New ‘smart-metal’ for earthquake-resistant structures
Super-elastic alloys and bendable concrete composites have been used for a first-in-the-world application in downtown Seattle, USA.
A bridge that bends in a strong earthquake and not only remains standing, but remains usable is making its debut in its first real-world application on a busy downtown Seattle highway. Memory-retaining alloy is the key to success in this pioneering
technology. “We’ve tested new materials, memory retaining metal rods and flexible concrete composites in a number of bridge model studies in our largescale shake table lab. It’s gratifying to see the new technology applied for the first time in an important setting in a seismically active area with heavy traffic loads,” said Saiid Saiidi, civil engineering professor and researcher at the University of Nevada, Reno.
“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.
Saiid Saiidi, civil engineering professor and researcher at the University of Nevada, Reno, at the large test laboratory.
A giant leap forward
The University partnered with the Washington Department of Transportation and the Federal Highway Administration to implement this new technology on their massive Alaska Way Viaduct Replacement Program. “This is potentially a giant leap forward,” Tom Baker, bridge and structures engineer for the Washington State Department of Transportation (WSDOT), said. “We design for nocollapse, but in the future, we could be designing for no-damage and be able to keep bridges open to emergency vehicles, commerce and the public after a strong quake.” Modern bridges are designed not to collapse during an earthquake but this new technology takes that design a step further.
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.”
Memory retaining alloys substantially reduce damage, allowing structures such as bridges to remain open even after a strong earthquake.
Seismic lab trials
In an experiment in 2015 one of Saiidi’s bridge’s moved more than six inches off centre at the base and returned to its original position, as designed, in an upright and stable position. At up to 250% of the design parameters the bridge still had excellent results. “It had an incredible 9% drift with little damage,” Saiidi said. WSDOT has decades of experiences building and refitting bridges to withstand earthquakes. However there are two cutting edge differences with the design of this bridge. Tom Baker explains: “The most crucial factor is reinforcing bars called SMA, Shape Memory Alloy. These bars remember what their original shape was.
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.
Memory-retaining nickel/titanium rods and specialized concrete composite are key to the flexible structures.