When a big earthquake strikes, it’s important that buildings stay standing to protect the occupants. In the past, a building that could stand up to a large quake would have to be demolished afterwards.
But in the last few years there has been a revolution in the way engineers think, and the goal now is to construct low damage buildings that move with the shaking, yet remain intact and fully useable.
When central Christchurch was hit by a devastating earthquake in February 2011 most buildings in the central city – with a couple of tragic exceptions – remained standing. But in the months that followed most of them were demolished. They had done their job of saving lives, but were damaged beyond repair.
In their place a new generation of earthquake resilient buildings is rising from the rubble, many of which are using innovative new engineering solutions.
It’s an active area of research for earthquake engineers, including a world-leading team at the University of Canterbury.
Among their ranks is Rutherford Discovery Fellow Geoff Rodgers, who brings a mechanical engineer’s sensibility to an area that has traditionally been the domain of civil engineers. In 2017, Geoff was awarded the Cooper Award, the Royal Society Te Apārangi’s Early Career Research Excellence Award for Technology, Applied Science and Engineering, for his work in this area.
When it comes to earthquake engineering, Geoff says that the underlying premise is that strength doesn’t equal performance. It’s a “difficult design balance” that has to take into account a complex interaction between the frequency of ground shaking during an earthquake and the period of a building (the number of seconds it takes for a building to naturally vibrate back and forth).
“We can’t modify the way the ground shakes – that’s the hand we’ve been dealt,” says Geoff. “But we can modify the way in which structures respond to that shaking.”
Another design balance is one of cost versus strength. It’s theoretically possible to build something that would withstand a large earthquake and sustain no damage – but it would be prohibitively expensive.
Geoff explains that until recently the approach to earthquake-proofing has been the life safety design principle, or the sacrificial damage approach.
“The thinking has been that in a large earthquake a building can’t be entirely damage free.”
Instead, he says, it was about carefully managing where damage would occur, and ensuring that damage to the structure doesn’t lead to collapse.
Unfortunately what damage occurred was usually inaccessible and unrepairable, and the only option was often demolition.
Geoff says a few different anti-seismic approaches are being explored.
A well-known approach in New Zealand is base isolation, particularly the lead-rubber Robinson bearings that New Zealander Bill Robinson developed, which are used in buildings such as Te Papa.
“There’s also things like friction pendulum bearings which essentially use a concave disc with a puck in them, that transmit forces but allow the building to slide.”
Base-isolation systems allow the building to slide independently of the ground. They are very effective, but suit low horizontal buildings built on flat ground.
Mind the gap
Rocking systems are a technique that Geoff says allow a building to move in a very controlled manner.
“It’s a small amount of gap opening that happens at structural connections.”
In practise this means a steel tendon connecting two building elements such as beams.
In a small earthquake the building acts as a single block, but in a large earthquake the connection opens up, allowing some controlled motion at the joints. Geoff explains that this lengthens the period of the building and “changes the way the building is excited by the ground shaking.”
It’s all very well to design a building that moves – but the occupants don’t want it to move too much, so designs like rocking systems also include damping devices to help dissipate some of the energy that would otherwise add to the swaying and shaking.
Some of these devices are sacrificial - they are readily accessible and can be easily replaced.
Lead extrusion dampers
Geoff has been developing and refining another damper called a lead extrusion damper. He uses the analogy of squeezing toothpaste out of a tube to explain extrusion, but says the key is to make the process repeatable.
You can think of the lead damper as a half-full tube of toothpaste, narrowed in the middle, sitting under a chair leg. As the chair rocks to one side, the toothpaste is squeezed through the narrowing to one end of the tube. As the chair rocks back in the other direction, the toothpaste is squeezed to the other end. It’s a repeating cycle that can happen as many times as needed, and at the end of the process the tube of toothpaste is still intact, and the chair is still upright.
The key is that the lead is relatively soft – although of course, it’s much stiffer than toothpaste!
Viscous fluid devices are another anti-earthquake system that are being used on modern buildings – think of something like mountain bike or car suspension, where the shock absorbers sit in a thick fluid, which in the case of buildings can be anything from oil to a silicon-based fluid. Geoff says that most of these devices for buildings have been very basic, but engineers are looking at optimising the viscous damper for individual buildings.
So there are many potential ways to create safer buildings that will ride out an earthquake and remain fit for use – and Geoff says many of these will be visible in new buildings around Christchurch, especially those on Cambridge Terrace.
“You go in the elevator lobby and the entire rocking frame is visible as are all the dissipative elements. And I think that’s probably due to our recent experience – that the public feel safer seeing the obvious signs of seismic bracing in a building.”
Find out more about earthquake engineering
Alison Ballance meets University of Canterbury earthquake engineer Stefan Pampanin and finds out about the shake table that is used to test new designs.
“Lego but with concrete components’ is how two University of Canterbury earthquake engineering students describe a precast flexi bridge designed to better withstand earthquakes.
And hear how and why earthquakes happen in this look at new findings from the 2016 Kaikoura earthquake.