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Introductory Overview
The report is framed around an increasing frequency of solar power plant development in hurricane-prone and other high wind speed locations. As photovoltaic modules have grown in size, the total wind force acting on each module has also increased, intensifying the load transferred through structural connections that support the modules. The report explains that wind loading on tracker-mounted modules is influenced by boundary layer wind behavior and that wind tunnel testing commonly shows pressure is not applied evenly across the module surface. In particular, the windward side of a module can experience higher pressure during gust events.
The authors note that the magnitude of peak wind pressure is generally well understood in design practice because design wind speeds can be determined from local building code maps or through site-specific studies. These design wind speeds typically correspond to a short-duration gust measurement (a three-second gust) associated with a defined probability of exceedance and return interval. In the U.S. code environment, the report describes this framework as providing a known level of reliability for project owners and insurers.
While peak gust design is widely addressed, the report identifies a gap in industry understanding related to fatigue loads. It defines fatigue failure as the formation and propagation of cracks due to repetitive or cyclical loading, where the cyclic stresses are typically below the threshold that would cause yielding in a single event. Hurricanes are described as capable of producing high gust wind speeds over extended periods, resulting in many load cycles at high amplitudes. The report explains that each cycle can extend crack growth incrementally, eventually leading to structural failure.
The report focuses on a specific structural interface that is presented as particularly susceptible to fatigue failure: the connection between the purlin (also referred to as a support rail, clamp, or MIB) and the torque tube that rotates PV modules throughout the day in a single-axis tracker system. Purlins are described as commonly manufactured from galvanized cold-formed steel in an omega (hat) shape. The report presents an example purlin-to-torque tube connection design without reinforcement and labels it as “Purlin Design A.” The authors state that steel gauge must be optimized to achieve an economical solution while maintaining desired reliability and that thinner materials can make the connection more susceptible to fatigue failure.
The report sets out to determine the magnitude and frequency of fatigue loading and evaluate the suitability of current approaches for assessing fatigue capacity, including code-based fatigue equations, finite element modeling, and full-scale testing. It positions the proposed testing methodology as a response to limitations identified in current standards and analysis tools.
Purpose
The purpose of the report is to explain how fatigue loading can accumulate on tracker structural components during hurricanes, quantify the expected cycle counts and pressures that may occur during a major wind event, and assess whether current testing and design standards are sufficient for tracker systems installed in hurricane-prone locations. The report also proposes a fatigue loading test procedure intended to more accurately reflect hurricane-relevant pressures, cycle quantities, and loading conditions, with the aim of improving reliability assessment for tracker systems and PV module assemblies.
Publishing Organization
The report is published by GameChange Solar and dated May 2025. It includes a disclaimer stating the report is presented based on analysis of data collected by GameChange Solar from third-party solar PV project owners and builders, as well as observations of GameChange personnel, and that it is provided without warranties related to reliance on the information. The report is presented as a white paper titled “Designing for Extreme Wind: Understanding Fatigue Loading on Purlins for Single Axis Trackers.”
Geographic and Industry Scope
The report addresses utility-scale solar PV power plants using single-axis tracker systems, with emphasis on installations in hurricane-prone and high wind speed regions. The analysis and example testing are tied to wind conditions in Florida, including a specific hurricane case study and a described test scenario for a Florida site. The structural focus is on the purlin-to-torque tube connection and the assembly behavior of PV modules, module frames, fasteners, and purlins under repeated wind-induced cycling.
Timeframe
The report references Hurricane Ian, which made landfall in Florida in 2022, as the example hurricane used for fatigue loading analysis. It describes hurricane-force winds extending up to 45 miles from the storm center and notes that wind gusts reached up to 110 mph near Sarasota Bradenton International Airport. The report indicates that high-resolution wind data from a meteorological tower at the airport provided a basis for the analysis performed by CPP Wind Engineering Consultants.
Section-by-Section SummaryExecutive Summary
The executive summary states that solar power plants are increasingly being designed and installed in hurricane-prone and high wind speed locations, requiring structural systems to sustain significant levels of repeated wind loading. The report describes fatigue loading as the accumulated effect of cycling wind loads over time and cites a study commissioned by GameChange Solar and performed by CPP Wind Engineering Consultants. The executive summary reports that fatigue loading at a sample site during an example hurricane reached over 8,000 cycles at pressures up to 1,400 Pa.
The executive summary further asserts that current testing and design standards for fatigue loading on single-axis trackers are either insufficient relative to the cycles and pressures identified by CPP or unable to accurately model the complex assembly of purlin, module frame, and module glass. It states that systems designed according to these standards may fail under real-world loading conditions. The executive summary introduces the report’s proposed fatigue loading test procedure as a method intended to more accurately reflect pressures, cycle quantities, and loading conditions experienced during long-duration wind events.
