What’s New Under the Sun?

ASCE/SEI Solar PV Structures Committee Manual of Practice

As the energy market transitions towards having a larger renewable energy component, the solar energy industry has experienced fastpaced growth. In particular, the solar photovoltaic (PV) industry has grown rapidly over the last decade, which has presented numerous challenges. One significant growing pain has been the lack of design guidance on solar PV structures from the building codes. The building code (referred to herein as the “Code”) is typically some version of the state building code based on the International Building Code (IBC) as adopted by the local Authority Having Jurisdiction (AHJ).

The practice of structural engineering is a technical endeavor, an art, and a business enterprise. The practice benefits when the Code establishes minimum and uniform standards that define a level playing field for producing structural designs and reviewing them for compliance. Another essential function of the Code is to emphasize public safety by defining critical aspects of the engineering “standard of care,” such as the expected basic safety and reliability of engineered structural systems. Unfortunately, these functions are held in delicate balance with and sometimes eroded by a competitive commercial environment in which constant pressure exists to economize/optimize structures.

The Wild West

Some structural engineering practitioners like to describe the current design landscape in the solar PV industry as the Wild West. While the solar industry may contain its share of outlaws and other ornery varmints (i.e., bad actors), it is the Wild West because of the lack of explicit Code design guidance for solar industry structures. The result is a lack of consistency and, in some cases, confusion. Here are a few examples:

  • Inconsistent interpretation and application of existing Code provisions that are not readily applicable to solar PV among designers, AHJs, and other reviewers;
  • Use of widely different methodologies with varying degrees of design safety, reliability, and effectiveness;
  • Inefficient permit review by AHJs due to lack of familiarity with or confusion about industry norms;
  • Inconsistent requirements among AHJs; and
  • General lack of design guidance on some design issues.

ASCE/SEI Solar PV Structures Committee

To address these challenges, the Structural Engineering Institute (SEI) of the American Society of Civil Engineers (ASCE) recently approved the formation of the ASCE/SEI Solar PV Structures Committee. The Committee comprises many solar PV energy industry stakeholders (engineers, AHJs, solar equipment manufacturers, owners, developers, and contractors). It is focused on the consistent and reliable design of structures that support solar PV systems. Structures supporting concentrated solar (i.e., a “mirror farm” focusing light on a central boiler), hot water solar power, or any non-PV-related solar power system are not under the dominion of this committee. The Committee’s mission is defined in its statement of purpose:

To promote advancements in the design, procurement, permitting, and construction of solar photovoltaic (PV) ground-mounted, canopy, and roof-mounted structural systems. The committee, made up of an interdisciplinary team of engineers, manufacturers, contractors, permitting officials, and owners, addresses issues in design and construction, shares lessons learned, develops design guides and standards, and advocates for the reliable and consistent design and development of solar PV power generation structures.

Currently, the Committee’s primary effort is working towards the mission of “…shares lessons learned, develops design guides and standards…” so that more consistency can be brought to the Wild West by producing a Solar PV Structures Manual of Practice (MOP).

The coming together of stakeholders in an industry to bring some level of consistency to the design process is not new. The Structural Design of Air and Gas Ducts for Power Stations was developed by the Energy Division of ASCE to bring design consistency to large-scale ductwork used in power-generating facilities. The Subcommittee on the Design of Substation Structures of ASCE developed the Substation Structure Design Guide to provide consistency amongst substation design engineers. When the utility-scale wind energy industry was facing similar circumstances a decade ago, stakeholders in the utilityscale wind industry formed a joint committee to address the challenge of bridging and reconciling the industry’s prevalent international design practices with U.S. Code requirements. Under the aegis of the ASCE and the American Wind Energy Association (AWEA), a guidance document was created: ASCE/ AWEA RP2011, Recommended Practice for Compliance of Large Land-based Wind Turbine Support Structures. It was introduced in STRUCTURE’s July 2013 issue in a piece titled Wind Farm Tower Design.

The Solar PV Structures Manual of Practice

The MOP is intended to provide needed design guidance to the solar industry for the following benefits: education, consistency, managed risk, and innovation – all contributing to more reliable solar PV power. The developing MOP currently has the following outline:

Chapter 1. Introduction: intent, limitations and scope, solar industry glossary. Chapter 2. Design Loads: recommended risk categories, wind, wind tunnel testing, dynamic sensitivity, aerodynamic instability, flood and scour, ice and hail, snow, earthquake, frost jacking (heave), effects of stowing, peer review.

Chapter 3. Corrosion: science of corrosion, rational application of unknowns and variables, basis for estimates, and factors of safety applying to solar PV structures.

Chapter 4. Structural Design: load combinations, resistance factors, serviceability and displacements, structural connections. Chapter 5. Foundation Design: Solar PV foundation design using driven piles and drilled piers.

