Wind Loads on Retractable Solar Panels

[AZengineer]

I am designing the foundation for an array of solar panels. There are four 15' x 18' panels attached to a continuous horizontal pipe support. Between each panel, there is a vertical column which connects the pipe support to the foundation. All four panels can rotate together about the horizontal support. Additionally, each panel can rotate perpendicular to the horizontal support - that is, the panel array can track bidirectionally.

The wind loads generated by the engineers of the superstructure use a fluid dynamics approach, and are much lower than the loads calculated by ASCE7-05 for solid signs and freestanding walls. The manufacturer of the panels states that the panels are designed for a reduced wind load, and that the panels automatically rotate into a horizontal position during a high wind event.

A couple of questions:

1. Are there any other engineers who feel comfortable designing a foundation for this retracted position? It seems to me that we should not rely on mechanical systems to reduce loads, and that the solar panels should be assumed to be in the worst case position, in which they seem to comply with the requirements for signs in ASCE7. Needless to say, difference in the foundation size for the two cases is enormous!

2. Does the IBC allow the use of fluid dynamic approaches? It seems to me that only the three methods in ASCE7 (simplified, analytical, wind tunnel) are recognized, along with the explicitly noted reference standards in the IBC (AF&PA WFCM, TIA/EIA-222, etc).

 

[bigmig]

Your first point is correct. Do not rely on a mechanical system to prevent a structural failure. In our climate, we have freeze/ thaw, so the problem we run into is that the panels catch a dusting of snow, which melts slightly, and then freezes. The panels would then be essentially frozen in place. Along comes a design wind load and the rest is played out in a court of law.

Solar panel "signage" fits pretty well into the parameters of the ASCE7. I have never used or seen fluid dynamics used on wind load calcs and would imagine that you would have a very difficult time explaining in a court of law why you ignored the ASCE7.

My experience working with solar panels and mounting systems is that they are typically undersdesigned per the ASCE7. If you are responsible for the foundation, tell the manufacturer to give you the load and design to that, and then cover yourself with a "by the way...." note that says the loads should be higher and you should not count on mechanical systems to prevent design loads, which I believe is also in the code.

 

[msquared48]

I agree with bigmig. Design to the worst condition to save the structure. Small insurance to pay up front for a long life of service.

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[beton1]

Agree also with bigmig. There is a big push to go green and
it is emerging technology. Until the panel manufacturers do full scale testing, there is no other alternative than to design with structural codes.

 

[paddingtongreen]

The answers seem right, and yet, we do design some building frames on the basis that the siding would strip off in an extreme wind.

Michael.
Timing has a lot to do with the outcome of a rain dance.