Rocking bridge jumps clear of earthquake damage
18 May 2007
Earthquake engineers have come up with an ingenious new way to make bridges tremor-proof. Their 'rocking' bridge design avoids damage by jumping right off the ground. The system can even be fitted to old bridges so they'll be safer in future.
Antenna investigates...
Making bridges earthquake-proof takes engineering to its limits, with strength and rigidity normally being the key. But now engineers from the University at Buffalo in New York have successfully tested a novel bridge design that rocks in response to earthquake tremors.
They hope it could be used to construct new quake-proof bridges or be retrofitted to improve existing ones.
The model bridge tower in the lab.
Image: Michael Pollino
How does the design work?
All bridges have towers that support the bridge deck. Usually these towers are securely fixed to deep foundations, especially in an earthquake zone. But engineers' new bridge design has an innovative and counter-intuitive feature - its towers aren't anchored down.
The Akashi-Kaikyo Bridge is the longest suspension bridge in the world. It has two supporting towers with a height of 298 metres above sea level.
Instead, the towers can lift a few inches off land, causing the bridge to rock to and fro. This stops vibrations from an earthquake building up, which would otherwise rip the towers right out of the ground.
The amount of rocking is carefully controlled by grounding structures that guide the towers safely back into position. Shock-absorbent 'dampers' can also be attached to each tower base to help restrict how high the bridge lifts and how fast it travels back into the ground.
A close-up of the model tower base.
Image: Michael Pollino
Shaking things up...
Lead engineer Michel Bruneau explains:
'Our approach is unconventional. Usually engineers concentrate on anchoring bridge towers down in earthquake-prone areas, which can be very expensive.'
'Our approach is unconventional. Usually engineers concentrate on anchoring bridge towers down in earthquake-prone areas, which can be very expensive.'
Michel Bruneau, a civil engineer from the University at Buffalo.
Image: University at Buffalo
The rocking bridge design was tested on this model bridge tower in the university's earthquake simulation lab.
Image: University at Buffalo
The team tested their innovative design on a model bridge tower a fifth of the size of the real thing. 'Once we'd come up with the concept we wanted to test our new theory to see if it worked,' says Michel. |
This is a close-up video of the tower base during testing.
Image: University at Buffalo
Weighing in at 9 tonnes and 6 metres in height, the mock-up was put through its paces in a specially constructed earthquake simulator. At each testing stage the quake shaking got worse, but the tower didn't give way under pressure. Even under conditions as bad as 6.7 on the Richter scale, the tower still held fast. |
Michel thinks that rocking bridges could really help to reduce the damage that earthquakes cause. 'Every bridge is different. But this new method will help us look at the specific conditions of each bridge to work out exactly what sort of structure we need to help make it earthquake-proof,' he says.
'Earthquake damage to bridges is a really big problem. For example, during the 1995 Kobe earthquake in Japan, pretty much every bridge in the area was damaged in some way.'
The Kobe earthquake occurred on 17 January 1995 and caused millions of pounds worth of damage to local buildings and bridges.
The idea of a rocking bridge isn't completely new; a similar design was used to retrofit the Lion's Gate Bridge in Canada a few years ago. But this team's technique has taken the concept to the next level.
Michel explains: 'These previous examples were built on a one-off, trial-and-error basis. What we have done is come up with a systematic approach that is general enough to be used in all sorts of bridge designs.'
Lion's Gate Bridge in British Columbia, Canada, used a similar rocking bridge design.
Ben Russell, Curator of Mechanical Engineering at the Science Museum, thinks the new bridges sound like a good idea. 'This approach to building bridges makes real sense. Allowing the bridge to flex offsets any damage that would otherwise be caused during an earthquake.'
'All sorts of approaches have been taken towards quake-proofing buildings and bridges. This seems to be part of a whole range of new quake-proof designs that are helping protect our structures and is a really fruitful area for research.'
Ben Russell, Curator of Mechanical Engineering at the Science Museum
Michel adds, 'Engineers are starting to warm up to the possibility of using rocking in their designs. We hope that this concept will be increasingly used to help design new bridges in the future, but it's early days yet. This is truly a trend of the future.'