Revised internal pressure calculation in AS/NZS 1170.2:2021 Part 2

[Tomfh]

This is a follow up from the following thread:

Revised internal pressure calculation in AS/NZS 1170.2:2021? - AS/NZS (au/nz) Code Issues

In the recently revised wind code, there is a change to the way internal pressures are calculated for structures with dominant openings. The 2011 version defined the internal pressure coefficient as the external pressure coefficient at the location of the opening [i.e. Cp,i = Cp,e at the...

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In it, someone asked:

For the Kl part, is it still only referring to cladding, cladding fixings, elements that supports cladding, and this fixing?

to which @rowingengineer replied:

For KL internal pressure it applies to all parts of the building. This has been clarified by the code committee.

Can anyone confirm/clarify this? Are local pressures at the locations of openings now applied to all internal surfaces simultaneously, including loading of primary framing members?

 

[rowingengineer]

The local pressure factor is ment to represent the external local pressure factor acting on the opening at the max event. There was a fair bit of testing done and a thesis was born and it does suggest that you are correct, the local variable pressure is still present with an opening. I think the guy was Mitchell.

However what the code didn't take from the testing was the flexibility of the structure.

 

[Tomfh]

Do you have any references? I can't find anything by Mitchell?

 

[human909]

You mean this video:

I thought it was Chris h on the NZ video where we asked but I can't find it ont hat video, but we asked one of the committee members during the presentations to clarify the intent.

The question was asked here too, but noanswer wasn't given:

What do you mean from how physics would work? I’m trying to understand, but it seems contradictory with what local pressures represent.

Local pressure factors represent short, localised spikes due to turbulent corner flow, enough to damage cladding, but not the structure. So how can that same brief pressure spike (say kL= 1.5,2,3), once inside, load the entire internal volume and frame? Where does the extra energy come from to do all that work, if it wasn’t present to begin with? Do the flow dynamics change radically if there’s an opening, with the local pressures persisting in the location of openings?

My gut feeling is to agree with you. Though my understanding is that local pressure are not necessary all "brief pressure spikes" eg corners. But I still generally agree with your thesis

However I have questions:
-Would air flow as is required to increase the pressure inside the building be sufficient and not affect the actual pressure at the local source? Unlikely IMO/
-Would the volume of the airflow required be sufficient to given buildings are leaky and cladding is flexible?

 

[Euler07]

I think applying local pressure factors to the entire internal pressure would be a radical change in the loading and in the design of buildings. Based on my experience, looking back at historical design from the last 70 years, I have not seen buildings have random catastrophic failure that would be expected with this drastic increase in unanticipated loading. But just my 2 cents.

 

[rowingengineer]

Euler,
Some emotional words, radical, catastrophic, I doubt that anyone would be able to pinpoint any of the building failures to internal being under appreciated by the difference of this calc.

Overall the change to the internal pressure with local pressure changes hasn't greatly changed the outcomes, the wind ward local pressure factore is 1.5 and the area is 0.25a2, for a 12x12x3.5 that is an a of 3.5, which gives an area 2.6m2, and if you opening is greater than this area in the example building than that KL is 1. They you compare this to background permeability you will be down the table, and a larger opening like a roller door will govern, that is unaffected by KL.

Internal suction, well that is a different story, it is higher generally, however I don't find this changes the worst design Case of CW + IP.



Tomfh,
Overall I don't find the outcomes significantly different, just a few extra calculations to show the larger opening is the governing rather than the small 0.25a2 and has a slight lower internal pressure due to kV.....


Overall I would encourage people to do the calcs and see if it really has a impact. A group of North Qld engineers spent a heap of time doing different setups of existing buildings to see if they could be certified as as1170.2-2021 and came to be conclusion that for most building the 0.7 was still a conservative/acceptable assumption for internal pressure coefficient. This was in region C.

 

[Tomfh]

Overall I don't find the outcomes significantly different, just a few extra calculations to show the larger opening is the governing rather than the small 0.25a2 and has a slight lower internal pressure due to kV.....

With small openings, kL can be 2 or even 3, which makes a huge difference. Applying a CPe of 0.9 with kL equal to 3 results in massive internal pressures, apparently acting across the entire internal surface.

What is the research showing it caps out at 0.7? That would help us a lot.

