Wind Loads on Solar Panels Generally, and Open-Roof Structures Specifically 1

[ArchGuy]

First, the general questions:
I'm calculating solar panels over roofs in Phoenix, Arizona using ASCE 7-16 provisions, and am relatively new to using ASCE 7-16 for solar arrays. I'm calculating the LRFD average wind pressure normal to the panels to be around 35 psf to 40 psf. I'm determining the Normalized Wind Area using the area of a single solar panel (which results in a higher GCrn coefficient). Are my numbers within the ballpark, and does that approach sound correct?

Second, the specific questions:
ASCE 7-16 gives specific provisions for solar panels mounted on roofs over closed structures. The ASCE 7-16 commentary specifically excludes open or lattice roof structures supporting solar panels from these provisions.

I'm designing an open, lattice-type structure supporting solar panels, and my gut instinct is that it is conservative to use the ASCE 7-16 closed-structure provisions to calculate wind loads on these solar panels for this open structure. My questions related to this specific topic are:

1. Is it that truly conservative to use the ASCE 7-16 closed-structure wind provisions for all scenarios where an open, lattice-type structure supports solar panels?
2. Are there any thoughts regarding an appropriate reduction (or increase) in wind loads due to the panels being on open, lattice-type structures versus closed structures?
3. Any literature or studies you could point me to regarding calculating wind loads on solar panels supported by open, lattice-type structures?

Thank you

 

[r13]

A search on "solar panel" on the forum yields "572 records matched your query of solar panel. You can refine the search by adding relevant words.

 

[Lomarandil]

I haven't read ASCE 7-16 yet, but think it's unlikely you are intended to use the enclosed structure provisions as you suggest.

You probably want to design as an open structure, neglecting the aerodynamic effects of any gaps between panels (although you only need account for the actual surface area present).

----
just call me Lo.

 

[moon161]

Back of envelope approximation, without safety factors applied 40psf is the SLS stagnation pressure of air at about 125 MPH, or the max lift of a flat plate at about 140 MPH. (F1-F2 tornado, F3-F4 hurricane)

If those loads include say a 2.0 safety factor, then you're looking at about 90 MPH stagnation pressure or about 100 MPH for max lift. (F1 tornado, F1-F2 Hurricane)

 

[Aesur]

@ArchGuy,

In my experience, open structures are designed as open structures, for C&C use Figure 30.7-1 through 30.7-3, for MWFRS use Figure 37.3-4 through 27.3-6. Now if you have an open structure on a parking garage or other roof, I would consider using Kzt for a cliff face and applying that to the open structure. I had this discussion with a wind tunnel testing facility many years ago when they were working on the SEAOC wind provisions (where the ASCE 7-16 wind provisions come from) and while it hasn't been tested, it seemed like an appropriate approach as the structure could act similar to a topographical feature.

Keep in mind that you will have different wind loading for each component, ie, a panel has less effective area, so it's attachments are designed for higher C&C psf loading, the rails or purlins will have a larger effective area and therefore lower psf loading and the main frames will again see a reduction in loading based on effective area.

Please also note the live load requirements on solar structures per ASCE7, these are newer to the 7-16 code and many engineers miss these as they can be a headache to find.

Please also note that previous projects I have completed in Phoenix required me to use Exposure C (per local amendments/building officials), unless the EOR could provide calculations proving exposure B is acceptable, which is tough to do many times due to "open patches", especially in Arizona.

I am not aware of any reference documents for open structure solar design, however if your structure is a carport type structure as I suspect, I would recommend looking up wind loading on carports as a reference. If you can provide a picture/sketch of what you have we could provide more help.

 

[ArchGuy]

Thank you for everyone's comments - I think designing as an open structure with a Kzt value for a cliff face in Exposure C is appropriate.
Per Aesur's request, the array is over a courtyard created by two existing L-shaped buildings. The array is supported on arches spanning over the courtyard, which are supported on the existing parapet walls. The solar panels will be about 40 feet above ground, and the existing wall roof height 18 feet above ground, with a 5-foot parapet. Please see drawings below of the array:

Any additional insights after reviewing these drawings?

 

[r13]

Impressive. No comment, but noted a minor discrepancy in the 3D image.

 

[ArchGuy]

Thank you r13. I think the rendered image with the solar panels is a little difficult to see the underlying structure clearly, but there is no discrepancy in the array. Both the rendered image and the isometric image were generated from the same Revit model, and the brace is consistent (assuming that brace is what you were pointing out as potentially inconsistent).