AS4055 Serviceability Wind Pressure - Why is it so low?

[MikeRD_Aus]

Hi,

Looking for some clarification on serviceability wind pressures for residential buildings.

AS 4055 seems to provide very low serviceability wind pressures relative to AS1170.
Serviceability pressures are approx ~40% of ultimate which seems to equate to a 1yr probability of exceedance in the equivalent AS1170.2 loads.

For ultimate load, using a probability of exceedance of 1/500 (as stated is AS4055 and typical for AS1170.2) I can get similar pressures between the two standards.

However, when using AS1170.2 the serviceability criteria I've generally used is based 1/25 probably of exceedance which usually equates to 68% of ultimate for a normal low rise residential building.

AS4055 provides no explanation (that I can find) of where the serviceability pressure/wind speeds are derived and what probability of exceedance has been used.

Hoping someone might know and can point me to a section in the standards/commentary that explains.
Or otherwise if you're able to provide an explanation.

Thank you.

 

[rowingengineer]

I don't know, but as4055 may have been derived before the amendment to 25yr from 20 or 15 year.

 

[Settingsun]

I can speculate that it's for economy, ie so people can actually afford a house.

Interesting that brick outhouses don't strictly need to be built like the proverbial brick outhouse.

 

[MikeRD_Aus]

I have just downloaded the 2021 version of AS4055 which actually provides some further detail in the commentary that was not included in the 2012 version.

It states that the annual probability of exceedance for the serviceability limit state has been taken as 1/25.

Still doesn't correlate with AS1170.2 unless for whatever reason the serviceability pressure is taken from the most favourable cardinal direction.
Testing a few scenarios the addition of the lowest direction multiplier from 1170.2 brings the site wind speed down in an 1170.2 calculation to match the AS4055 site speeds for serviceability.

I'm not sure I understand why it would be appropriate to then apply that load in any direction using the AS4055 method.

 

[Agent666]

I know in NZ they often take a shielding factor Ms= 0.85 into account for residential design in the suburbs due to generally being surrounded by similar buildings Maybe similar has been applied here.

https://engineervsheep.com

 

[rowingengineer]

Good old branz.

There is a direction factor of 0.95 applied to both ult and service. It isn't md but is similar and applied to both sheeting and building in reg C unlike as1170.2.