Solar Module Mechanical Load

[andrew.tkelly]

This is a bit of a structural/civil engineering question, but I figured I'd ask it here since it ultimately pertains to an electrical system.

Within a solar module's installation manual, it generally shows you a few ways to mount the module. Each mounting method is accompanied by a different mechanical load rating. I find that most traditional rail-mounting methods give the module a rating of 5400 Pa. However, if you move the rails to the extremities of the module, it reduces the mechanical load it can hold to about 1800 Pa.

My question is this: How can I know if the wind forces in my area are enough to surpass this 1800 Pa rating? I'm in the SF Bay Area, so I know that ASCE 7-10 standards require PV systems to withstand a 3-second gust of 110 mph (92 mph in the ASCE 7-16). I'm basically scratching my head, wondering, "Since I'm using this mounting method, will I need some kind of engineering stamp now?"

 

[Carultch]

This is a bit of a structural/civil engineering question, but I figured I'd ask it here since it ultimately pertains to an electrical system.

Within a solar module's installation manual, it generally shows you a few ways to mount the module. Each mounting method is accompanied by a different mechanical load rating. I find that most traditional rail-mounting methods give the module a rating of 5400 Pa. However, if you move the rails to the extremities of the module, it reduces the mechanical load it can hold to about 1800 Pa.

My question is this: How can I know if the wind forces in my area are enough to surpass this 1800 Pa rating? I'm in the SF Bay Area, so I know that ASCE 7-10 standards require PV systems to withstand a 3-second gust of 110 mph (92 mph in the ASCE 7-16). I'm basically scratching my head, wondering, "Since I'm using this mounting method, will I need some kind of engineering stamp now?"

The short answer is 5400 Pa is likely "safe", and 1800 Pa is "iffy". Wind loads in the calculation exceeding 5400 Pa (112 psf) are rare, but wind loads exceeding 1800 Pa (37.6 psf) may happen, and will depend on the results of the structural engineer's calculation. So if you can, plan on mounting it close to the "quarter points" or using the frame holes close to the quarter points, so the frame can both span and cantilever, instead of spanning its entire length.

The long answer is that you'd need to get your structural engineer involved. The applicable version of ASCE has a formula for how to convert the 110 mph into a basic wind pressure for the site, occupancy category, height, and wind exposure category. You then have to apply wind coefficients that account for the way wind distributes across a building, lands on the panels in question, and ultimately becomes a net load on the panel in both directions. There are prescribed load combinations that specify how wind loads "add up" with the self-weight (i.e. dead load) and other environmental loads such as snow and seismic.