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Structural Design Requirements for Solar Installations

Learn more about structural design requirements for solar installations in the City of Portland.

Dead and Live Loads

The solar installations and their supporting structures shall be designed in accordance with section 1607.13.5 of the OSSC. The self-weight of the photovoltaic panels and modules and ballast (if any) shall be treated as dead load. Roof Live load shall be determined per section 1607 of the OSSC. You may submit questions about this page online.

Snow Loads

Snow loads shall be based on section 1608 of the OSSC. Snow load on the photovoltaic panels shall not be taken less than be 20 psf x Importance factor, Is. in accordance with section 1608.2.3 and 1608.2.4 of the OSSC. Snow drift created by the photovoltaic installation shall be considered

In accordance with section 1607.13.5.2 of the OSSC the structure of the building supporting the photovoltaic panels or modules shall be designed to accommodate the full solar photovoltaic panels or modules and ballast dead loads, including concentrated loads from the support frames in combination with the loads due to live loads, snow, wind and seismic. Load combinations shall be per section 1605 of the OSSC.

The structure of the roof that supports the solar photovoltaic panels shall be designed to accommodate the full solar photovoltaic panels and ballast dead load, including concentrated loads from the racking support or standoffs due to all applicable dead, snow and wind loads

When designing the supporting roof framing for loads including roof live loads, snow loads or wind loads, the supporting structure must be designed for the following two conditions per section 1607.13.5.1 of the OSSC (1) all applicable dead, snow and live loads assuming the photovoltaic panel system is not present and (2) the applicable  dead, live, snow and wind loads including any concentrated loads from the photovoltaic panel system from the racking support or standoffs. Roof live loads applied to the area covered by photovoltaic panels where the clear space between the panels and the roof surface is 24inches or less need not be considered in this case.

Wind Loads

The design wind loads for solar photovoltaic arrays shall be based on the latest edition of the OSSC and ASCE 7. The following methods may be used to determine the wind loads on solar arrays:

  1. Wind loads determined in accordance with sections 29.4.3 and 29.4.4, or 30.13 of ASCE 7;
  2. Wind load determined in accordance with Wind Tunnel tests per section 1609.1.1, ASCE 49 and Chapter 31 of the ASCE-7;
  3. For low profile solar photovoltaic array on flat roofs, using the Structural Engineers Association of California (SEAOC) Report PV2-2017, Wind Design for Solar Arrays.

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:

  1. 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;
  2. 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:

  1. Scope of peer review with limitations defined;
  2. The status of wind tunnel test at the time of review;
  3. Conformance of wind tunnel study with requirements of ASCE 49;
  4. Conclusion of the reviewer identifying areas that need further review, investigation and/or clarification
  5. 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. 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.”

Seismic Loads

Rooftop solar photovoltaic panels arrays and their attachments shall be designed for forces and displacements determined in accordance with section 13.3 of ASCE 7    

The solar panels 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 for ballasted systems provided that they comply with the requirements of section 13.6.12 of ASCE 7 summarized below  following:

  1. The installation is on Risk Category I, II and II structures six stories or less in height;
  2. The maximum roof slope supporting the array is less than or equal 1 in 20 ( this may be reduced to 1 in 12 provided an independent peer review is conducted);
  3. The height above the roof surface of the center of mass of any panel is   is less than the smaller of 3 feet and half the least spacing in plan dimension of the panel supports;
  4. The arrays shall be designed to accommodate the seismic displacement of the array relative to the roof surface as required by ASCE 7 section 13.6.12;
  5. Each panel shall be interconnected to resist a horizontal force of 0.2 Sds Wpi where Wpi is the weight of each component; otherwise each portion of the array shall have the minimum separations to accommodate seismic displacements;
  6. Panel framing and supports are designed for a seismic force path from the center of mass of each component to location of frictional resistance equal to the lesser of Fp from section 13.3.1 of ASCE 7, and 0.6Wp;
  7. Panels are located on the roof such that all parts of the panel are located a minimum of 2.0 X calculated seismic displacement distance but not less than 4ft away from the roof edge or offset OR the panels are placed on roof that bounded by parapets 12” or higher designed to resist impact loads from the from the panel. All electrical cables are designed to accommodate the calculated seismic displacement;
  8. 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.

Alternatively, the City of Portland accepts designs and installations that are shown to comply with the procedures contained in   Structural Engineers Association of California publication PV-1, “Structural Seismic Requirements and Commentary For Rooftop Solar Photovoltaic Arrays.”  

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