Wind Uplift-Solar Panels 1

[JoeBaseplate]

Please see the attached rough sketch. Existing prefab building, built in mid 80s, new solar panels to be installed @ one corner.

My question is how would I determine wind pressure, specifically uplift, on these panels. The panels are not sitting right on top of the roof, there is a 3” gap. I was wondering if the leeward wind pressure on the roof could then be applied to these panels. So in essence I would double my leeward uplift on some portion of the roof.

 

[JedClampett]

I'm not sure you're going to find an analytical or code approach for something this specific. You could try ASCE 7-05 or 7-10 to see if they have something close, but you're going to have to extrapolate some building or component case. It might be right and it might not.
Beyond that, if there's budget, you could try a wind tunnel approach.

 

[a2mfk]

This question is asked pretty often on the message board, and with the move towards green energy and solar, and the tax incentives that go along with it, it is going to occur more and more often. I think the industry itself should get together and do some wind tunnel tests...

If it is placed that close I would tend to think from a basic fluid mechanics standpoint that your suction would not be THAT much higher than normal roof pressures for those zones, but what does that mean- 10%? 20%?

If placed a few feet off the roof surface, I would tend to model it as a separate open monoslope roof structure. But I will keep listening if others have good ideas...

 

[SrVaro]

There has been some research into this. Most of what I have found indicated that a few inches off the deck (up to ~6) is not a concern for increasing the uplift loads on the existing building itself. But it does change from a uniform load to a point load at the clip to the roof panel from the panel. But I argue that the standing seem roof is attached to the purlin with a clip and therefore also not uniform. The main item I have seen is to not locate the panels within the edge or corner zones, where the uplift is the greatest. With so few panels this should not be a problem.

 

[paddingtongreen]

For what it's worth, I take the view that the air is free to flow and any pressure quickly equalizes above and below the pane. That could be criticized because it is not conservative, so allow say 25% net pressure or suction.

This follows the same principle that has us put vents in our eaves, to allow pressures to equalize.

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

 

[cap4000]

At 20 feet up and at the corner location I would use ASCE 7-05 either Figure 6-11B or 6-14A just to be conservative. ASCE is very silent on your direct question. I would use the C&C pressure coefficients since your going to need calculations.

 

[a2mfk]

So Paddington- would you then use ASCE open monoslope roofs for you pressures on the solar panels that are off the roof surface to allow free flow of air?

 

[gte447f]

Most manufacturers of solar racking systems, publish guidelines that prescribe a method for calculating wind loads on solar modules that are flush mounted like this (I know it isn't really flush mounted, but that is how the solar industry refers to this type of mounting). In my experience, the methods employed by the racking manufacturers are identical to ASCE 7. They simply assume the loads are applied to the solar module surface rather than the roof surface. The 3 inch gap between the roof and the modules is neglected and considered immaterial.

 

[JoeBaseplate]

gte, the pnl manufacturers show their wind and even snow loads perpendicular to the face of the panel. On one project when I called their designer to point out that on a 20 degree tilted panel, the snow load should be vertical and not perpendicular to the face of pnl.........it sounded like it was news to him. I don't trust the numbers they use.

 

[a2mfk]

Joe- gravity acts vertically!??? Dammit......

 

[csd72]

Define vertically! What you call downwards is actually upwards if you use somewhere like Australia as the reference point.

Actually it acts towards the centre of the earth so if you look at the ends of the panel the components of gravity are inclined towards each other and will actually induce a compression in the panel from the force component.

 

[JoeBaseplate]

Csd, spoken like a true engineer.

 

[OHEcoEng]

Sorry I'm a little late getting in on this, but hopefully, this will help. It's a very recent guidance document written on this exact subject.

I have been providing it to code agencies that question my calculation methods, and all of them (so far) have been quite receptive.

Mike

 

[JoeBaseplate]

OHEcoEng........thank you for the link. Very useful info.