Updates on ASCE 7 Standard for Solar PV SystemsFind out how the ASCE 7 standard affects wind load, seismic load, and tornado load considerations for solar photovoltaic (PV) systems.
At SEAC’s February general meeting, Solar Energy Industries Association Senior Director of Codes and Standards Joe Cain presented an update on structural load requirements affecting solar photovoltaic (PV) systems in the ASCE 7 standard.
Cain provided an overview of the 2016 edition, ASCE 7-16, a referenced standard in the 2018 and 2021 editions of the International Building Code (IBC) and the International Residential Code (IRC). He highlighted significant changes in the 2022 edition, ASCE 7-22. In addition, he drew attention to notable code development issues affecting various configurations of PV systems, including rooftop and ground-mount systems, and shared several resources for more information.
The 2024 edition of the IBC and IRC, due to be published later this year, will include ASCE 7-22 as a referenced standard. Cain, a SEAC Assembly member, also serves as a voting member of the ASCE 7-22 main committee and a participant on the wind load, seismic load, and tornado load subcommittees for the American Society of Civil Engineers (ASCE).
ASCE 7-16 for PV Systems
The 2016 edition of ASCE 7 has been in effect for about three years. It has three more years remaining before the standard is superseded by ASCE 7-22.
ASCE 7-16 introduced substantial increases in the component and cladding pressure coefficients used to calculate wind pressure in various wind zones. This change had a big impact on rooftop systems.
ASCE 7-16 defines the weight of solar panels, their support system, and ballast as dead load. Load combinations must be used in structural calculations. (Sections 3.1.5 and 4.17.2)
ASCE 7-16 requires modeling for live load offsets under various conditions. If any portion of system rises over 24 inches above the roof surface, you need to model live load in that portion of the system. If an entire system is no more than 24 inches above a low-slope roof, you don’t model live load at all. However, for portions of the roof not covered by PV system, uniform live load must be included. Calculate load cases with and without PV, including 300-lb concentrated load for all roof surfaces subject to maintenance workers. (Section 4.17)
Ballasted, unattached PV systems on low-slope roofs have to meet seven conditions to comply with seismic load requirements in Section 13.6.12. For low-profile systems, the height of the center of mass of any panel above the roof surface must be less than half the least spacing in plan of the panel supports, but in no case greater than 3 feet.
ASCE 7-16 provides an exception for ballasted systems in some instances where the maximum roof slope is no greater than 1 in 20. The IBC, on the other hand, provides a basic maximum roof slope threshold of 1 in 12. Where model code disagrees with a referenced standard, the model code prevails.
Section 13.6.12 also establishes maximum expected displacement for PV systems that can be calculated using a formula in the standard, or shake table testing or non-linear response history analysis. Another option added in the 2018 IBC development process allows “approved analysis” to the list of allowed methods, meaning any method that is “acceptable to the Building Official.”
To comply with seismic load requirements, mounting systems have to be interconnected and able to distribute lateral forces without deforming as they might slide. These criteria will discourage odd array shapes that are unable to distribute seismic forces.
Parapets and curbs near ballasted unattached arrays might need to be designed for impact forces. Signage or markings must note the boundary where sliding may occur.
For unattached PV systems, wiring methods must be flexible enough to allow for system displacement. National Electrical Code (NEC) Section 690.31(F) has a corresponding Informational Note on flexible wiring.
Changes in ASCE 7-22
The 2022 edition of ASCE 7 includes an update to Section 13.6.12 that says, “The solar panels shall not be considered as part of the load path that resists the interconnection force unless the panels have been evaluated or tested for such loading.” This new subsection has the potential to eliminate from the marketplace some ballasted systems where PV panels span to individual, isolated mountings.
ASCE 7-22 has new qualifying criteria and nomenclature for wind design of fixed-tilt ground mount systems but not systems with single-axis trackers.
ASCE 7-16 Section 31.6.1 on wind tunnel testing is replaced by ASCE 7-22 Section 31.5.2. Wind Tunnel Test Criteria has been relocated to ASCE 49.
ASCE 7-22 introduces a new chapter, Chapter 32, on tornado loads. Historically, in general, hurricanes are the cause of more property loss, and tornadoes are the cause of more deaths.
Chapter 32 applies only to buildings and structures in Risk Categories III and IV. The greater the Effective Plan Area, the greater the likelihood of a tornado strike. Should any ground-mounted PV system be deemed Risk Category III or IV, the language in Section 126.96.36.199 clarifies that it is not the area of an entire solar facility that is used as the Effective Plan Area.
Code Development Issues
Cain identified several code development issues for SEAC to monitor.
Strong guidance exists for low-profile systems on low-slope roofs. However, Cain is keeping an eye on the edge factor used in wind design. (ASCE 7-16 Section 29.4.3) He said the criteria associated with edge factor accidentally got into another method. In addition, more guidance is needed for averaging area based on rack stiffness and load sharing, Cain said.
More work is also needed to accommodate large-format modules. At the time the wind design test method was developed, everyone thought 6.7 feet was generous enough to envelop all PV modules in the marketplace. Now, that’s no longer true, Cain said. How can ASCE 7-16 29.4.4 be modified to address large-format modules?
More study is also needed for Elevated PV Support Structures. A wind pressure design method is needed. The flexibility of PV panels and the structures themselves must be better understood.
Research by the Structural Engineers Association of California (SEAOC) formed the basis for key provisions of ASCE 7-16. See the following white papers for research on seismic design, wind design, and gravity design. They are essential companions to ASCE 7-16.
- Structural Seismic Requirements and Commentary for Rooftop Solar Photovoltaic Arrays
- Wind Design for Solar Arrays
- Gravity Design Loads for Rooftop Solar Photovoltaic Arrays
For wind tunnel test results that supported code development for PV systems parallel to the roof, see the Journal of Wind Engineering and Industrial Aerodynamics article, published in 2015, “Wind loads on photovoltaic arrays mounted parallel to sloped roofs on low-rise buildings.”
The SEAOC Wind Design Manual Based on ASCE 7-16 is now published.
More study is needed for “flush mounts” parallel to the roof. For reference, see “Wind Loads on Rooftop Photovoltaic Panel Systems Installed Parallel to Roof Planes,” published at the 2016 SEAOC Convention Proceedings.
Guidance is available for ground mounts. See “Wind Loads on Utility Scale Solar PV Power Plants.”ASCE’s Solar PV Structures Committee is developing a first draft of a comprehensive manual of practice.