In last week’s experiment at the university's earthquake engineering laboratory, the bridge withstood multiple simulated earthquakes and showed little signs of stress.
The two-span model was shaken to test the performance of new shape memory materials such as nickel titanium and copper-aluminum-manganese alloys. The shape memory alloys, in contrast to steel rebar used in conventional construction, are super-elastic. They can be distorted about 20 times as much as the steel components they replace before they reach their elastic limits.
The bridge design is the culmination of years of testing and simulations. It combines shape memory alloys, such as nickel-titanium and copper-aluminum bars, with rubber and carbon-fibre shells around the columns and includes special fibre in the concrete to ensure that it remains operational even after devastating earthquakes."We have solved the problem of survivability, we can keep a bridge usable after a strong earthquake," said civil engineering professor Saiid Saiidi. "With these techniques and materials, we will usher in a new era of super earthquake-resilient structures."
The 50t, 21m-long bridge precast and then built atop three 4.25m by 4.25m hydraulically driven shake tables and tested in a simulation that mimicked the 1994 Northridge earthquake in , California.
Researchers used 230 sensors and gauges to monitor the stresses on the bridge and its components. "It had an incredible 9% drift with little or no damage," Saiidi said. "I'm excited to see the results and pleased with how well the bridge performed under extreme conditions. We subjected this bridge to a series of earthquakes, took it apart, and reassembled it before the final experiment. There's a lot of data analysis ahead of us, but the initial result shows success."
The bridge moved more than 150mm off center at the base and returned to its original position, as designed, in an upright and stable position.
The shape memory alloys are expensive, but the shake table experiments show where they can best be deployed to keep costs down while keeping bridges up, according to Saiidi. He has introduced the copper-aluminum manganese alloy, which is more easily machined, as an alternative to higher cost titanium.
