Resilience measures are required to protect structures against earthquakes, wind and fire.
At the hybrid Australasian Structural Engineering Conference 2022 last week, a panel discussion on Engineering resilience for extreme events looked at three overlapping and interactive hazards – earthquake, wind and fire.
In the midst of climate change, resilience can no longer be overlooked when it comes to designing and updating structures.
This is what three pre-eminent experts think needs to happen going forward.
Preparing for earthquakes
Every year, there is a magnitude 5 earthquake in Australia. The challenge, however, is predicting where the next earthquake will occur, says Swinburne University Professor John Wilson, an expert in earthquake engineering.
While Geoscience Australia coordinates much of the country’s seismic hazard work through updated maps of seismicity, they heavily rely on historical events.
“The question is: are past events good predictors of future events? I’d say probably not,” says Wilson. “We [therefore need] to have an element of safety when it comes to designing our structures.”
In 2015, an update to the Australian Earthquake Loading Standard AS 1170.4 included a minimum threshold seismic design coefficient of Z=0.08, with all Australian structures now required to be designed accordingly.
However, with the National Construction Code (NCC) only factoring in new buildings, Wilson says this could be problematic for big east-coast cities such as Melbourne, Sydney or Brisbane.
“When an earthquake strikes, it actually strikes a lot of buildings,” he says. “There’s debate whether we need something extra for a city where a lot of economic damage could result from such an earthquake, because so much of the economy is concentrated in one area.”
As a country in a lower seismic region, the most vulnerable buildings in Australia are masonry structures.
“There has been a trend with our structures to reduce column sizes and wall thicknesses to increase net lettable areas (NLAs) – so we’ve been making our columns work a lot harder,” says Wilson.
Heavily loaded structures don’t have much drift capacity, which can be catastrophic in the event of an earthquake.
“If a structure has sufficient drift capacity to absorb those displacements, it will stand up,” says Wilson. “If it doesn’t, gravity takes over and it collapses.”
The twisted railway line (pictured below, bottom right) which occurred as a result of the 6.5 magnitude Meckering earthquake in 1968 should serve as a warning of the vulnerability of our structures.
“I always find it pretty staggering that in a lower seismic region, you can still get events that do that,” he says.
Windy cities
The serviceability wind speed for Melbourne is about 130 km/hr, with the ultimate limit state wind sitting at around 162 km/hr.
Wind speeds between that range are where resilience efforts should focus, says wind expert Dr Geoff Boughton, Adjunct Associate Professor in the College of Science and Engineering at James Cook University.
“We want buildings to resist wind and still be put back into service reasonably quickly afterwards,” says Boughton. “That’s what resilience is all about.”
This requires buildings to be designed to the limits outlined by the NCC with some additional smarts, so that damage doesn’t compromise functionality.
“The last thing we want is [for] something like a hospital to be unserviceable,” adds Boughton.
Inspection capability is an important part of speedily returning buildings to serviceability. If damage does occur, buildings should also be easily accessible for maintenance and replacement.
“That’s a smart way we can incorporate into design to produce a building that [provides] safety, but also protects itself and maintains its functionality over the course of its life.”
Fighting fire
Fire safety engineering is a relatively young discipline in Australia, says Kate Nguyen, leader of RMIT’s Innovative Fire and Facade Engineering Group.
However, given Australia is one of the most fire-prone countries on earth, this discipline will likely become more prominent.
“When we look at buildings and fire resistance, it’s not just about one element,” says Nguyen.
“We need to understand how a construction material will behave in fire – how it will burn, how quickly it will catch fire and how fast it will burn.”
There is room to understand more about the behaviour of buildings in fire conditions, including through recent RMIT research which aims to understand how different wind speeds and directions facilitate or reduce the spread of external facade fires.
“When we combine the simulation between wind and fire, we use an artificial neural network to improve the accuracy of the simulation,” says Nguyen.
“We use different algorithms to understand how the system behaves as a combination of the influence between fire dynamics and wind loading, so we will be able to better capture that.”
It’s essential for contemporary discussions about extreme weather to factor in cascading and concurrent events, says Nguyen. For example, after bushfires, the landscape becomes drier and more vulnerable, leading to further events such as floods and landslides.
“In the optimisation of the design, we need to consider the multiple risks that buildings may be exposed to during their lifetime,” she says.
Time for a rating system
Since they were introduced by the Green Building Council of Australia in 2003, Green Star ratings have successfully improved the sustainability and longevity of buildings, says Wilson. All three panellists think it’s high time a similar system was introduced in Australia for resilience.
“It’s a complex area, but if you can break it down into its component parts and have a point system, you get a score [which] provides a quantification,” he says.
A resilience rating system would be a movement away from adherence to compliance, says Nguyen, which is “the minimum standards we need to meet rather than the optimum design”.
Rating systems are also familiar to the general population, making them an easy selling point, says Boughton.
“Awarding a resilience rating to buildings would be a great tool that assists in our communication with the people who ultimately have to pay for them.”
When it comes to the cost of building resilience into structures, Boughton says it depends on the building. Let’s take a warehouse that might be designed for low internal pressure as an example.
“If we design that building for higher internal pressures, it might add something in the order of 2 to 5 per cent of the cost of the build,” he says.
According to Wilson, this cost framework is similar for earthquake resilience.
“You can spend a little bit more under the capital cost to end up with a much [more] resilient structure,” he says. “But it has to be done at the conceptual stage and not down the track.”
Planning for the right risk
Considering the different types of risk to which your structure might be susceptible entails understanding environmental vulnerabilities. However, it also depends on the characteristics of the building, says Boughton.
“Something like a large warehouse is generally a lightweight structure [which] doesn’t present a huge earthquake risk, but it can present a large wind risk,” he says. “Whereas some high-rise buildings are much more of an earthquake risk than a wind risk.”
Resilience against wind requires buildings to be as heavy as possible, whereas for earthquakes it’s the opposite.
“For both, [however] you want to tie it together so that it hasn’t got a chance to come apart,” says Boughton. “So, I believe there is some common ground.”
Another important aspect in designing for resilience is where we built. Building locations and the selection of the building site are probably the most important factor in reducing the risk of all the above.