On December 28, 1989, thirteen people were killed and more than 160 were injured in the Newcastle earthquake (aftermath pictured above) which measured 5.6 on the Richter scale and damaged more than 35,000 homes, 147 schools and 3,000 commercial/other buildings.
One year earlier, Australia’s largest ever recorded quake measuring 6.6 on a modern extension on the Richter scale known as moment magnitude or Mw occurred in Tenant Creek. Twenty years earlier in 1968, a 6.5Mw quake in Meckering around 130 kilometres east of Perth injured more than 20 people.
Whilst earthquakes are commonly associated with countries located on tectonic fault lines such as New Zealand or Japan, they occur more frequently in Australia compared with what many think.
Indeed, according to Geoscience Australia:
- Australia typically records around 100 earthquakes which measure greater than 3.0Mw each year.
- Earthquakes above magnitude 5.0Mw such as the Newcastle quake referred to above occur around once every two years.
- Earthquakes measuring 6.0Mw or above typically occur around once every ten years.
This raises questions about how Australia is performing in building for earthquake resilience and how we can improve.
For answers, Sourceable spoke with Emeritus Professor Adrian Page, a professor of civil engineering at Newcastle University and a leading international researcher in structural masonry along with Mark Sturgess, a principal and structural engineer at civil, structural, building services and sustainability engineering consultancy Northrop.
Speaking particularly of commercial, public and multi-residential buildings (Class 2-9 buildings), performance requirements for structural reliability are outlined in Part B1 of Volume Two of the National Construction Code (NCC). In essence, these stipulate that buildings and structures must perform adequately under reasonably expected actions or events and must withstand extreme or frequently repeated events. Along with wind, rain and other things, these ‘events’ specifically include earthquakes.
Compliance with this is most commonly achieved through deemed-to-satisfy provisions outlined in the aforementioned part. These, in turn, reference several Australian standards.
Of these standards, the most significant in determining the level of robustness which is required is Australian Standard AS/NZS1170. This outlines the procedures and criteria for structural design and the effects of external loads on structures and their elements. In particular, AS1170-4 determines the earthquake force which is to be applied to the building along with the loads which need to be applied to the structure and designed for.
Once the required loads have been determined, the aforementioned DTS provisions then reference various other standards which outline specific design measures to be applied according to the type of structural material being used. Such standards include AS3700 (masonry), AS3600 (concrete), AS4100 (steel) AS 2327 (composite steel/concrete), AS 1664 (aluminium) and AS1720 and AS1684 (timber).
When an earthquake hits, Sturgess says shaking in the ground causes the building’s foundations to move and the building mass which sits atop the foundation to move in a sideways direction and attempt to accelerate. This effect is particularly severe where heavier structural materials such as concrete or masonry are used.
To prevent such movement, rigid elements within the structure are needed. In multi-storey buildings, this is often achieved though concrete horizontal beams which surround areas such as lifts and fire stairways. Another option is a portal frame consisting of the concrete column and a concrete beam. Where lighter structural materials are used such as steel, diagonal bracing is applied. These take the horizontal load which is placed on the building as a result of the sideways movement and transfer this down toward the earth.
Asked about the importance of designing for earthquakes Sturgess says prior to becoming a structural engineer, he did not appreciate the prevalence of earthquakes in Australia. Since Australia does not lie on a tectonic fault line, he says earthquakes are more readily associated with places such as New Zealand or California in many people’s thinking.
As demonstrated above, however, earthquakes do happen in Australia.
Moreover, predicting where these are likely to hit is difficult. Whilst more commonly known interpolate earthquakes typically occur nearby plate boundaries, Australian earthquakes are what is known as intraplate earthquakes. These occur within the interior of a tectonic plate and result from weaknesses within the plate itself. Whilst some areas such as Adelaide appear to be more seismically active compared with others, Sturgess says predicting where and when quakes will occur is fraught with difficulty. For this reason, it is important that buildings in all areas are designed to withstand earthquake forces.
This is particularly critical for buildings which are less than forty storeys in height. Above this level, Sturgess says requirements in respect of wind design are such that design measures which are needed to achieve the required wind load resistance will also be sufficient to also achieve required levels of earthquake resistance without additional measures needing to be applied for earthquakes.
