Bridge supports that can rock around during an earthquake instead of remaining fixed to the ground could prevent injuries by dampening movement, according to US researchers.
A team of engineers at the University at Buffalo in the US developed the technique as a way of disconnecting structures from the violent shaking that occurs during an earthquake.
During recent trials, a 9.1-metre-tall steel tower was placed on top of a 鈥渟hake table鈥� that simulates the juddering caused by an earthquake. Instead of remaining still, however, the tower鈥檚 supports were unfixed, so that their feet could move up to 10 centimetres into the air during shaking. In others test, the legs were also connected to viscous dampers. These use thick fluid to reduce the violence of the vertical motion, reducing overall movement to about 5 cm.
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Watch a video showing a 6.1-metre-tall, 8.2-tonne tower . Another video shows a , which is connected to dampers.
Lollipop rock
A large weight was placed on top of the tower to simulate a top-heavy bridge, making the total structure weigh 9.1 tonnes. The shake table beneath was programmed to copy the motion of different types of earthquakes, including the most costly quake ever to occur in the US 鈥� striking Northridge, California, in 1994, and measuring 6.7 on the Richter scale.
鈥淚t looks like a 30-foot lollipop shaking around,鈥� says lead researcher Michel Bruneau, who worked on the project with PhD student Michael Pollino. He says the rocking of the supports is 鈥渜uite strange because it is not a movement we are used to seeing in a structure鈥�.
Adding the dampers can balance the benefits of rocking against the danger of the structure completely toppling over, says Bruneau. In one trial, a bucket of water on top of a tower did not even slide during the simulated earthquake. 鈥淵ou wouldn鈥檛 have cars being thrown off a bridge built this way,鈥� he says.
Cheaper foundations
Such a structure would have two main advantages over a conventional one with deep, rigid foundations, Bruneau says. 鈥淭he very strong forces of a quake are no longer passed on to the structure; it would also be possible to do away with such costly foundations.鈥�
Bruneau notes a similar rocking design is featured in the Lions Gate Bridge in Vancouver, Canada, but says: 鈥淭heirs was a one-off experiment 鈥� we are developing a deeper understanding to make it easy for anyone to design a bridge with this technique.鈥�
Angus Low, a director and bridge designer at engineering firm Arup, agrees that the method has potential. 鈥淭his is an intriguing idea and I can see it being particularly useful for retro-fitting a structure against earthquakes,鈥� he says. 鈥淛ust working at ground level would be much easier than working on the whole structure.鈥�
Low adds that decoupling a structure from ground movement is a familiar idea to bridge designers, 鈥渂ut it is normally achieved by making the structure able to deform so its movement is not directly linked鈥�.
