There are regions of the world that have both high irradiance for solar power and high risk for damaging earthquakes. California, perhaps, best exemplifies this combination of features. However, places such as Japan, which is investing heavily in solar power, are also prone to earthquakes.
Throughout the U.S., building codes are generally based on the International Building Code (IBC), published by the International Code Council. Recent versions of the building code have said that in a moderate- to high-seismic area, all of a building's equipment – unless it is really light or tiny – must be positively attached to the building.
‘Positively attached’ means that it must be physically secured with fasteners of some sort that do not rely on friction or weight to keep the equipment in place.
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Rob Ward, head of structural engineering at SunLink, says the code was not written with photovoltaic systems in mind, per se. However, PV arrays are problematic because of how much space they occupy on a roof.
‘For residential solar, for instance, on a sloped roof, you are going to attach the array anyway,’ Ward says. ‘It's not an issue. You have frames that have to support the PV modules. You just have to make sure that in a seismic zone, you've got a capacity for swaying in any direction – it's not too difficult.’
Where it becomes interesting is when you have PV on commercial flat or low-slope roofs. The perfect examples of such roofs – big box retail buildings, warehouses and the like – have exactly the sort of real estate that developers are looking to put solar on. In non-seismic regions, the unattached, ballasted roof-mounted system has evolved as the ideal solution for covering such roofs with solar panels.
At the same time, the building code is saying that low-slope and flat-roof arrays in seismic areas have to be positively secured – which brings us back to coastal California.
Attaching commercial arrays to such desirable roofs from an irradiance standpoint adds significant complexity and costs for the installer. According to Ward, many commercial arrays on rooftops would require a large number of roof penetrations to secure the entire structure.
‘You can do it, but it's a lot more roofing work,’ he says. ‘Those penetrations have to be done properly because they are a potential source of water intrusion. They have to bring in another trade to put the penetrations in. The systems have to align with framing in order to get the attachments in the right place.’
In 2010, aware that the seismic codes could act as a drag on solar growth in such a promising market, Ward and some colleagues at other racking and mounting manufacturers began to question whether positive attachment was really necessary.
‘The code says attach in a seismic zone, but what if you don't?’ he says. ‘What's going to happen? What's going to go wrong?’
Ward and his team began looking for the answers to these questions in computer models. California has a lot of seismic sensors in place, many of which are on rooftops. The sensors on the ground and on buildings contribute to an earthquake history database.
‘We went and got some earthquake records and made models of PV mounting systems that attach to the roof with friction and can slide,’ Ward says. ‘We figured out that you can characterize the displacement and quantify how far the array is going to move on a particular kind of roof under different levels of earthquake shaking.’
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Subsequently, Ward and his colleagues had the opportunity to test out their models in the physical world. With a grant from the California Public Utilities Commission, the team was able to put some of its full-scale models on the shake table at the University of California at Berkley. The testing proved that the computer models were correct.
With this information, Ward and his colleagues approached the Structural Engineers Association of California (SEAOC), which produced some recommendations on how to design unattached mounting systems for earthquake performance. SEAOC also produced recommendations on how to deal with wind loads for such systems.
‘Those recommendations have been rattling through the code-writing process,’ Ward says. ‘The 2015 version of the IBC has some language that says this approach is valid for PV for low-slope roofs in seismic areas.’
The question is, how are these approaches to be implemented in practice? Ward says the specific provisions the IBC alludes to are in development and should appear in 2016 or the year after. It might be a few more years before those provisions are actually ‘baked into the code.’ In the meantime, there is something of a gray zone. The IBC says unattached can be fine for earthquake zones, but the particulars are still up in the air.
‘The building code also says that if you can present evidence that your approach meets the intent of the code – you're getting it done but not actually following the letter of the code – your design is permissible,’ Ward says.
For this reason, authorities having jurisdiction have a lot of control over how stringently attachment provisions are to be applied in seismic areas. Like many states, some local jurisdictions have clear guidelines about what is permissible with regard to roof mounts in seismic areas – others less so.
Though this brings a lot of uncertainty into the market, it also represents an opportunity for those developers and engineers with enough experience and skill to make their pitch to the authorities.
