The new AS/NZS1170.2:2021 - Solar panel/PV loads on roof structure

[nicbarbs]

Hi all, long time listener first time caller.

I have spent a chunk of time with solar panel/solar racking suppliers in the last few years. It is well known that the solar panel loading in Appendix B.6.1 (Formerly Appendix D6) are a bit vague, as they only cover flush mounts on residential roofs within strict conditions. Many suppliers extrapolate this slightly with advice from wind engineers, or they seek wind tunnel testing for particular conditions and generally establish coefficients. This all seems reasonable and I am generally comfortable with the principles and parameters defined.

The biggest struggle is defining the loads on the base structure. Justifying that a solar array does or does not increase the overall load on a structure beyond its weight. We typically consider:
[ul]
[li]1.2G+1.5Q - if we've allowed Q=0.25kPa, the overall weight of panels (~0.15kPa) can fit within the live load allowance, with enough allowance for temporary stacking, hail etc[/li]
[li]0.9G+Wu(reversal) - It is a goal to justify any additional wind action is countered by the extra weight of panels (~0.15kPa)[/li]
[li]1.2G+ψcQ+Wu - Generally live load is ignored for roof (ψc = 0). So this clause makes clear we need to account for extra weight of panels, regardless of wind load.[/li]
[/ul]

The biggest unknown is the load on the ROOF STRUCTURE induced by the panels. Clause B.6.1 snuck in this note (Attached) which everyone seems to ignore when discussing solar panel loads. Every google search only discusses B.6.2 which covers non-tracking tilt ground mounts. And hence I cannot for the life of me interpret this subclause.

The most simple interpretation is that say my racking has a Cshp of -1.7, I can apply a factor of +0.6 on my roof? ie a downwards pressure? But it could act simultaneously, so could the net Cshp on my panel be -1.1? Or is it +0.6 for an outward pressure, causing a net Cshp of -2.3 on my panel?

I cannot find any discussion on this note and it makes absolutely no sense to me.

I have seen wind tunnel results where the pressure on taps directly applied to the roof with panels are much higher than the 'area average net' pressure between taps above and below a panel. And I cannot for the life of me interpret the expected physical behaviour of the roof under a panel.

Does anybody know the origin of this new subclause comment? Or shed light on expected behaviour?

 

[nicbarbs]

Just one bump on this post, it's interesting there is nothing publicly available about this. I asked MEL Consultants and Windtech but got no feedback.

 

[rowingengineer]

Basically air gets trapped between the solar panel and the roof, this acts like an internal pressure pushing up and down, up in the panel down on the roof. For this case the solar panel to receive both an external suction and a pressure which in the first case would be -1.1 + .6 roughly to make -1.7. and the roof would get +0.6 and the reversed when sign reversed.

 

[PersonalProfile]

The following may be of interest, if I have understood the enquiry, though I am not a subject matter expert etc:

1. Proposed modifications, I added emphasis, according to recent draft for public comment AS/NZS 1170.2 Amd 2, attached,

2a. ...Nett pressures acting on the panels, obtained in combination with the pressures on the roof, are transmitted to the roof structure via the fixings (brackets and railing)....

2b. Also attached example.

Ex:

https://www.awes.org/product/wind-l...zs-1170-2-wind-actions-wind-loading-handbook/

3. Geoff, a committee member for AS/NZS 1170.2, participates in the following forum, refer following example, that perhaps you can request on the forum.

AS4055 Serviceability Wind Pressure - Why is it so low?

https://eaxchange.engineersaustrali...-bce0-4091-897b-4fccbefb6472&tab=digestviewer

Geoff has a PhD in Structural Engineering for work on wind loads on housing.

He is a member of a number of Australian Standards Committees including the committee for AS 1684, and the committees for the wind loadings standards, AS/NZS 1170.2 and AS 4055.

Ex:

https://www.jcu.edu.au/cyclone-testing-station/our-researchers/associate-professor-geoff-boughton

Attachments:

https://files.engineering.com/getfi...-e6e074a6f8d2&file=DR_AS_NZS_1170.2_Amd_2.png

https://files.engineering.com/getfi...-4b24-a849-ad5aa95deb51&file=AWES_Example.png

 

[PersonalProfile]

I interpret the following video by another committee member to state that the intent of the current note is to ensure the net load difference is negligible.
This appears to be contrary to the earlier reference material, so I am lost.
I propose is a moot point considering that the proposed amendment to Note 2 regarding roof mounted solar panels, that apparently states that the engineer should obtain specialist advice.

https://youtu.be/6gnRaAN9SOY?t=2143

Regards.

 

[nicbarbs]

Thank you for the diligent research PersonalProfile, it's definitely raised some more thoughts.
Appreciate this is not your area necessarily but you've dug up some information I had not!

I think the blanket statement from John in that video is not entirely correct, we cannot conclude "that coefficients become 'similar to a bare roof'". I understand his idea that a maximum external pressure on an enclosed building is -1.1 to Section 5 of the code - although this is for tall buildings, for squat buildings as section B6 shows this peak would be -0.9. And he is correct that peak panel force -1.7 (+0.6) = -1.1 which is similar to a bare roof. But this -1.7 can apply in the 'Central' zone of a roof, where the roof load from Section 5 would reduce to say -0.2. Hence we would need to apply -1.1 in this zone, ie ~5x the roof design pressure to section 5.

As you mention, the draft amendment seems to scrap this entirely, so I'm glad to know there's some debate at a committee level as well. It's certainly not black and white.

Out of curiosity, did you have a link to the draft amendment 2? Or was that snapshot from elsewhere. I cannot find the draft online now that comments have closed (as of January 2024) and can't find comments or information on possible adoption of the AMD.

 

[PersonalProfile]

I could not find a link to the Amd 2 in public domain, and I interpret that I am not permitted to distribute the document that I have access to.

The AWES states, apparently prior to Amd 2:

Leitch et al. (2016) presented the external pressures on the baseline building roof and the external pressures on the roof under the solar panel array and the nett pressures on the solar panel array. These results show that the panels at the leading end of an array are subjected to larger nett pressures than those in the middle of the array. Furthermore, these nett pressures may be larger than the external pressures on the adjacent part of the roof.

Leitch states

Because the gap under the panels and between the rails is so narrow, it acts in a similar manner to a conduit, with both the maximum (acting into the surface) and minimum (suction) pressures being very similar for both the top of the roof and the underside (bottom) of the panels.

Ex:

https://www.researchgate.net/public...tched_roofs_and_acting_on_the_underlying_roof

I speculate that the committee is aware of research that is more recent that concludes otherwise, and/or is inconclusive etc.

Regards.