By Steve Bauer, Special To Solar Power World
It is no secret that building codes have not addressed photovoltaic systems until recently. This is particularly true for the structural racking components of a solar array. Many ask, when applying ASCE 7 methodology, what is the best way to generate accurate wind and seismic loads? Is wind-tunnel testing worth the investment? If so, how do you maximize dollars and ensure it’s being done correctly? These are all good questions the solar community is actively addressing.
In 2012, the Structural Engineers Association of California Solar Photovoltaic Systems committee generated two documents to provide guidance for the application of solar PV systems (links to purchase these documents are at the bottom of this article). This committee, chaired by Ron LaPlante of the California Division of the State Architect and Vice-Chaired by Joe Maffei of Maffei Structural Engineering, set out to define a clear methodology for designing racking systems.
The seismic document, Structural Seismic Requirements for Rooftop Solar Photovoltaic Arrays (SEAOC PV1-2012), outlines multiple approaches for designing both attached and unattached arrays. Attached arrays are designed to resist the lateral seismic force (Fp) specified in ASCE 7-10 Chapter 13. It is permissible to include roof friction to help resist these forces in some circumstances.
This document also outlines a design basis for unattached systems. Since the primary risk associated with unattached systems is movement (and falling) of an array, a prescriptive method can predict and mitigate system movement through appropriate edge distances and system spacing. Unattached systems rely on friction to resist movement. Therefore, specific guidelines and testing requirements are provided to determine appropriate coefficients of friction. Guidelines are also provided for shake-table testing or nonlinear time history analysis for systems that do not qualify for evaluation using the prescriptive method.
The wind document, Wind Design for Low-Profile Solar Photovoltaic Arrays on Flat Roofs (SEAOC PV2-2012), sets forth guidelines for designing rooftop systems with adjustment factors for edge distance, normalized wind area and shielding. The methodology essentially assumes that an array of many rows is being evaluated to take advantage of the system shielding effects. Wind loads are then factored up around the perimeter and at system breaks to account for higher localized wind forces.
Dr. David Banks from Cermak Peterka Petersen (CPP) and Dr. Gregory Kopp from the University of Western Ontario both contributed data from dozens of wind-tunnel studies to ensure these recommendations were formulated from the most current body of knowledge of wind behavior around various types of buildings. The wind document also provides guidelines for the creation of private wind tunnel studies for individual systems. As it is not reasonable to perform a wind tunnel study for every project, a common set of guidelines was needed to set minimum safety standards within the industry. The elements of the SEAOC guidelines have been simplified into code language and submitted to ASCE 7, through a subcommittee chaired by Dr. Kopp. The proposal is working its way through the approval process and is expected to be adopted into the 2016 edition of ASCE 7.
To help understand and promote awareness for these new codes, we are soliciting your feedback:
• Are the new guidelines clear and attainable?
• Have you had success permitting with the new guidelines?
• Do you believe there is a perceived cost impact of the new requirements?
Please submit your input to info@unirac.com. This information will be tabulated and reported in a future edition of Solar Power World. It will also be shared with the SEAOC PV Committee for consideration in future PV code development activities.
Steve Bauer is the technical director for Unirac.
Download Links:
Structural Seismic Requirements for Rooftop Solar Photovoltaic Arrays
Wind Design for Low-Profile Solar Photovoltaic Arrays on Flat Roofs
MattT says
-The guidelines are attainable.
-If your racking system adheres to the guidelines then permitting will not be an issue
-Value, safety and quality of racking trumps cost when making a long term capital investment such as rooftop PV.
So far, the PV industry has engaged in a race to the bottom with price influencing every decision rather than quality, system performance and reliability being the main influencers in making sound investment decisions. We are well into a period of seeing system failures and underperformance (due to sub standard racking), building fires that won’t be fought by fire departments due to the risk and uncertainty of sub standard PV racking and systems being blown apart in high winds or hurricanes that they were not engineered to withstand.
When will the Insurance and Finance industries step up and demand that smart and safe standards drive the PV industry? Why does this have to come from the Fire department?? I respect the FD’s efforts to date for taking on this important issue, but come on, solar isn’t new anymore.
The lack of implementation and delays of 2703 will continue to allow unsafe, unreliable and cheap construction to prevail.
Dan says
I’m curious as to the point of this “special”. Clearly the guidelines are being developed to help ensure that systems are safe and have long term reliability. Currently there is no racking system standard as 2703 seems to be in perpetual development, and even that does not address structural. Boundary layer wind tunnel testing, stiffness testing, coefficient of friction testing, and shake table (for seismic) testing should all be requirements for ALL ballasted racking systems to ensure that they will not be the system point-of-failure in the future.
The guidelines as proposed are attainable, although some racking systems might not like the results of their own testing.
Permitting should not be an issue going forward when this is adopted into the 2016 edition of ASCE 7 if the racking system being utilized for a project has tested and provides ballasting information in accordance with the test results.
Yes there is cost impact of the new requirements – that being the cost that racking manufacturers spend on the testing and engineer their product to conform with the requirements. That cost might be passed along in additional cost of the racking product, but if that is the necessary cost to ensure a system is safe and will last 25 years then that cost should be negligible to the project financier. That cost might be an additional 5% of total system cost which means projects are still financeable.
As the industry becomes more mature racking will become the next subject of scrutiny, as we’ve seen poorly designed and engineered racking cause system fires, roof collapses, and panels to blow off of roofs. Anything that can be done to minimize failures caused by racking should be of utmost importance to PPA companies, insurance companies, and end users.