Chapter 6. Construction Quality Assurance: special inspections, preconstruction/pull testing of piles, construction/production pile testing. Chapter 7. References.

Solar PV Structure Types

The MOP will cover the following types of solar PV systems. Ground-mounted PV (GMPV) support structures:

  • Fixed-tilt rack systems. See Figure 1. This system consists of PV panels supported on a steel purlin-beam framework supported on a foundation consisting of driven steel piles, helical anchors, ballast foundations, and sometimes cast-in-drilled-hole piles.
  • Single and Dual Axis Tracker systems. See Figure 2. These systems consist of PV panels attached to either a racking system, such as on a dual-axis tracker, or a torque tube, such as on a single-axis tracker, that allows the superstructure assembly to rotate mechanically and actively tilt toward/track the sun during daylight hours. The supporting foundation typically consists of driven steel piles, helical anchors, shallow foundations, or ballast foundations.
Figure 1. Typical ground-mounted fixed-tilt rack system. Courtesy of ASE.
Figure 2. Typical single-axis tracker system. The torque tube (centered longitudinal tube member, left) rotates (twists) to facilitate tilting the solar PV panels mounted to the superstructure. This way, the tracker system can tilt toward and track the sun. Courtesy of Anubhav Tandon; Array Technologies Inc.

Rooftop PV (RPV) support structures:

  • Building roof mounted. See Figure 3. This system consists of PV panels supported on a low-profile framework that is either ballasted on the roof or mechanically anchored to the roof.
Figure 3. Combination rooftop and carport/parking lot shade structure solar PV installations are an ideal use of area at large retail establishments. Courtesy of Target Corporation website.

Elevated PV (EPV) support structures:

  • Parking lot carport roof mounted. See Figure 3. This system consists of PV panels supported on a roof framework supported on vertical cantilevered columns attached to the top of either piles, cast-in-drilled-hole piles, or spread footings. This configuration allows the solar structure to serve as a parking lot shade structure and produce power.

Floating PV (FPV) support structures:

  • Floating solar PV. This aquatic system consists of a low-profile framework similar to a rooftop framework system but is attached to the top of floats. The floats are then fastened together, and the entire system is anchored to keep it in place on the body of water.

Design Challenges and Recommended Practices

What may be apparent is that solar PV structures are relatively lightweight and flexible compared to other nonbuilding support structures. As such, they tend to be dynamically sensitive structures that, in many cases, exhibit resonant vibrations, galloping, and flutter even at low to intermediate wind speeds far below the Code’s extreme design wind speed. The MOP will attempt to address the many design challenges that the industry faces due to the unique aspects of these structures.

The MOP will provide a snapshot of the current state of the solar PV structure design practice and recommend solutions to a myriad of structural design challenges. The following are some of the significant topics that will be explored in the MOP.

Wind

Due to the characteristics of solar PV structures and their corresponding structural behavior, one of the biggest challenges is establishing Code-compliant wind loading design criteria. Also, the direction from the ASCE 7 on wind loading is wide-ranging. The MOP will provide guidance on how to utilize the existing

ASCE 7 standard design recommendations while filling in key areas that were too new to get into the most recent version of the ASCE 7 standard. These key areas include additional guidance on calculating more refined wind loads in coordination with ASCE 7 and other solar industry standards and how best to apply ASCE 7 wind parameters such as directionality and pressure coefficients to a solar PV structure while considering its unique structural behavior.

Many ground-mounted solar PV structures are relatively lightweight and flexible. Due to this unique trait, these structures are dynamically sensitive and are at a high risk of experiencing aerodynamic instabilities that have been known to result in structural failures. The MOP will provide guidance on dynamic sensitivity, including evaluating aerodynamic instabilities, wake buffeting, and vortex shedding. There will also be guidance on aerodynamic instability for low tilt, stiffness-driven instability (static or cyclical torsional divergence and torsional galloping); high tilt, dampingdriven instability (flutter, vortex lock-in, torsional flutter, stall flutter); wind tunnel testing for instability (section model testing and aero-elastic model testing); and numerical assessment by computational fluid dynamic (CFD) simulation.

Ballasted roof-mounted PV systems that have undergone wind tunnel testing to establish loading requirements are sensitive to vertical lift, which can negatively affect the applicability of the wind tunnel test results. As a result, the current provisions for the Effective Wind Area (as defined in ASCE 7) on solar arrays may be unconservative due to this effect. The MOP will provide guidance on testing rooftop array structures intended to reduce the risk of excessive lift of the array, leading to a more accurate determination of the Effective Wind Area.

Frost Jacking

While the weather becomes increasingly warmer in many areas, the ground still freezes in many parts of the U.S. during winter. Since ground-mounted solar PV structures are relatively lightweight with relatively deep piles, they are susceptible to frost jacking. Frost jacking is an upward, permanent heave that takes place during freeze and thaw cycles and can damage solar PV arrays due to non-uniform differential displacement.