1. Introduction
The introduction describes how increased deployment of solar PV in high wind regions, combined with growth in PV module size, increases wind forces and structural demand on module support systems. The report explains that wind load magnitude for a given wind speed is relatively well understood due to wind tunnel testing, but emphasizes that wind pressure is unevenly distributed across modules, with the windward side typically experiencing higher pressures during gusts.
The report describes design wind speed determination as typically based on building code maps or site-specific studies, and characterizes design wind speeds as corresponding to a short-duration gust event associated with a return interval. It notes that this framework provides a known level of reliability. However, the report highlights that fatigue loads during hurricanes are less understood, and that hurricane wind conditions can generate many load cycles at high amplitudes on tracker components, which can drive fatigue crack growth over time.
The report identifies the purlin-to-torque tube connection as a key fatigue concern and describes purlins as galvanized cold-formed steel components typically shaped like a hat or omega profile. It introduces “Purlin Design A” as an example industry design without reinforcement and states that thinner steel gauges can increase fatigue susceptibility. The introduction closes by indicating that the report will address fatigue magnitude and frequency and assess different approaches for determining fatigue capacity, including design equations, finite element modeling, and full-scale testing.
1.1 Expected Magnitude and Cycle Count of Fatigue Loads
This section describes a study conducted with CPP Wind Engineering Consultants to estimate fatigue loading experienced by a single-axis tracker during hurricanes. The report states that CPP used wind conditions from a site during an example hurricane combined with wind tunnel data, and performed a rain-flow analysis to determine cycle counts and pressures on the windward half of the module.
The report identifies Hurricane Ian (2022) as the example hurricane selected for analysis and notes that Ian was tied for the fifth strongest hurricane on record to hit the contiguous United States at the time. It states that gusts reached up to 110 mph near Sarasota Bradenton International Airport and that meteorological tower data from the airport provided a high-resolution dataset used in the analysis.
The reported results indicate the sample site likely experienced over 8,000 cycles with pressures up to 1,400 Pa, with over 1,700 cycles at amplitudes over 600 Pa. The report notes that the full CPP study is available on request. The report uses these cycle counts and pressure levels as a basis for evaluating whether current fatigue testing standards are sufficient.
2.0 Are Current Means to Address Fatigue Sufficient?
This section evaluates whether existing standards and analytical tools adequately address fatigue loading on tracker purlin systems. The report examines mechanical cyclic load test standards, fatigue equations in cold-formed steel design standards, and finite element modeling as approaches that may be used to evaluate fatigue risk. The report’s overall conclusion in this section is that existing methods are either insufficient or not well suited to the complex assembly behavior of module and purlin connections under hurricane-style loading.
2.1 Mechanical Load Test Standards
The report describes IEC 62782 as an example test standard for cyclic loading of PV modules. It states that IEC 62782 calls for mounting a PV module on support rails using appropriate mounting means and subjecting it to a uniform 1,000 Pa load cycled on and off at 3 to 7 cycles per minute for 1,000 cycles. The report argues that full-scale tests should be performed on an assembly consisting of both the PV module and the supporting purlin, rather than only on an individual purlin, because assembly testing better represents the load path through the module and accounts for the stiffness contribution of the module frame.
The report describes testing sponsored by GameChange Solar and performed by RETC. Under strict IEC 62782 conditions, the report states that Purlin Design A showed no visible cracks after testing. The report provides details that these purlins were Grade 80, 21-gauge steel meeting ASTM A653 requirements, and that the purlins supported the PV module using bolts at the 400 mm mounting hole locations in the module frame. It also notes there was no reinforcement between the purlin and the torque tube. The report states that because no cracks were visible after IEC testing, fatigue crack propagation was not flagged as a concern under the standard test conditions.
The report then describes enhanced cyclical testing of the same purlin design using the same equipment and loading frequency as IEC 62782 but with a higher cycle count intended to better align with CPP’s hurricane-derived cycle counts. The report states that after only 2,000 cycles at 625 Pa, crack propagations were observed at multiple locations in the purlin. The report notes that this cycle count is still less than the total cycle count expected during a hurricane per the CPP analysis.
The report further notes that purlins were not the only source of failure during testing. It states that other samples exhibited cracking of the aluminum module frame under cyclical loading. This observation is used to reinforce the report’s emphasis that the full assembly of module, hardware, and purlin must be analyzed to identify potential failures.
The report summarizes that purlins tested per IEC 62782 showed no signs of failure, while those tested under simulated hurricane loading showed crack propagation and structural failure. It concludes that IEC 62782 is not an appropriate means to determine the fatigue capacity of purlin systems installed in hurricane-prone locations and states that fatigue failure could result in on-site damage.
