Wind loads for a fire water tank 1

[sdz]

According to AS 2304:2019 Water storage tanks for fire protection systems, Clause 4.1
"The design life of water tanks for fire protection systems for the determination of design
actions within this Section shall be a minimum of 25 years and a minimum importance level
of 2 as specified in AS/NZS 1170.0.
NOTE: Tanks providing water for fire services are considered equally as important as the
buildings they protect and hence the minimum importance level is given as two but could be
higher (refer to AS/NZS 1170.0). This then leads to the annual probability of exceedance for the
various design actions (refer to AS/NZS 1170.0)."

Going to AS1170.0 Table F2 this gives for wind ARI=250 years

However going to the NCC-BCA a structure with importance level 2 has to be designed for wind ARI=500 years. (The tank would be a Class 10b)

Which one do I have to use? It makes a difference of about 14% in wind load which in the case I'm designing for means the difference between needing tie downs or not.

 

[Agent666]

I'm not sure how it works in Australia, but if you go less than 50 years design life in NZ there is an expectation/requirement in our building code that you must remove the structure at the end of say 25 or whatever design life you design for.

50 years implies infinite life basically in terms of the minimum baseline for acceptable risk. Otherwise everyone would be designing for 25 years and leaving structures in place for 50+ years which is not the intent.

Note it says minimum 25 years, they also imply 'equally as important as the building served', so possibly imply the same design life as the building potentially should be used.

The way I look at it is, if say the real life of the tank was 20 years before it needed replacement, and it was replaced over and over, its as if this life of the service provided was continuous. So 50+ year design life should apply in terms of the risk applied to this function. Simply having 25 years at lower risk, followed by 25 years at lower risk followed by 25 years at lower risk is not the same as continual cover for 50 year level of risk if you know what I am getting at.

It's a little like cladding, say it has a 20 year design life before replacement, you don't design it for 20 year level of loading if it is part of a building that requires as a whole 50 year design life.


Ask your client for agreement whatever you choose if there is choice, as they (and more importantly their insurers) need to accept the risk and understand any constraints. Insurers are obviously risk adverse so would likely have an opinion. Overall the loading increase is minimal so I'd just suck it up.

 

[human909]

It isn't entirely clear to me that a water tanks does classify as a Class 10b building. Nor could I find the reference that the ARI design needs to be 500 years. Though please do correct me as I not fully abreast of the BCA.

That said it seems to me to be prudent to design the fire tank to have hold downs. Surely the cost can't be prohibitive.

 

[apsix]

I would say the the probability of the tank being empty and also subject to the design wind event is negligible, hence tiedown shouldn't be required.

 

[sdz]

apsix, For tanks with a steel plate floor like tanks to API650 you can count a part of the contents to resisting uplift and overturning but the tank I'm designing has walls only and a flexible liner that is ground supported inside the tank so even if full contributes very little toward stability.

 

[Settingsun]

Sounds like it should be the ARI the building was designed for.

 

[apsix]

sdz, my AS1170.0 Table F2 for wind has ARI = 500 years. Do you have the latest version? Mine includes Amendment 5, issued in 2011.

Since there is no tank floor it looks like tiedown is required.

 

[sdz]

apsix. Actually I was looking at an older version with Amdt 4 but I have a copy with Amdt 5 so i should make sure I look at that one in the future.

So looking at the correct version Table F2 for 25 years, Importance Level 2 gives 1/200 for wind load.