Wind Design for Solar installations
The design wind loads for solar photovoltaic arrays are not covered by prescriptive methods contained in the ASCE 7 or the current provisions in the Oregon Structural Specialty (OSSC). In the absence of detailed guidance from ASCE 7 or the OSSC for wind loads, designers often use an hybrid approach of the ASCE 7 components and cladding tables for enclosed buildings and main force resisting system tables for open structures which could lead to un-conservative results particularly considering the size of the edge and corner roof zones.
For low profile solar photovoltaic array on flat roofs, the Structural Engineers Association of California (SEAOC) has produced two reports PV1-12 addressing the seismic structural design and Report PV2-2012 addressing the wind design. The City of Portland will accept designs based on the methodologies outlined in these two documents.
Solar Installations Designed Utilizing Wind Tunnel Tests and Peer Review Requirements
Wind loads on Roof-mounted solar arrays are permitted to be determined by using wind tunnel tests as generic loads applicable to a range of buildings. Wind tunnel tests shall satisfy ASCE 49, “Wind Tunnel Testing for Buildings and Other Structures”
Solar installations designed using this method will require an independent peer review. The peer review shall be independent and objective, technical review by knowledgeable reviewer(s) who shall be:
- Experienced in performing wind tunnel studies on buildings and similar systems, and in properly simulated atmospheric boundary layers.
- Familiar with the technical issues and regulations governing the wind tunnel procedures of ASCE 49 as it is applied to systems similar to solar photovoltaic arrays that use generalized wind tunnel data for design.
- Independent from the wind tunnel laboratory that performed the test and report and shall bear no conflict of interest.
- The peer reviewer shall be acceptable to the Bureau of Development Services.
The peer reviewer shall submit a report which shall include as a minimum, statements regarding the following:
- Scope of peer review with limitations defined
- The status of wind tunnel test at the time of review
- Conformance of wind tunnel study with requirements of ASCE 49
- Conclusion of the reviewer identifying areas that need further review, investigation and/or clarification
- Statement from the reviewer that in their opinion the results of the wind tunnel tests have correctly been applied to the specific situation/project and the final design conforms to the requirements of ASCE 7.
A source for peer reviewers is the American Association for Wind Engineering’s (AAWE) boundary layer wind tunnels list (http://www.aawe.org/info/wind_tunnels.php ). The following is a summary of the AAWE list with other known boundary layer wind tunnels added. This is not a comprehensive list, but is provided as an aid.
- Cermak Peterka Petersen (CPP), Inc.
- Colorado State University
- Concordia University, Montreal
- Force Technology
- I.F.I Institute
- Rowan, Williams, Davies & Irwin (RWDI)
- Texas Tech
- University of California Davis
- University of Iowa
- University of Maryland
- University of Minnesota
- University of Washington
- University of Western Ontario
Other sources of peer reviewers are the ASCE “Wind Loads on Solar Collectors subcommittee” or the voting members of the ASCE 7 “Subcommittee on Wind Loads”.
Requirements for Seismic Design
Rooftop solar photovoltaic panels arrays shall be bolted, welded, or otherwise positively fastened without consideration of frictional resistance produced by the effects of gravity, except that solar photovoltaic arrays without attachment to the roof structure are permitted, provided that they comply with the following:
- The maximum roof slope supporting the array is less than or equal to 5 degrees.
- The height above the roof surface of the center of mass of the solar array is less than the smaller of 3 feet and half the least plan dimension of the supporting base of the array.
- The arrays shall be designed to accommodate the seismic displacement of the array relative to the roof surface as required by ASCE 7.
- Each array shall be interconnected with the strength to slide as an integral unit; otherwise each portion of the array shall have the minimum separations to accommodate seismic displacements.
- The coefficient of friction between the array and the roof shall be determined based on testing considering weather conditions including the effects of ice or cold weather on friction