2.2 Fatigue Equations in Design Standards
This section discusses cold-formed steel design standards that include fatigue provisions and evaluates their applicability to tracker purlins. The report identifies AISI S100 as the applicable U.S. standard for cold-formed steel parts and states that fatigue loading is addressed in Section M of the 2016 edition. The report explains that AISI S100’s fatigue methodology expects the purlin to be idealized as a cantilevered beam extending from a fixed connection with the torque tube in order to calculate stresses that can be compared to a maximum design stress range derived from tables and equations.
The report states that under Table M1-1 of AISI S100, the purlins qualify as Stress Category 1, which applies to “as received base metal and components with as rolled surfaces, including sheared edges and cold-formed corners.” It then states that Equation M3-1 yields a theoretical design stress range of 158 ksi for a component enduring 8,000 cycles. The report argues that this theoretical stress far exceeds the maximum stress calculated from static analysis of a design-level wind gust and concludes that the AISI S100 calculation becomes overly conservative when applied to single-axis tracker purlin design.
The report explains that a key reason AISI S100 is not accurate for this application is that it primarily governs parts of a single material, whereas the purlin and module connection is a complex assembly involving steel, aluminum, glass, and other materials. It states that while AISI S100 can accurately model cyclical loading of a purely steel part such as a torque tube, it is not applicable to module and purlin testing and design in the described context.
2.3 Finite Element Modeling
This section discusses finite element analysis as a structural design tool and highlights limitations for predicting fatigue and brittle failure modes in the tracker purlin connection described. The report states that 3D finite element analysis is useful for identifying ductile-type failures such as yielding, but that current software solutions do not show adequate correlation to test data when identifying brittle failure modes. The report also notes that modeling the purlin-to-torque tube connection is particularly difficult because the modeler must account for residual stresses from cold forming, galvanizing, and bolt preload created by tightening fasteners.
The report provides an example FEA model of Purlin Design A under loading that resulted in failure during mechanical testing. The modeled loading includes a downward pressure of 625 Pa on the module, corresponding to interior module loading during an 86 mph wind event. The report states that the analysis was run in Solidworks and that maximum stresses were on the order of 50 ksi, below the 80 ksi yield stress of the material. The report concludes that the FEA model would not predict fatigue failure even though mechanical testing demonstrated the part did not have sufficient capacity to survive the expected cycle counts. The report therefore characterizes this analysis method as not accurately highlighting the risk of purlin failure under fatigue loading during a hurricane.
3.0 Proposed Test Programs
The report proposes a fatigue testing methodology intended to better reflect the pressures, cycle counts, and loading characteristics experienced during hurricanes and other long-duration wind events. It presents the approach as a service-level fatigue load sequence and states that strength-level static loading procedures should still be followed separately. The report frames the proposed test program as a response to the limitations identified in existing standards and analytical tools.
3.1 Fatigue Loading Approach
The proposed methodology relies heavily on full-scale testing. The report states that existing analytical means are inaccurate and describes a service-level fatigue load sequence to be performed as dynamic mechanical load testing. The report notes that the static loading procedure described in IEC 61215 Section 4.16 should still be followed in addition to the proposed fatigue procedure.
The report recommends that the procedure ideally be performed on a test rig large enough to hold two modules so that the purlin between those modules is loaded in a way that reflects real-world conditions. It states that if only one module is used, each purlin would experience loading corresponding to only one connected module rather than the more typical condition of being connected to two modules. In that case, the report states that the pressure on the module should be doubled to match loading conditions for a shared purlin.
The report describes the fatigue sequence as consisting of five rounds of dynamic mechanical load tests. Each round includes a set cycle count and pressures scaled to expected on-site pressures. The report states that 3 to 7 cycles per minute are applied, consistent with IEC 62782. Each DML cycle is described as one load pulling on the front of the module glass followed by a return to neutral position, simulating uplift wind gust loading on the rear side of the module. The report notes that wind direction may change during hurricanes, inducing loading on either the front, back, or both sides of the module, and states uplift loading is used as the most conservative scenario.
The report presents consolidated cycle counts and base pressures derived from CPP analysis and describes these as reduced into three load levels to simplify test programming. It states that each cycle range is increased by a factor of 1.2 to account for additional wind loading beyond a single simulated hurricane during the system’s design life. The report gives an example that CPP analysis shows over 6,000 cycles at pressures at or below 600 Pa, which are consolidated into two rounds of 600 Pa at 4,000 cycles each.
The report explains that project-specific pressures can be scaled from base pressures by using the squared ratio of the project design wind speed to the CPP report wind speed of 110 mph. It provides an example calculation for a project with 120 mph design wind speed, resulting in a first-round pressure of 714 Pa using the described scaling relationship.
3.2 Example Test Results
The report describes testing performed at RETC using the proposed procedure for a site with a design wind speed of 107 mph in hurricane-prone Florida. It states that the full test report is available on request and provides a summary of results and test parameter derivation.
