AS2304 and seismic loads

[sdz]

Here's something that just tripped me up.

AS2304:2019 - 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

AS2304:2019 - 4.6.2.1 General
Kp = probability factor for earthquake from AS 1170.4, Table 3.1
The value of Kp for fire tanks shall be 1.25

However if you go to AS 1170.4, Table 3.1 you find
P=1/800 kp=1.25
P=1/500 kp=1.0 (BCA Importance Level 2)
P=1/250 kp=0.75 (nominal design life of 25 years)

Unfortunately there is no commentary on this which leaves me thinking that the value in 4.6.2.1 General is a mistake.
I'd appreciate any clarification.

 

[human909]

I agree that does sound like an oddity. However there is no direct contradiction in those two clauses. It clearly says:

• a minimum of 25years
• kp = 1.25

I don't see any reason to believe that the nominated kp value is a mistake. The fact that they clearly specify it would indicate that it is deliberate. Given that fires can very much result from seismic events it seems quite reasonable that the kp value is increased for fire protection systems.

I would suggest that the "minimum of 25 years" comes from a general design life for such structures/equipment. While I can't quote the codes or standards, from my observation the minimum design life for "mechanical type" structures seems to be 25years. I say this having viewed many design documents from large mining and government jobs. They typically specify 50 years for structures and 25 years for mechanical structures such as tanks, silo etc...

 

[sdz]

I agree that does sound like an oddity. However there is no direct contradiction in those two clauses. It clearly says:

• a minimum of 25years
• kp = 1.25

I don't see any reason to believe that the nominated kp value is a mistake. The fact that they clearly specify it would indicate that it is deliberate. Given that fires can very much result from seismic events it seems quite reasonable that the kp value is increased for fire protection systems.

I would suggest that the "minimum of 25 years" comes from a general design life for such structures/equipment. While I can't quote the codes or standards, from my observation the minimum design life for "mechanical type" structures seems to be 25years. I say this having viewed many design documents from large mining and government jobs. They typically specify 50 years for structures and 25 years for mechanical structures such as tanks, silo etc...

It's not the 25 years that specifies the design criteria, it's "importance level of 2". Referring to the NCC/BCA then gives Earthquake 1/500 probability with kp=1.0 to AS1170.4. In addition the importance level would be increased if necessary to match the structure it serves. So a fire tank serving a hospital could be importance leve 4.

Otherwise I agree that the 25 years can refer to non structural criteria.