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Seismic Focus Part 1: Why do we have seismic requirements in Australia?

Australia does not sit across more than one tectonic plate like New Zealand does, so does not have as many severe earthquakes. However, Australia still has about 100 earthquakes per year of magnitude 3 or more. Earthquakes of magnitude 5 or more, occur on average every one to two years.

Thankfully, due to Australia’s low population density, and the fact that most of the population is clustered along the coast, we have escaped being affected too much.

However, due to the risk of a severe earthquake such as the Newcastle earthquake of 1989 that killed 13 and injured 160, earthquake codes have been developed.

Why more attention on earthquake codes recently?

The fire cladding issue recently highlighted due to Grenfell and Melbourne apartment fires, has focussed attention on compliance in general and more persons are becoming aware of just how stringent the earthquakes code AS1170.4 is.

As certification bodies and professionals are examining building documents more closely in the light of this, more documentation is being asked for.

Proving compliance with earthquake regulations is not as easy as you would expect.

What areas in Australia are most at risk from Earthquakes?

Earthquakes in Australia can be considered low probability but high consequence events. This is because most Australian cities are unprepared for earthquakes, and insurance policies for Australian buildings generally include earthquake cover.

See map below which shows the relative earthquake risk for different regions of Australia. Only Canberra has a higher earthquake risk than most cities in the world, on account of its closeness to one of Australia's most active faults, at Lake George.

Note this map shows the relative chance of a region experiencing a hazardous amount of ground movement based on a 10% chance over a 50-year period. This can be simplified as a 1/500 annual exceedance probability (AEP). The 1/500 AEP is defined in the National Construction Code.

Also of note, is that up until 2018, many areas in Australia were over-categorized in terms of risk, compared to cities in other areas of the world, but this has now been corrected in the 2018 revision of the National Seismic Hazard Map of Australia.

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The 2018 National Seismic Hazard Map of Australia. (Source: Geoscience Australia)

What does the NCC say about seismic compliance?

The NCC of 2019 requires all buildings to comply (to varying degrees) with AS 1170.4-2007: Structural Design Actions Part 4: Earthquake actions in Australia.

This requires allowance for earthquake loads on both structural and non-structural parts and components of a building.

The NCC also classifies buildings into four importance levels, which are takin into account when determining the design of a building in accordance with the code.

Most commercial buildings are Importance Level 2.

Commercial buildings where more than 300 people can assemble in one area, and schools with more than 250 people, are Importance Level 3.

Importance level 4 includes hospitals and other buildings that are essential to post-disaster recovery, such as ambulance stations.

Note houses are not required to be seismic compliant unless over 8.5m high to the top of the roof.

What building types are most affected by the seismic regulations?

As well as location and importance level as mentioned above, there are two other factors that affect how a building must be designed for earthquake compliance.

These are soil class and building height.

These four factors are combined into Table 2.1 of AS1170.4-2007, to determine one of three Earthquake Design Categories or EDC.

A flowchart for determining the correct Earthquake Design Category from Table 2.1 is given below.

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Flow diagram—Design procedure for determining EDC.

Australia has three Earthquake design categories, covering low level, low risk commercial buildings up to multi-level, higher risk buildings that contain many people.

The implications of these three categories, as well as some information on typical bracing and framing methods for interior components, will be covered in a separate blog.

Watch the following videos as they explain and showcase some of the worlds biggest earthquake tests:

As always, feel free to reach out to our friendly team, either to book further free training sessions, or speak to us about any questions you have that may still be unanswered.

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