Wind Loading on Balcony Perforated Aluminium Screens - AS/NZS 1170.2

[Warramoos]

Hi All,

We're reviewing the wind loads on a perforated privacy screen that free spans between the slabs of a building screening off the balconies. The screens are of various porosity and shape. Some screens are movable (i.e slide along the front of the balcony) but most are fixed. Very similar to the photo below.

Our query: Does anyone know or have experience with what wind loading these screens would need to resist. AS/NZS 1170.2 doesn't cover something like this, so we've had to try to come up with something that fits the standard logically, but wondering if there is other literature or "best practice" that others in the industry may be using. A couple of thoughts.

1) We initially considered it as a hoarding or self standing wall, but this doesn't really fit as the free stream wind cant get under or over it. And because there is a wall a couple meters behind it couldn't really get much suction as the free stream wouldn't be traveling fast behind.

2) We're currently considering it to be a cladding. It would only receive windward pressure, and internal pressures would be zero. But would also need to have local pressure coefficients applied to these. We feel confident with this but think its probably extremely conservative, especially as there is no porosity factor on a windward wall in AS1170.2. There is a porosity factor for a hoarding though.

Any thoughts or pointers would be very helpful

Many thanks,

 

[ilyas415]

Hi Warramoos,

Your thinking is correct.

Q1: It's hard to tell from the pictures, but is there a solid facade a few meters behind these perforated screens?
Q2: Is it a low-rise building? Is it taller than the average height of surrounding buildings?

If yes to Q1, then in reality the space behind the screens will pressurise in a wind storm. That means that the net pressure coefficients across your screens will reduce.
As you said, treating them as hoardings and self-standing walls would be conservative, since they will not have a strong leeward pressure.
The windward pressure will drop slightly compared to a solid surface.

However, consider that the wind might act at an angle of attack like +/- 5 degrees from the horizontal, which means the projected tributary to the wind would be reduced quite a lot. Therefore I would ignore the pressure reduction effects due to the porosity. It is also possible that building users may cover up the perforations on the screen with furniture or other objects.

At a minimum I would design using the full external pressure coefficients for solid windward walls. I wouldn't say this is extremely conservative (there is a whole number of reasons why the pressures may still be as high as considering external+internal pressure which I can dive into if you like),... and I would suggest that you do NOT use directional reductions (i.e. directionality factor = 1.0).

Finally, I might encourage you to consider wind tunnel testing if: it sits in your budget; if there is a history of wind-related cladding failures in your area; and, if you want need to make additional savings by trying to lower the pressures (which is always a bit of a dangerous game to play with cladding elements).

Any questions, please let me know.


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Ilyas

 

[Warramoos]

Thanks Ilyas for your comments. Very much appreciated.

Yes there is a wall 3 or 4 meters behind the screens. This wall is the glazing line for the unit. The screen is mounted on the front edge of the balcony. Its a low rise building only 5 stories and fairly long in regards to its height.

With respect to pressure coefficients - we've neglected the "internal pressure coefficients" as the deck is essentially open on one face. When we look at table 5.1(B) considering the balcony edge being open and the other 3 sides fully sealed. The result is Cp,i= Cp,e which technically cancels each other out. So the worst load case is to neglect cp,i and only consider Cp,e. Or am I reading this incorrectly?

We've also adopted local pressure coefficients of Kl=1.5 for the windward positive pressure, and Kl=2 for the side load negative pressure (for those panels that sit in this region). The worst case patch loading of this results in applying the full value over the full screen. No savings there.

Is there anything else that we may have missed? I understand its difficult without all the information - but hopefully what I've mentioned above makes sense.

Unfortunately wind tunnel testing is out, as typical construction deadlines don't allow the time to go down that path.

Thanks so much for your help.

Cheers,

Warramoos

 

[ilyas415]

Hi Warramoos,

Just to confirm, this is the image I have of your building:

If that's right, then you should not be looking at Table 5.1(B). That table is saying that, if you have a significantly large opening on your building envelope, the interior of your building (e.g. rooms and corridors) will see significant positive pressurisation and therefore an increase in your internal pressures. For a building with dominant openings, according to Table 5.1(B), that means you would need to design the wall facade panels to Cnet = Cpe - Cpi, where Cpi = +/-Cpe because of the positive pressurisation caused by wind flowing into the openings.

The terms do not cancel out. Here is an example. Consider a facade panel on such a building. Let's say the external positive pressure on it is Cpe,+ve = +0.8. You have to combine it with the negative internal pressure. So...

Cp,net = Cpe,+ve - Cpi,-ve = +0.8 - (-0.8) = 1.6

A good example of dominant openings might be an aircraft hanger. Sometimes the hanger doors get left open during the passage of a strong storm, so the inside of the hanger positively pressurises. As a result, the hanger walls and roofs see much higher net pressures than they normally would (because of the higher internal pressures).

Hopefully that makes sense. Anyway, sorry to take the discussion on a tangent. The bottom line is that your balcony area (if my sketch is correct) does not represent a dominant opening, because you have an sealed facade behind it.


I had a look at AS/NZS 1170.2:2011. I haven't used this standard in a whlie, however, you should be careful. Yes, you can ignore Cpi, but do not ignore Cpe.

I think you're Cpe will be around 0.8 based on table 5.2(A). The Kl factor is a pressure amplification factor which you apply to your Cpe, and accounts for the location of the panel on your building. So for positive pressures, your Kl value would be 1.5. And for negative pressures, you would use Kl = -2.0 like you said. So you're effective positive Cpe would be Cpe*Kl = 0.8*1.5 = 1.2, and I THINK your negative will be: Cpe*Kl = 0.8*-2.0 = -1.6 (this is based on a fairly quick look that I had on the standard).

