Design Pressure of Architectural Perforated Panels 5

[kranic24]

I'm looking for suggestions on the "correct" wind pressure to apply to perforated aluminum panels acting as cladding. In particular, the panels that are installed at corners of a building or even wrap around a building.

Does anyone have technical documents or case studies that prove effects of certain hole patterns on perf panels?

My engineering judgement tells me that you cannot develop negative wind pressure with perforated panels at the corner if the percent open is large enough. However, I am unable to determine what percent open I would need to eliminate the negative pressure.

Thanks for your suggestions or any documents to support your response.

 

[SteelPE]

Is this for a roof screen? I am having a hard time visualizing your application.

 

[kranic24]

SteelPE, thanks for the reply. Not a roof screen, that would be a completely different loading.

My typical condition is cladding installed over subgirt tube/zee framing on the wall. Its simply aesthetic. Most of these panels are 3-4' tall by 4-6' wide lapped together or have some form of 1/2" joint system. (Reference CENTRIA Ecoscreen for more info)

My inquiry is when the panels exist within the 'a' corner zone dimension of a building. I'm trying to support my theory that the panels would not be able to develop negative pressure. Thanks.

 

[JedClampett]

You could do a analysis based on effective "k" factor for the hole pattern. The ASHRAE Design manuals have examples for perforated plates. Or you could go to Idelchek and get the loss from that.
But why refine the calculation that much? Consider it solid (it's not that conservative) and use that value.

 

[JAE]

I think there would be some reduction in the pressure but it certainly won't be zero.

If you have a thin cable perpendicular to the wind direction you get as much drag as a full airplane wing (per my aerospace engineer friend).

It is not a direct linear function of percent open vs. wind pressure either as the shape of the holes, the hole/opening edge condition, etc. all affect the drag on wind running through the panel.



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[SteelPE]

So I take it you are in charge of designing the perforated "rain screen"? In this instance I would approach it the same way you would any other type of panel and ignore the perforations.

I know there is a "rain screen" system out there where they can place images on the outside of the panel by varying the size and type of perforation (they also do something with the ingot leftover)

http://www.dri-design.com/dri-design/imaging

Maybe you could contact them to see if you could pry some information out of them.

 

[kranic24]

Jed, in my field of work, a 10-20psf difference can make a large impact to what is adequate or failing. I would agree, that if you are not too concerned with the slight increase in pressure, be conservative and treat as a solid plate.

JAE, I completely agree and this is the main point to the thread. My hope was to get dialogue about other engineers experience. I am hoping there is some actual tests conducted or technical documents that I can reference to justify my design pressures I use.

As you mention JAE, and I agree, the hole pattern and shape makes an impact to the amount of pressure. I'm trying to develop an educated procedure.

One may argue also that the panel will pressure equalize but I am leery of making this argument. Per ASCE 7/10, making this argument is difficult to back up.

Thanks again everyone.

 

[kranic24]

SteelPE, great suggestion except Dri Design would contact their engineer, which is someone like me

 

[XR250]

I would design as a solid cladding

 

[JAE]

The only thing I could also suggest is to put the material in a wind tunnel and test it at various angles (the perforations turned at an angle could affect it too).

That way you have something to go on. Otherwise, you probably need to use a solid surface in your calculations per ExcelEngineering or at the very most assume 90% of the wind force as a conservative assumption.

Part of this assumption depends on how much of this stuff you have on the building and how sensitive your assumption is to the total design/stability of the structure.

If it doesn't affect the overall wind forces (MWFRS) as much perhaps you can be a bit less conservative. If your assumptions (70%, 80%, 90%) significantly alter the overall, then I'd go safe.

[blue]When in doubt, make it stout.[/blue]

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[Gus2015]

Wouldn't the system pressure equalize eliminating the pressure on the panel?

 

[JLNJ]

I believe ASCE has some guidance for perforated signs, but you may find that you nedd very large holes indeed to keep from developing any negative pressure.

One item you might scrimp on is the factor of safety. Normally, wall panel loads are quite high to keep the panels from tearing and allowing rain to damage the building's innards. If part of a screen wall tears away, the consequencess seem less severe.

 

[Trenno]

In the AUS codes - a freestanding wall / screen perforated (50% solidity ratio) will get you a 25% reduction in wind pressure.

 

[Gus2015]

If you were creating a F.E.M. with this reduced pressure would you only apply the pressure to the solid surface, or would you apply an increased pressure to the solid surface?

 

[dcarr82775]

There is certainly a reduced pressure on the perforated panels. Maybe contact someone like CPP in Fort Collins, Colorado. Wind tunnel testing guys who have looked into this.

 

[brut3]

I work in the metal cladding industry and we run into these conditions quite often. The short answer to your question is that there is no singular document that will address the pressure loss, strength reduction, and stiffness reduction through a perforated sheet. There are many different factors that come into play from the open area that is introduced from the perforation pattern to whether or not the the sheet has any sort of geometry to it. All of these factors will effect pressure loss and strength reduction of the panel.

That being said, there has been research done for the Industrial Perforators Association who commissioned O’Donnell & Associates, Inc. to try and come up with some sort of method to effectively calculate some of these properties. Below is a link to an article which summarizes the findings of the study performed by O'Donnell & Associates.

http://www.fergusonperf.com/pdfs/tolerances-options_strength.pdf

Although this article may help if your trying to take reductions as reference in 30.1.5 of ASCE 7-10, it by no means addresses all of the different factors that need to be considered for each particular application. From my experience the design of perforated panels ultimately comes to using the professional engineers own engineering judgement as to what reductions in wind pressure and strength should be taken.

If you do find something that may better address your issue please share as I'm always looking for more information to substantiate my own calculations.

 

[kranic24]

brut3, we have used the IPA numbers for years and agree that it is good documentation to build your judgement on. We can all agree that perforated panels take substantial judgment to design and are typically allowed to have a reduction in pressure.

However, to get back on track to the main goal of this discussion was to ask what other engineers thoughts were on the negative pressure effects of perforated panels. Can you really develop a negative corner zone pressure on moderately open panels (say 50% for argument)?

 

[KootK]

Negative corner pressures are a cross wind phenomenon. They can be mitigated via pressure equalization but that very mechanism itself involves air movement perpendicular to the screen. So it boils down to this: if air were blowing perpendicular to the screen, would it feel any pressure? Of course it would. It's just a matter of how much.

I'm working on an art piece involving perforated aluminum. We're considering a load case that involves icing concurrent with a statistically sensible concurrent wind speed. For that case, we've deemed it prudent to assume that the ice makes the aluminum essentially solid.

I like to debate structural engineering theory -- a lot. If I challenge you on something, know that I'm doing so because I respect your opinion enough to either change it or adopt it.

 

[kranic24]

Koot, thanks for the reply.

I completely agree that if ice is present, treat the perforated panel as closed and used concurrent ice wind withour open area reduction.

 

[BobKK]

I am having the same issue though in my case the panels are quite large. They are triangular shaped with 3m sides. The perforated area is around 50%. I believe if I take it as a solid member, there would be 2 folds over estimation. One I would be assuming that there is no reduction in the internal pressure and secondly I would be applying the pressure on the full area and not just the perforated area. If we use the full pressure but apply it only on the perforated area, the total force is reduced by 50%. But I am not sure if that is the right approach.