For buildings of less than forty storeys, however, he says this is not the case. With such buildings, specific measures in respect of earthquakes are needed to ensure that earthquake resistance requirements referred to above are met.
Page says the need for earthquake design can be seen through the Newcastle quake. Whilst there was an earthquake loading code included within the state’s building regulations at the time, he says the city had previously not been deemed to have any risk and earthquake design within the city’s buildings had not been required.
As things happened, thirteen fatalities and four billion dollars’ worth of damage occurred. This could have been worse. The quake struck between the quiet Christmas and New Year period during the morning. Of the thirteen who died, eleven were killed in the Workers Club building which collapsed. That night, the auditorium within that building was scheduled to hold a rock concert for a large number of people.
Such an example demonstrates what could happen should an earthquake strike a populated area in cases where buildings had not been designed for earthquake events, Page says.
Asked about how earthquakes impact buildings, Page uses the analogy of an item sitting on top of a bowl of jelly. When the jelly moves backwards and forward, so too does the item atop it but with a lag. This generates forces within the building that need to be transferred down to the ground and for which clear load paths are needed to ensure that this can happen effectively.
Asked about Australia’s current performance in designing buildings and structures for earthquake resistance, both Sturgess and Page agree that buildings which are constructed under current NCC provisions should deliver a robust level of protection. Speaking particularly of concrete buildings, Sturgess says this is particularly the case after revisions to the concrete standard last year involved significant design changes for earthquakes.
Nevertheless, both point to a challenge with older buildings. Many of these either may not have had any specific seismic design attached to them or may have been designed for earthquakes but not in an adequate manner. Many older buildings designed in the 1970s and earlier, Page says, may not have been adequately designed for earthquakes – especially masonry buildings. Some ambulance and fire buildings in older towns, for example, may potentially be unsafe in a quake.
On this score, Page and Sturgess would like action in several areas.
First, Sturgess would like to see building owners required to strengthen their buildings to meet a minimum standard for earthquake resistance as part of the approval process for a change of use of that building. Whilst bringing old buildings up new building standards may not be practical, Sturgess would like an approach similar to that in New Zealand where buildings undergoing renovation need to be brought up to between 33 percent and 67 percent of the seismic capacity required by new buildings.
As things stand, Australian Standard AS3826 sets out minimum requirements in respect of strengthening existing buildings for earthquakes in accordance with the strength limit in each state. As well, Sturgess says many consultants also insist to their clients that buildings be strengthened up to the level specified in this standard.
Nonetheless, the standard is not referenced in the NCC. Accordingly, any owners who have their buildings upgraded for seismic resistance do so by choice rather than being forced to do so.
Page would like to see a review of the earthquake capacity of post-disaster buildings such as hospitals. Whilst this was a recommendation contained within a 1990 report of an Engineers Australia Committee which had the support of the NSW Government and on which he sat after the Newcastle earthquake, Page is unsure whether or not this actually happened.
Beyond this, Page says there are several ways that buildings could be made to be safer which are not overly expensive. As an example, he says free standing elements such as parapets or chimneys which have substantial mass and could fall to the ground in an earthquake event could be secured.
Sturgess, meanwhile, would like better alignment between the concrete standard and the masonry standard. Whilst the aforementioned new version of the concrete standard AS3600 has delivered improvements in concrete design, he says this has created misalignment between this and the standards required for masonry design under AS3700 which has not had any recent updates in respect of seismic design. This, he says, means that design requirements for concrete structures are more onerous compared with those for structures designed with reinforced blockwork.
As well, there needs to be greater education and awareness about earthquake design procedures among property owners. Experience in Christchurch showed a misunderstanding in thinking that engineers were designing buildings to be functional in the aftermath of an earthquake, Sturgess said. In fact, buildings were being designed simply to avoid collapse and loss of life. This did not mean that the building would not require demolition if a quake happens.
Now that awareness of this is growing, Sturgess says some building owners in New Zealand are having buildings designed in such a way that there are elements which absorb the shock which can be replaced rather than having the building completely demolished.
Whilst not lying on a tectonic fault line, Australia does have earthquakes.
To protect building occupant safety, designing for these is critical.