Frost jacking occurs when the three “Ws” happen at the same time:

  • Winter (soil temperatures below 32° F),
  • Water (in the soil), and
  • Wicking (soil types that pull up in-situ moisture from below its elevation, typically clays and silts).

If one of these “Ws” is not present, frost jacking will not occur. Once the potential for frost jacking has been evaluated, it is sometimes possible to reduce the risk of frost jacking forces through mitigation measures. However, one of the challenges with mitigating frost jacking is determining the strength of the bond that develops between a pile and frozen soil (i.e., adfreeze bond stress) to use in design. The MOP will provide an overview of the mechanics of frost jacking, including the driving forces behind adfreeze bond, and provide engineering solutions to mitigate frost jacking by addressing the primary mechanisms of action and reduce the risk of a pile experiencing uplift.

Corrosion

Utility-scale solar PV projects may have hundreds of thousands of steel piles driven into the earth. While most structural engineers understand the general importance of corrosion above and below grade, few have subject matter expertise specifically in corrosion. The MOP will provide an overview of corrosion mechanisms, testing, and rates. The MOP will also provide guidance in determining service life projections and implications of safety factors with respect to data uncertainties.

Even on larger projects where a specialized “corrosion engineer” subconsultant may be retained, it is prudent that structural engineers understand the subconsultant’s findings and recommendations as they relate to the structural design of the pile foundations. The MOP will provide information on relevant corrosion protection techniques, such as corrosion rate calculation methods incorporating the Romanoff similitude procedures, protective and sacrificial coatings, cathodic protection, modification of the environment, and proper testing, design, and material selection.

Rooftop Solar

Rooftop loads from the solar racking occur as concentrated loads, linear loads, and uniform loads. Considering only aggregate uniform load effects when performing a structural capacity evaluation of the existing roof may overlook roof framing and decking capacity limitations. The MOP will provide design guidance on the importance of considering concentrated, linear, and increased uniform load effects from PV racking when performing capacity checks of the supporting decking and roof framing elements.

Rooftop solar design and construction have outpaced the development of building Code requirements for this work. Various solar industry guidelines and standards, such as the Structural Engineers Association of California (SEAOC) rooftop solar guidelines, have been written to bridge the gaps while the Code catches up. Also, many local jurisdictions have enacted ordinances specific to rooftop solar to serve as local design requirements. As a result, structural design requirements for rooftop solar evaluation are scattered and not well consolidated. The MOP will consolidate the pertinent Code provisions, industry guidelines, standards, and ordinances for reference by rooftop solar design engineers and others.

Determining existing building roof dead loads and structural capacity can be critically important and should be based heavily on confirmed facts and lightly on assumptions. The MOP will provide best-practice methods for determining existing roof load and capacity information for rooftop solar construction. These methods will cover the investigation of existing/as-built drawings, site investigation, destructive and non-destructive testing, and other tips and tricks.

The MOP will also provide guidance in determining the capacity of the often overlooked roof deck on existing buildings and the mechanical attachments to the roof deck. In addition, the MOP will discuss ICC Evaluation Service report AC467, Acceptance Criteria for Proprietary Attachment Systems of Photovoltaic (PV) Arrays to Roof Assemblies, in which the allowable capacity of the attachment system is based on a safety factor of five. Also discussed will be the importance of reconciling the modeling of roof construction in the test configuration versus the subject roof’s as-built field condition.

Conclusion

Over the past decade, there has been rapid growth in the solar PV industry. As often happens in industries that require unique nonbuilding structures to support the main equipment, the Code is not able to adequately address the solar PV industry’s specialized design needs. Following the tradition that has been set in other industries like fossil fuel power generation, substation design, and wind power, the newly formed ASCE/SEI Solar PV Structures Committee is developing a Manual of Practice that will provide design guidance to the solar PV industry and allow industry stakeholders to design and develop consistent and reliable structures. Check the ASCE website (https://www.asce.org) for Manual of Practice development updates.

References

Gartner, Steve. (2020), “Wind Activities on Solar PV Structures: A New ASCE Committee Has Blown into Town,” The Wind Engineer, November 2020, Newsletter of American Association for Wind Engineering, November 2020, pp. 14-16.

Manning, Jon, and Gartner, Steve. (2021), “New ASCE committee will focus on advancing the reliability of solar PV structures,” PV Magazine USA, June 25, 2021.

About the author  ⁄ Nestor A. Agbayani, P.E., S.E., SECB, M. ASCE

Nestor A. Agbayani, P.E., S.E., SECB, M. ASCE, is a Principal Engineer at Agbayani Structural Engineering. Nestor may be reached at nagbayani@sbcglobal.net.

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