At what separation distance are solar panel uplift (or similar roof c&c) and roof uplift additiv 1

[Skaard]

Hi all,

As per the title, I am struggling to find a clear answer on this one. I have found several sources that indicate how to determine the load that the solar panels are subject to, but I cannot find anywhere a clear answer as to what the existing roof structure is subject to, particularly addressing whether or not the roof structure is subject to additional uplift load due to the presence of the panels creating a pressure differential.

If you fastened something to a roof that was 10m above it (I know this is ridiculously far but it illustrates the point), then there would be two separate and independent instances of uplift load that you would need to add together. If a solar panel was directly fastened to the roof cladding with no gap, then I would confidently consider them as a single surface/skin and calculate uplift as per a standard roof scenario. I suspect there is a transition zone between the two where the effect of the panels changes relative to the separation distance, or something similar.

Does anyone have any advice, and even better any reference materials that discuss this?

The only documentation I have on this is the linked study which to my knowledge is what AS1170.2 AppD is based on. I can infer from this document that the forces from that appendix account for both the roof and the panel as that is how they were tested in the wind tunnel, but that is an inference, I don't know exactly how/where their load cells were located, and would therefore still love to hear others opinions and hopefully find something that is more explicit on this.

In case it is relevant I am in AUS, but understand this forum is NA based and am more than happy to look at NA documents or otherwise and make my own determination.

 

[canwesteng]

I would say at the point when they are additive is when you are also no longer treating the solar panel as components and cladding, but as a it's own structure. Then the uplift on the solar panel should be less than calculated for c&c, because there are pressures on both sides of it. Exactly where that happens might be hard to determine, but you could look at shielding provisions and see if that gives you rational basis to start with.

 

[phamENG]

Skaard - based in the US here so I'm not familiar with the code you referenced. I'd say that most practical applications are not going to have sufficient separation to worry about. If you want to figure out anyway, you'll need to look at some air foil aerodynamics. This is outside of my knowledge as a structural engineer and going back to my flight training, but try reading up on ground effect as it relates to aircraft. When a plane is close to the ground, the pressure differential across the wing is increased due to the wing's proximity to the ground. It's what gives a plane the last little float that a pilot can use to touch down softly. The separation point where ground effect becomes insignificant would be analogous to the separation distance you're looking for. From my experience flying a little high wing four seater, it was about 15 feet off the ground (I think - haven't flown in the better part of 10 years). Maybe somebody from the aeronautical side of the house could pitch in.