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# AS1170.2:2021 Roof local Pressure "a"

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## AS1170.2:2021 Roof local Pressure "a"

(OP)
Can anyone shed light on this. Could this be a mistake in our code.
Following the rules of 5.4.4 for a large shed. If approximate b=50m and d=50m but h=10, then "a" works out to be 10m. if we increase b and d by .5m, "a" magically jumps to 20m. This large increase makes no sense. This would be almost 1/2 the building length or width.
Reviewing the American code (30.3.-2A ASCE7-22 page 319) "a" for the 50.5m b & d, h=10m would appear to be 1.2xh=12m. Why is there such a major transition in "a" with the new AS1170.2:2024 code vs the old wind code.
Regards,

### RE: AS1170.2:2021 Roof local Pressure "a"

It looks like you're right. I guess we won't know the reasoning for a long time since it's almost impossible to ask questions on standards.

This is similar to the new Kv factor, where minor architectural changes can have drastic effects on the wind loading. It's almost impossible to implement some of these clauses...

### RE: AS1170.2:2021 Roof local Pressure "a"

Surely this is just really badly worded as standards writers can only do!

Can read it as min of 0.2d or 0.2b or 2h if you use some 'standards writers' imagination... And remove the first comparison inequality if the second one applies.

### RE: AS1170.2:2021 Roof local Pressure "a"

I cannot directly assist, though:

1. Consider discussing on the following forum,

https://eaxchange.engineersaustralia.org.au/Higher...

I propose the following is an example of a comprehensive response to a wind enquiry of this forum.

https://eaxchange.engineersaustralia.org.au/commun...

2. AS/NZS 1170.2 is issued draft for public comment, refer following, closing tomorrow 2023-12-25. That clause is not specifically included, though perhaps you can question.

https://comment.standards.org.au/Dashboard

### RE: AS1170.2:2021 Roof local Pressure "a"

You're referring to
"For roofs, the value of a is the minimum of 0.2b or 0.2d, if (h/b) or (h/d) ≥ 0.2; or 2h if both (h/b) and (h/d) < 0.2. ", where a will jump from 0.2b (10m) to 2h (20m).

That's the way I read it too. As to why? I have no idea.

### RE: AS1170.2:2021 Roof local Pressure "a"

Interesting, so clearly not an error!

### RE: AS1170.2:2021 Roof local Pressure "a"

(OP)
Thanks PersonalProfile for your investigation. It appears there is slight unconservatism in the load of the 2011 code between 0.5h and 1h, but potentially this is made up for by conservatism in the loaded area (not sure). 2021 code on the other hand would appear to be conservative between .5h and h. Between 1h and 2h, the product of kl.Cpe appears unconservative for both codes. Certainly food for thought. Unfortunately this means for h/b and h/d ratios of less than 0.2, our local pressures on purlins need to be evaluated further into the building. Yes Agent666, this does look like it was done on purpose. Bugger. Have to change all my spreadsheets.

### RE: AS1170.2:2021 Roof local Pressure "a"

PersonalProfile, that graph isn't clear to me because it doesn't separate Kl from Cpe. What it does show is that there is a big step change at x/h=1.0 and the code does not provide a good match to the test results.

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