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 “shake table” that simulates the juddering caused by an earthquake. Instead of remaining still, however, the tower’s 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.
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 – striking Northridge, California, in 1994, and measuring 6.7 on the Richter scale.
“It looks like a 30-foot lollipop shaking around,” says lead researcher Michel Bruneau, who worked on the project with PhD student Michael Pollino. He says the rocking of the supports is “quite strange because it is not a movement we are used to seeing in a structure”.
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. “You wouldn’t have cars being thrown off a bridge built this way,” he says.
Such a structure would have two main advantages over a conventional one with deep, rigid foundations, Bruneau says. “The 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.”
Bruneau notes a similar rocking design is featured in the Lions Gate Bridge in Vancouver, Canada, but says: “Theirs was a one-off experiment – we are developing a deeper understanding to make it easy for anyone to design a bridge with this technique.”
Angus Low, a director and bridge designer at engineering firm Arup, agrees that the method has potential. “This is an intriguing idea and I can see it being particularly useful for retro-fitting a structure against earthquakes,” he says. “Just working at ground level would be much easier than working on the whole structure.”
Low adds that decoupling a structure from ground movement is a familiar idea to bridge designers, “but it is normally achieved by making the structure able to deform so its movement is not directly linked”.