 

[rowingengineer]

I do believe your referring to internal suction, not pressure, and your now referring to an opening in the roof with a KL=3 and 0.9, this would be a rare building, and permanent or debry roof opening are rare, and given that debry impact is reduced for most low pitch building to the point that most roof coverings should be able to pass this requirement thru testing under impact, this outcome is very unlikely.

Again I would suggest doing the calculations, rather than postulating alternative high order outcomes. Anything sounds bad if you suggest outcomes that are not required or real.

i haven't had a reason to include roof openings in any of my buildings to date in the KL region, even with skylights, and can only think of tiles on and a pitched roof, or permanent open skylights however here your tiles area would be like 0.5m2 and background permeability would surely be more than this. So maybe a permanent open skylights??? But would you allow these in you kl3 area. Maybe in an importance level 4 building, but I would suggest ply under sheeting and limit the opening to Debry holes so again wouldn't govern, maybe, that is the only case that maybe would be possible that a roof opening could be in the KL region..... I struggle to see a real world outcome.

 

[Euler07]

Euler,
Some emotional words, radical, catastrophic, I doubt that anyone would be able to pinpoint any of the building failures to internal being under appreciated by the difference of this calc.

As an example, I was just considering ridge vents that are not uncommon in Region D. I assume this would be local region RA4, leading to Kl = 2.

Or a PA door or small window that's failed. Even if the building is designed for full internal pressure, the local pressure factor would be 1.5. I would assume most buildings would need to be designed for a window failure and a local factor or 1.5?

A group of North Qld engineers spent a heap of time doing different setups of existing buildings to see if they could be certified as as1170.2-2021 and came to be conclusion that for most building the 0.7 was still a conservative/acceptable assumption for internal pressure coefficient. This was in region C.

Do you have a link to the studies? Thanks.

 

[Tomfh]

I do believe your referring to internal suction, not pressure, and your now referring to an opening in the roof with a KL=3 and 0.9, this would be a rare building, and permanent or debry roof opening are rare, and given that debry impact is reduced for most low pitch building to the point that most roof coverings should be able to pass this requirement thru testing under impact, this outcome is very unlikely.

Again I would suggest doing the calculations, rather than postulating alternative high order outcomes. Anything sounds bad if you suggest outcomes that are not required or real.

Yes, suction, negative pressures. That’s where flow detaches, turbulence increases, and local pressures are worst.

You seem to be saying that if we avoid the more extreme cases, e.g. by relocating openings, or prohibiting them in high-suction zones, or increasing background permeability, then internal pressures aren’t too severe. And sure, if you design around the worst-cases, the numbers come down. But that’s not what I’m asking. I’m talking about the worst cases AS1170.2 numbers where such openings DO exist and have NOT been designed out.

See, our role includes blanket certification of certain cladding products against the full AS1170.2 wind envelope. We can’t just “move the skylight” and be done with it, we have to assess performance using the actual code figures, assuming worst-case configurations. As Euler07 noted, these conditions really exist in buildings: ridge vents, skylights, windows, etc.

We've been treating the code at face value and applying these values to cladding and cladding supports, even if it’s hard to see how a brief turbulent gust could evacuate a building down to the peak local pressure factor intensity and hold it there sufficient to load the entire interior.

But back to my original question: are we applying those theoretical kL of 1.5, 2.0, 3.0 to the internal pressure coefficient for the frame too? You originally seemed to be saying the committee confirmed it, which is what I’m trying to clarify.

 

[rowingengineer]

Euler,
I do a fair few building in high wind zones with ridge vents, I am yet to have an issue with local pressure factors once you take averaging into account or m2 rules etc.

In regards to the study, you can reach out to he Queensland works department, they maybe able to direct you the people that did it for the cyclone shelters, I was only provided the outcome because I had a cyclone shelter fitness of design reconfirmation, where this was a hot topic.

Tomfh,
I'm saying the worst case numbers are purely theoretical.

As for sheeting pressure certification I do a heap of this as well as roller doors, and I do not have the struggle you are referring too, we state airbox pressure adjusted by kt factors, nothing more.... If it is AS4055, the internal coefficient in this code are amended.

Might be worth reaching out to a committee member at JCU they are always helpful or Chris at checkwind is also a guy that can ask for you. he is/was on the AS4055 committee. I had hoped that they would have added clarification in amendment 2 but all they seem to be adding is the stuff from WA that JCU has been working on.