On October 17, 1989, a 7.1 magnitude earthquake struck San Francisco, causing part of the upper level of the city’s heavily trafficked Bay Bridge to collapse, among other damage and casualties. Similarly, in 1964, a 7.6 magnitude quake struck Niigata, Japan, causing a nearly total collapse of the Showa Bridge there.
Bridges also have collapsed under the weight of major quakes in China, Chile, Turkey, and other countries. That’s why a group of researchers from various universities, funded by the National Science Foundation (NSF), are working to ensure that fewer (or no) bridge structural failures will happen in the future with the development of new structural materials based on what are called shape memory alloys.
The current combination of concrete and steel is financially and typically structurally sound for quakes below 7.0 on the Richter scale. But bridges made from this material don’t hold up as well for stronger quakes because bridge columns rely on the steel and concrete to dissipate energy during earthquakes of this size, said Misha Raffiee, an undergraduate student at the California Institute of Technology, in an
article on the NSF website. This can create permanent damage, rendering the columns weak and unstable or even unusable.
“Under earthquake loading, engineers allow for damage in column hinges to dissipate energy and prevent total bridge collapse,” she wrote. “While that practice is widely accepted, the effects of hinge damage can interfere with disaster recovery operations and have a major economic impact on the community.”
To remedy this, engineers have been seeking stronger materials that can withstand the impact of stronger quakes. Specifically, a project led by civil engineer M. Saiid Saiidi of the University of Nevada in Reno, which Raffiee also collaborated on, discovered that nickel titanium, or nitinol, could be such a material. The material is currently used predominantly in flexible eyeglass frames.
The reason SMAs are good materials for this purpose is that they can withstand severe strain and yet still maintain their original state, either through heating or superelasticity. Nitinol, in particular, stands out among the SMAs tested by researchers in that it doesn’t need a heat source to return to its original state as do most SMAs. Instead, it has the property of superelasticity, which makes it between 10 to 30 times more able to stretch under the pressure of an earthquake than typical metals used in bridge structures, such as steel, Raffiee wrote.
Researchers examined three types of common bridge columns, traditional steel and concrete, nickel titanium and concrete, and nickel titanium and engineered cementitious composites (ECC) (which include cement, sand, water, fiber, and chemicals), in order to test nitinol’s performance under stress. They used OpenSEES, an earthquake simulation program developed at the University of California, Berkeley, to model the columns digitally. They tested them, then built physical column models and tested those on the shake table at the George E. Brown, Jr. Network for Earthquake Engineering Simulation (NEES).
Researchers reported “promising” results of the tests. “The nickel titanium/ECC bridge columns outperformed the traditional steel and concrete bridge columns on all levels, limiting the amount of damage that the bridge would sustain under strong earthquakes,” Raffiee wrote.
One drawback in using the new materials is cost -- it would be about 3 percent higher to use nitinol in bridge construction than the materials currently used. But this cost could be recuperated in long-term maintenance and other benefits in using the material, according to Raffiee. “Not only would the bridge require less repair, it would also be serviceable in the event of moderate and strong earthquakes,” she wrote. “As a result, following a strong earthquake, the bridge would remain open to emergency vehicles and other traffic.”