum clamp load using test data from representative joints assembled with the same tools and procedures specified in their manuals, and to qualify alternative fastening systems — such as lockbolts or tension-controlled fasteners — that do not rely on torque as the primary control variable.
  
• The forthcoming ASCE/SEI Solar PV Structures Manual of Practice — Expected in late Q1 2026, the MOP classifies PV mounting structure joints into traditional structural joints (½-inch and larger bolts, governed by the RCSC Specification) and alternative structural joints (sub-½-inch fasteners in thin-gauge steel, aluminum extrusions, clamps, and proprietary connections). For traditional joints, the MOP requires compliance with RCSC, which treats the fastener assembly, installation method, and tightening tool as an integrated system and explicitly allows non-torque methods such as tension-control bolts and DTI washers. For alternative joints, the MOP requires the Engineer of Record or mounting system designer to rationally design the connection, specify minimum pretension rather than torque in drawings and manuals, and demonstrate that the selected tightening method can consistently deliver controlled pretension in field conditions. The MOP also requires daily verification of tightening torque using representative fasteners in a tension calibrator when torque control is used.
  
• Industry training needs across roles — The report outlines targeted education opportunities for designers (understanding pretension scatter in design assumptions, specifying complete tightening methods, awareness of non-torque alternatives), EPCs and contractors (recognizing gaps in installation manuals, managing fastener friction as carefully as torque, matching tool selection to joint criticality), and inspectors (understanding the limits of torque checks as pretension indicators, verifying that project documents actually define critical joints and acceptance criteria, establishing escalation pathways for repeated deviations). The report suggests that organizations like NABCEP, SEIA, and ASCE/NCSEA could develop these into CEU-eligible, role-specific training modules.
  
• Emerging tool and fastener technologies — Looking beyond near-term improvements, the report examines ruggedized precision tightening tools engineered for PV site conditions (IP65 or higher, serviceable in the field), smart tools with real-time torque and angle feedback, data logging, and traceability features that shift quality control from post-installation audits to in-process verification. On the fastener side, the report covers the potential for load-indicating washers sized for sub-½-inch alternative structural joints (currently unavailable), Belleville washers as a near-term visual pretension indicator, lock bolts that achieve pretension through pin elongation rather than torque, and spring-loaded module retention devices that eliminate threaded fasteners entirely for module attachment. Each technology is assessed for readiness, barriers to adoption, and the conditions under which it could gain traction.
  

This report frames a fundamental shift in how the PV industry should think about bolted joint quality: torque is the method, not the objective — pretension is the outcome that governs joint capacity. The standards, tools, and engineering frameworks to close the gap between design intent and field execution are now emerging. What remains is for the industry to act, beginning with the joints that carry the most structural and electrical consequences.