If you want me to review your calculations once your done, I'm happy to give them a better check.

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Ilyas

 

[Warramoos]

Thanks Ilyas. Yes our thinking is along the same lines. Huge thanks!

Just on the internal pressure coefficients - as we're only interested on the load on the screen, we were considering the "balcony" as a room and disregarding the unit behind. If you consider the Balcony as a room then the screen wall is the dominant opening - which is why we were thinking Table 5.1(B). We're trying to fit our situation into the code - its not ideal, but if feels right. Then based upon this table if Cpe=Cpi (because the ratio >6 as one wall has openings and the others don't) then both are the same sign so they either push into the screen from both sides under windward load, and pull away from the screen under side loading. Thus the net coefficient for Cpe+Cpi = 0 on the Screen wall only. Other walls have different combinations which could be adversely affected - but we're only interested in the screen wall.

If they open the door to the balcony under an extreme wind event - then that changes things up even more - but we're not going there at this stage...

 

[ilyas415]

Hey Warramoos,

Glad to be of help and sorry for the delayed reply. A lot of drinking today and yesterday...

I would be very careful here. In theory, yes you are right, the positive pressurisation of that balcony space would reduce the external wind loads acting on the screen considerably. However, this may not necessarily effect the negative net pressures on the screens too much.

Addiitonally, this internal pressurisation does not happen instantly. There is a time lag between the external gusting pressures and the internal (i.e. the pressure in the large volume needs time to increase to be equal to the external pressure). So it is unconservative to consider the net pressure to drop to zero. Check out page 19 and 20 of the following PDF:
[URL unfurl="true"]http://www.wind.arch.t-kougei.ac.jp/info_center/ITcontent/tamura/5.pdf[/url]

Additionally, if wind is coming from another wind direction not normal to your building face, the pressure inside the balcony space won't always remain pressurised as you like. The air will be occasionally recyled by the wind outside (i.e. occasionally, the air will escape from the confined space, and you will lose the pressure equalisation). This is an effect similar to Helmholtz resonance effect you get in cars...

https://www.youtube.com/watch?v=cc58aeUEWi0

Finally then, I would suggest that you take the net pressure coefficient as equal to the external pressure, i.e. Cp,net + Cp,e Cp,net = Cp,e

Ilyas

EDIT: made a minor equation error at the end.

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Ilyas

 

[Earth314159]

In a case like your, I would say that the porosity helps a bit but I wouldn't likely take it into account. There is no wind blowing through the space, so you are just dealing with the pressures. In a case where wind is blowing through, I would say that the porosity would not help reduce the load significantly. I went through the process of recalculating the wind speed through a bill board type condition with openings. The pressure is proportional to the square of the speed. The wind speed around the openings increases which increases the pressures. The total load was still less but not enough to account for.

It also reminds of a net at a golf driving range that blow over. The net was mostly openings (90 to 95% open) but there was enough force to bend over fairly large steel posts.

I would suspect in your case, the total load is reduced by the total amount of openings.

 

[Tomfh]

I’d assume Cpn ~ 1.5 including local pressure effects.

porosity wise it’s effectively solid.

 

[Warramoos]

Thanks Everyone. Really appreciated your comments. It's great to know I'm heading in the right direction. Thankyou very much.

 

[Agent666]

To offer an alternative perspective, I'd tend to treat this type of thing as a hoarding.

I'd also tend to (and have in the past) treat the glazed balustrade just behind in the same manner, treating it as a long hoarding.

While the presence of the wall behind may ultimately reduce the loads, I feel that if the arrangement you are looking at does not strictly conform to all the code limitations or is simply not covered then it pays to give the illusion of conservatism in whatever approach you take. Often this simply means making a judgement call and making sure other people can logically come to the same conclusion when presented with the evidence of how you came to the conclusion of determining the load to accept for the design of the element.

Reality is if this got damaged slightly due to wind, I don't think too many people would be that worried, especially if the top and bottom tracks were sufficiently robust to retain it if it ended up bending in the middle or something. You wouldn't go around strengthening all the undamaged screens in the building if some got damaged in a storm. People are more accepting of things like fences and screens being damaged due to wind/storm, etc, compared for example with say damage occurring to the primary structure in earthquakes. Think of how many roofs seem to blow off in mild storms that have in no way approached ULS levels, we aren't all running round enacting some new measures to tie down roofs on every single house. It simply seems to be accepted by the public that these things occur from time to time. It's almost the complete opposite of earthquakes, if we notice a vulnerability it tends to be addressed in codes, design guidance, etc. This is probably a function of earthquakes generally causing more widespread damage and hence economic loss than the odd storm.

End of the day you could probably argue that this sort of thing, being secondary structure and all, could be argued away as being importance level 1 if for the fact it were not attached to an importance level 2 structure.

So as long as the approach you take accounts for the known effects such as local pressure coefficients, wind hitting it at an angle (usually this results in some increase in the coefficients as its acting more like a wing with higher 'lift'), and with a sprinkle of conservatism. Then who am I to say as a peer reviewer that you are wrong, if it's something that I could also come to the same conclusion when presented with your decision logic.

I'd tend to allow the reduction due to porosity, simply because its allowed in the context of a hoarding.

 

[Tomfh]

I don’t think it makes that much difference whether you view it as cladding or as hoarding. It seems to come out much the same. Either way it’s a rectangular shape thing blocking the wind.

The holes are too small for porosity to make any differences.


As for all the roofs that start blowing off when the wind approaches SLS, I’ve always found that somewhat amusing. I’d love to see what happens if we had an ULS wind.