Swinburne University of Technology - Melbourne Australia

Researchers engineer Australia for earthquakes

Story by Rebecca Thyer

For two of Australia’s leading seismic engineers, the demolition of part of an inner-Melbourne housing estate provided a rare chance to do what engineers dream about: push a building sideways until it collapses, essentially testing how well it performs in earthquake-like conditions.

Their opportunity came about when, through their involvement in a collaborative Australian Research Council (ARC) Discovery Project involving the University of Melbourne and Swinburne University of Technology, they heard that some of Carlton’s historic four-storey walk-ups were being demolished. As researchers, as well as engineers, Professor John Wilson and Associate Professor Nelson Lam jumped at the chance of adding to their earthquake knowledge.

Professor Wilson, director of Swinburne’s Centre for Sustainable Infrastructure, says opportunities to assess buildings’ earthquake performance do not come along often. So, with the University of Melbourne’s Associate Professor Lam, students from both universities and one of the Victorian Office of Housing’s engineers, Fiona Hodges, he set about engineering an earthquake.

First, the buildings were demolished to first-floor level. Then by adding weights and applying force in a horizontal direction, the buildings were made to sway – simulating the typical action caused by an earthquake. By pushing the buildings sideways, the team tested how much displacement the structures had; that is, how much they could move without collapsing.

“We were pleasantly surprised to find they had more displacement capacity than we had predicted, so we are now using that information for further investigations and to model this behaviour,” Professor Wilson says.

Although earthquakes might seem a low priority to many Australians, they do pose a significant risk. Australia’s major insurers regard them as sufficiently risky to annually transfer $200 million to $300 million to re-insurance companies overseas to offset their exposure. The re-insurance companies rate an earthquake in Sydney within their 20 top risk exposures worldwide.

“Australia is not silent when it comes to earthquakes,” Professor Wilson says. “We have a magnitude five earthquake somewhere in Australia every year and a magnitude six every five years.”

Because a lot of these happen in remote areas, damage is limited. “But when they occur in or near an urban area they can be quite catastrophic. The 1989 Newcastle earthquake was a magnitude 5.6 earthquake yet killed 11 people and caused $2 billion worth of damage. On a world scale it is a small earthquake, but it shows what happens if you have a bullseye event.”

That earthquake provided the impetus for the long-time collaborators’ research, which is helping to provide an Australian context to earthquakes and the damage they cause. It will also help assess whether Australian buildings meet a new earthquake building standard.

Adopted in January 2008, the standard means all new buildings, with the exception of private houses, will have to be designed to withstand earthquakes.

Associate Professor Lam says the objective of the ARC project is to provide realistic predictions on the performance of buildings deemed vulnerable by the new standard and, through that, whether retrofitting is needed.

“It is not really helpful to housing authorities and the like to say: ‘All these buildings need to be pulled down because they do not comply with new earthquake standards’,” he says. “This is impractical. Instead, our research helps contribute to general knowledge by saying that many of these buildings are OK, they just need some improvement.”

Both Professor Wilson and Associate Professor Lam believe the work is crucial given that the vulnerability of many Australian buildings is rated using information better suited to regions of high seismicity, such as New Zealand and California.

“It is important for us to have an Australian focus,” Associate Professor Lam says. “It is why we have been working on both the behaviour of buildings and on the physical properties of Australian-type earthquakes.”

He says the work has been multi-disciplinary. “We understand how the ground motion behaves and are studying intensively how structures relate to these ground motions.”

Initially the team looked at brick buildings because they suffered – and therefore caused – the most damage in the Newcastle earthquake. Now, through the ARC project, soft-storey buildings are being investigated.

Soft-storey buildings have an open space at the bottom – often used for car parks and retail space – and rely heavily on slender columns to resist horizontal forces at the ground floor level. Without structural walls to act as bracing elements in an extreme event, they are banned in high-seismic regions because large earthquakes cause column entries to fail and buildings to collapse.

Associate Professor Lam says the new Australian building standard does not prohibit these buildings, but contains little information that exclusively addresses this construction type. “Our ARC Discovery Project started after the draft of the new standard was completed, but our research will potentially contribute to future editions.”

Soft-storey buildings are common in Australia, so the project team set out to investigate the buildings’ potential seismic performance and whether they met the new earthquake standard. However, it was work with the Office of Housing that really created the means to test existing buildings’ performance.

Swinburne and the University of Melbourne were asked by the Office of Housing to check some of the office’s high-rise buildings to see if they complied with the new standard.

Ms Hodges says there was no seismic code when the buildings were built and although they met all engineering standards at the time, “we wanted to check how they performed according to the new code. So that is how this whole thing started.”

She says the demolition provided a “once in a lifetime opportunity” to calculate buildings’ responses to earthquakes.

Professor Wilson says the team essentially wants to establish buildings’ displacement capacity. “If you push them sideways, how many millimetres can you keep pushing them till they suddenly collapse? This is what the demolition project allowed us to do.

“From an earthquake perspective, displacement collapses buildings. If a building drifts too much then the gravity loading takes over and it collapses. It’s all about stability.”

Professor Wilson says by the project’s end in 2010 the team hopes to be able to say that although soft-storey buildings are not suited to high-seismic regions, they have their place in low to moderate-risk regions. “In Melbourne and other cities, building standards for wind will often be enough to protect buildings from earthquakes. I think we may be able to confirm this.”

 “What we have found so far is that buildings that were once thought to be very dangerous or vulnerable are not really as bad as they seemed,” Associate Professor Lam adds.

Soft-storey buildings are also very popular in Asia and both Professor Wilson and Associate Professor Lam are working with colleagues in Singapore and Hong Kong on earthquake impacts. “The work will have relevance in Asia where areas are very urbanised and have lots of soft-storey buildings.”

New standard

All new buildings in Australia, with the exception of private houses, will have to be designed to withstand earthquakes following the adoption of new building codes specially created for Australian conditions.

Traditionally, historic seismicity has been used as a predictor of future earthquake events and, on this basis, building standards have been developed. However Professor Wilson, who chaired the committee that designed the new Australian Earthquake Loading Standard, says the earthquakes in Newcastle and Tennant Creek highlight the need to upgrade the building code.

“It will mean that engineers build buildings that are sufficiently robust to tolerate any significant ground shifting, by either increasing their lateral force capacity by making them stronger or by making them more flexible to withstand displacement.”

Earthquakes: energy unleashed

Earthquakes result from a sudden release of strain energy in tectonic plates, which has accumulated from the relative movement of the plates on the earth’s molten mantle.

About 90 per cent of earthquakes occur on plate boundaries and are known as interplate earthquakes. The remaining 10 per cent occur away from the plate boundary and are called intraplate earthquakes.

Although Australia is far away from its tectonic plate boundary, the continent still experiences earthquakes because its plate is very compressed – it is moving northwards at about 70 millimetres each year.

Professor Wilson says that just north of New Guinea, Australia’s plate hits the Pacific plate. “So it is under massive compression and we don’t know where the next earthquake will hit or how big it is going to be.”

Earthquakes in Australia can be considered low-probability, high-consequence events, he says. The largest earthquake recorded in Australia was a magnitude 6.9 and occurred in WA. The 1988 Tennant Creek earthquake highlights how much there is to learn about earthquakes in regions of low to moderate risk, such as Australia. “In 1988 we thought Tennant Creek was so stable that it was considered as a site for a nuclear waste depositary. But then it had three 6.3 to 6.8-magnitude events in 24 hours,” Professor Wilson says.