To determine expected fatigue loading for the site, the report states that the 300-year return interval design wind speed of 107 mph was converted to a 50-year return interval service-level wind speed of 90 mph. This conversion is described as intended to match the tested wind speed to the project’s design life of 35 years, as described in AISI S100 Chapter M. The report states that test pressures were determined by taking the squared ratio of the CPP report wind speed (110 mph) to the 90 mph test wind speed and presents a table of DML cycle counts and pressures for interior and perimeter conditions.
The report states that the load is applied only to one side of the module to mimic unbalanced wind loading observed during wind tunnel testing and on-site loading. It indicates that other DML criteria match IEC 62782. At the end of the sequence, the report states that parts are inspected for signs of damage and that failure criteria align with UL 2703 Section 21.6. It states that there shall be no visual permanent deformation that may adversely affect system safety or compliance. Minor plastic deformations or dimples in cold-formed steel components are described as acceptable, while cracking or micro fissures visible to the naked eye greater than 1 mm and propagating through the entire thickness of the purlin are described as constituting failure.
Cross-Cutting ThemesFatigue Loading as a Distinct Design Challenge in Hurricane Regions
A central theme of the report is that hurricane-prone solar installations face a fatigue challenge that differs from single-event peak gust design. The report emphasizes that repeated cycling at meaningful pressure amplitudes can accumulate into fatigue damage over time, leading to crack growth and eventual failure even when individual cycles remain below yielding thresholds. The reported cycle counts and pressures derived from hurricane conditions are used to illustrate that fatigue loading may be more severe than typical cyclic test assumptions.
Assembly-Level Behavior and Multiple Failure Modes
The report repeatedly highlights that the purlin-module connection is a complex assembly involving multiple materials and components, including steel purlins, aluminum module frames, glass, fasteners, and torque tubes. The report’s discussion of module frame cracking during cyclical testing reinforces that fatigue failure risk may appear in more than one component, and that evaluating a single part in isolation may not capture the full range of structural vulnerabilities.
Mismatch Between Standardized Tests and Hurricane-Representative Loading
The report identifies a gap between standardized cyclic test procedures and hurricane-representative fatigue demands. It describes IEC 62782 as involving 1,000 cycles at uniform load conditions and contrasts this with the higher cycle counts and varying pressure amplitudes identified through hurricane-based analysis. The report presents enhanced cyclical testing results as evidence that fatigue cracking can emerge under higher cycle counts even when standard tests show no visible damage.
Analytical and Computational Limits in Predicting Fatigue Failure
The report argues that code-based fatigue equations and finite element modeling may not provide reliable predictions for the fatigue performance of the purlin-module connection under hurricane-style loading. It describes the AISI S100 fatigue equation outcome as overly conservative when applied to the complex assembly context and states that FEA may not predict fatigue failure despite mechanical evidence of crack propagation under repeated loading. The report attributes these limitations to simplified assumptions, single-material modeling frameworks, and difficulty capturing factors such as residual stresses and bolt preload.
Strategic and Forward-Looking Insights
The report presents full-scale service-level fatigue testing as a tool intended to improve confidence in tracker hardware configurations for hurricane-prone projects. It indicates that a test approach reflecting hurricane-relevant pressures, cycle counts, and unbalanced loading conditions can provide evidence of whether a module-purlin-hardware assembly has sufficient fatigue capacity. The report also implies that reliance on existing standards alone may not be sufficient for assessing fatigue risk in regions where long-duration extreme wind events are expected.
The report includes a comparison between an unreinforced purlin connection design (Purlin Design A) and an improved configuration (Purlin Design B). It states that the improved design showed no deformation and no cracks after the service-level fatigue test sequence, and concludes that fatigue failure is not expected even during a hurricane wind event for that tested configuration. The report identifies Purlin Design B as the standard preassembled purlin design provided by GameChange Solar and describes it as Grade 80, 18-gauge steel meeting ASTM A653 requirements, with module support through bolts at the 400 mm mounting hole locations.
Closing Synthesis
This report presents hurricane-driven fatigue loading as a distinct and potentially under-addressed reliability risk for single-axis tracker solar installations. Using hurricane-based cycle and pressure analysis and comparing standard test outcomes to enhanced cyclical testing results, it argues that common cyclic test standards may not reveal fatigue cracking that emerges under more representative loading sequences. The report further concludes that simplified code-based fatigue equations and finite element modeling may not adequately capture fatigue failure mechanisms in complex module-purlin assemblies. In response, it proposes a service-level dynamic mechanical load testing methodology that consolidates hurricane-derived loading into programmable test rounds and provides scaling logic for site-specific wind speeds and defined inspection-based failure criteria.
