Increasingly, contractors and solar installers are being asked to install solar panels on existing metal buildings. Building owners are aware of the cost benefits of a switch to solar power, and pre-engineered metal buildings represent a substantial portion of new U.S. commercial and industrial construction. Additionally, metal buildings tend to have a low roof pitch and significant surface area, which makes them a good place to locate solar panels.

Retrofitting solar onto a metal building; however, often entails structural challenges. The problems arise for the same reasons that metal buildings are attractive in the first place. In addition to being light, strong and easy to assemble, they tend to be efficiently engineered, i.e., designed to the absolute limit the building code will allow.

Common Problems
Because metal buildings are designed for such tight weight tolerances, it is necessary to be cautious about adding weight to the structure. When reviewing an existing structure for the installation of solar panels, it is necessary to analyze the effects of seismic, wind, snow, live and dead loads, and the combination of these loads, as required by the building code. Seismic and wind forces create the lateral loads on a building, while snow, live and dead loads are the gravity loads.

Gravity loads. The International Building Code (IBC) allows for the removal of the live load on a building where solar panels are installed; thus, in areas where the snow load is less than the live load, it is easy to justify the added weight of solar panels.

When a building is in an area where design snow load exceeds the design live load; however, justifying the added weight of solar panels becomes more difficult. The International Existing Building Code (IEBC) allows for a 5% increase in gravity loads to existing buildings. Because metal building roofs are constructed with lightweight materials, the use of this 5% increase does not typically provide enough capacity to justify installing solar panels.

Solar panels that are not flush with the roof (tilted panels) may cause snow drifting, which increases gravity loading, requiring additional analysis to determine if the roof can support the solar panels.

Lateral loads. The IEBC allows for a 10% increase in lateral loads to an existing building. When adding solar panels to a building, it must be determined if the lateral-force resisting system of the building is adequate. In seismic areas, the added weight of the panels increases the seismic loads on the building.

Flush-mounted solar panels cause a negligible increase in the wind loads. However, tilted panels will increase wind loads and must be analyzed for lateral impact on the structure. ASCE 7 now includes requirements for wind uplift that are higher than loads typically used in the design of metal roofs, resulting in additional challenges of providing for proper panel attachment.

In summary, whenever solar panels are added to a metal building, it must be determined if the building can resist the additional lateral and gravity loads.

Finding Answers
Building owners typically turn to solar installers, contractors or the metal building manufacturer for assistance in retrofitting solar panels on metal buildings. However, conducting a detailed analysis of a metal building’s load-bearing capability generally lies outside the expertise of most solar installers. It isn’t the kind of problem they’re normally called upon to solve.

Metal building companies often do not want to deal with the analysis and possible retrofit required when adding additional load to an existing building. Their expertise is in the efficient initial design and construction of metal buildings. Dustin Cole, PE, SE of Chief Buildings, Grand Island, Neb., says, “We do not know the condition of the existing building. This starts with the amount and type of suspended loads that have been installed and how that aligns with the collateral load, if any, specified in the design criteria. The condition of the building also addresses missing parts and pieces, for instance sag angles or flange braces, and the overall condition due to rust or poor maintenance.”

First Steps
Before beginning a solar-to-steel retrofit, it is necessary to have accurate information about the existing structure. The most economical way to get that information is from the original design drawings. If the original design drawings are not available, an experienced contractor or metal building engineer must go to the site to gather the required information.

This information includes photographs of the interior to determine actual collateral loading (plumbing, electrical, HVAC, etc.), the general layout of the building and the exact sizes of all steel members (length/span, spacing, and member thickness/shape). Obviously, the basic solar installation drawings showing proposed panel locations are also required.

Analysis
Once all the necessary information is provided, a gravity and lateral analysis of the existing building is conducted. In some situations, the building may be able to support the solar panels with no changes. However, if the building cannot support the proposed solar array, then there are several options. Determining the appropriate option requires careful communication between the solar installer, contractor, engineer and the building owner.

Sometimes, reducing or relocating the solar panels on the roof allows the installation to proceed. Alternatively, it may be required to reinforce/retrofit (or sister) the secondary framing (purlins) or primary framing (moment frames or other structural members).

Structural engineers who have expertise in the design of metal buildings and are familiar with the challenges of retrofitting solar can help. They can effectively compile the appropriate information, analyze the building, design retrofits and/or provide reasonable options and provide guidance on the installation of solar on metal buildings.

By: Jacob S. Proctor, S.E.