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Should we Design Earthquake-Resistant Buildings in Australia?
Understand the science behind earthquakes and have a quick view of seismic codes in the construction industry.
We think we don’t have earthquakes in Australia, and there is a reason behind this thinking.
Australia does not lie on a tectonic plate boundary.
To understand what it means, let’s have a look at the map below. There are seven major tectonic plates on earth. Put simply, when two plates collide, the result is an earthquake. Think of countries like Japan, Indonesia, Chile, and New Zealand which are close to the edges of tectonic plates and experience severe quakes quite frequently.
If the Meckering earthquake rang the first alarm in the Australian construction industry, after the Newcastle earthquake people started to question the efficiency of building codes. The time had come to review industry regulations with people’s safety and the resilience of buildings in mind.
Standards Australia, founded in 1922, published the first earthquake code (AS 2121) in 1979. This code was limited to buildings and houses in the Meckering area in Western Australia.
– Earthquakes of magnitude 3 or more ─around 100 earthquakes each year.
– Earthquakes of magnitude 5.0 – 6.0 occur on average every 1-2 years
– Earthquakes of magnitude 6.0 or more occur about every ten years.
Here is how GeoScience Australiaexplains the cause of earthquakes in Australia:
Australia sits on the Indo-Australian plate which is being pushed north at a rate of about 7cm per year and is colliding with other plates (the Eurasian, Philippine, and Pacific). This causes the build-up of mainly compressive stress in the interior of the Indo-Australian plate which is released during earthquakes.
At this point, there should be no doubt that we need to take the seismic hazard seriously while constructing buildings and infrastructure, because what we design now “is likely to be around for the next 100-200 years”, and we have the responsibility to minimize the impact of any future earthquakes on our communities.
Where are earthquakes most common in Australia?
Regions near Perth, Adelaide and the east coast are the most seismically active in Australia.
Earthquake activity in Australia since 1973, showing the clusters of seismically active regions.
There are earthquake hazard estimates for the ACT region, too. Canberra, for instance, facesa high earthquake hazard because of its proximity to Lake George, one of Australia’s most active faults.
In fact, most of the earthquakes in these regions are minor but they can also cause significant damage if buildings are not designed to withstand the earthquake loads.
Who measures earthquakes in Australia and how is the data used by the construction industry?
Geoscience Australia (GA), a government agency, is responsible for providing 24/7 monitoring of seismic activity in Australia.
GA owns a project called National Seismic Hazard Assessment (the latest one being theNSHA18), which determines the level of ground-shaking hazard across Australia, identifies higher hazard areas and provides this information tothe Australian Building Codes Board (ABCB) and Standards Australia.
Standards Australia uses this datato map out the seismic areas of greatest earthquake hazard and includes necessary updates in AS1170.4 Structural design actions, part 4: Earthquake actions in Australia.
Today, the NCC outlines the performance requirements for commercial, public and multi-residential buildings (Class 2-9 buildings). Note that structural reliability provisions are specified in Part B1 of Volume Two of the NCC.
Designing structures for earthquake loads in Australia.
First, let’s see why earthquakes cause damage.
When an earthquake hits, shaking in the ground causes the building’s foundation and the building mass to move in three directions ─ up and down, left and right, forward and back.
This means the building components should have sufficient strength and ductility to withstand the load reversals applied by an earthquake.
Sign in toABCB’s website to access the NCC and see the full details of Volume One and Two requirements.
In terms of the height of a building, buildings of less than forty storeys should be given more attention, because Class 1 buildings (which are higher than forty storeys) have robust requirements to withstand wind forces. These requirements are sufficient for the structures to also “handle” earthquake loads.
Seismic load indicates how much seismic energy (energy released during an earthquake) a structure would need to endure an earthquake.
We need to introduce two concepts before we proceed: dead loads and live loads.
Dead load is the self-weight of a building, i.e the weight of the construction material and the structural elements of a building (beams, floor slabs, roof, columns, and walls). Dead loads are static so they place continuous and permanent forces on a structure.
Conversely, live loads are variable loads. They include furniture, people, equipment, vehicles – anything that’s related to the intended use and capacity of a building. Being moving or moveable loads, live loads are temporary and can vary greatly.
Non-structuralbuilding elements include: non-load-bearing walls, chimneys, parapets, partitions, ceilings, as well as mechanical and electrical components (smoke control systems, fans, air conditioning equipment, elevators, etc.)
This problem becomes even more pressing in the case of post-disaster buildings. These buildings are of strategic importance and will pose significant consequences if the continuity of their services is not ensured (such as hospitals, power generating stations, communications facilities, etc.).
Designing earthquake-resistant buildings is a tough challenge and requires team efforts to be carried out successfully.
Builders, engineers, designers, architects, manufacturers, certifiers, and building control authorities — all of them should be well-trained and experienced in the construction field to tackle the existing problems and ensure compliance with laws and regulations.