“Using these materials substantially reduces damage and allows the bridge to remain open even after a strong earthquake.” Saiidi, who pioneered this technology, has built and destroyed – in the lab – several large-scale 200-ton bridges, single bridge columns and concrete abutments using various combinations of innovative materials, replacements for the standard steel rebar and concrete materials and design in his quest for a safer, more resilient infrastructure. “We have solved the problem of survivability, we can keep a bridge usable after a strong earthquake,” Saiidi said. “With these techniques and materials, we will usher in a new era of super earthquake-resistant structures,” said Saiidi.
A giant leap forward
In the lab, bridge columns built using memory-retaining nickel/titanium rods and a flexible concrete composite returned to their original shape after an earthquake as strong as magnitude 7.5. “The tests we’ve conducted on 4-span bridges leading to this point aren’t possible anywhere else in the world than our large-scale structures and earthquake engineering lab,” Saiidi said. “We’ve had great support along the way from many state highway departments and funding agencies like the National Science Foundation, the Federal Highway Administration and the U.S. Department of Transportation. WSDOT recognized the potential of this technology and understands the need to keep infrastructure operating following a large earthquake.”
Seismic lab trials
The concept is not new; for instance titanium frames for eyeglasses are flexible and return their original shape when bent. However building a bridge with SMA technology is more complicated. The second factor is that you not only need the bars to return to their original shape, but also the concrete surrounding the bars. We are using a highly flexible concrete which contains a lot of fibres so as it moves it doesn’t crack and fall away.” The idea of combining SMA with flexible concrete in bridges started 15 years ago in the University of Nevada, Reno, testing laboratory. “The tests we have done in the laboratory have really pushed things to the limits,” said Saiidi. “We have simulated very strong earthquakes, well beyond the kind of earthquakes that are likely to occur during the lifespan of the bridge.”
Standard bridges fail under these conditions, but the laboratory tests prove that with shape memory alloys and composites the bridge will move back into its original shape. The Seattle off-ramp with the innovative columns is currently under construction and scheduled for completion in spring 2017.