An expressway off-ramp that will bend in a strong earthquake but remain standing — and usable — is under construction as part of a huge bridge/tunnel project in downtown Seattle.
It marks the real-world debut of two new technologies working together: memory-retaining rebar and flexible concrete composites.
The concept has been pioneered by Saiid Saiidi, a civil engineering professor at the University of Nevada in Reno.
Over time, Saiidi has built and destroyed (in his laboratory) several large-scale bridges, single bridge columns and concrete abutments using various combinations of innovative materials as replacements for standard steel rebar and concrete. He has also incorporated the innovative designs that make use of those materials. All of that has been part of his quest for safer, more resilient infrastructure.
Central to his work is the large-scale shake table in his lab. He’s used it to test new materials, memory retaining rods and flexible concretes in a number of bridge model studies. His work has spanned about 15 years.
For example, he built a four-span model of a bridge using innovative materials, including a concrete composite using glass and carbon fibres, then tested it on the shake table.
To produce an accurate simulation, he fed the computer that controls the table data recorded during the 1994 magnitude 6.9 quake in Northridge, Calif.
During the test, the model was hit with more and more powerful simulated quakes over several days. Eventually it was hit with shaking that produced three times the acceleration of the Northridge quake.
The model survived in good condition.
That test was only one of a series that generated a lot of interest among senior bridge engineers. About 50 engineers and industry representatives attended that test. Another 100 or so watched it live via the web.
I’ve watched a video of one of Saiidi’s bridges as it moved more than six inches off centre at the base and returned to its original position — as designed — upright and stable.
Using computer-controlled hydraulics, the engineering lab can increase the shaking caused by the simulated quake. In the video, Saiidi turned the dial up to 250 per cent of the design parameters and still achieved excellent results.
"It had an incredible nine-per-cent drift with little damage," he said.
Modern bridges are designed so they won’t collapse during an earthquake, but Saiidi’s design goes a large step further. In his earthquake lab tests, his columns returned to their original shape after the shake table generated an earthquake as strong as magnitude 7.5.
Now Saiidi’s design is ready for prime time and its stage is in downtown Seattle.
He says 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."
The university has partnered with the Washington State Department of Transportation (WSDOT) and the U.S. Federal Highway Administration to implement the new design on the replacement of the Alaska Way Viaduct, the centrepiece of which is a two-mile-long tunnel. But there are also bridges and ramps.
One of the off-ramps will be supported by two of Saiidi’s columns.
The design has the potential to be a "giant leap forward," says Tom Baker, a bridge and structures engineer for the department.
"We design for no collapse, 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," he says.
In Seattle, his columns were made using memory-retaining nickel-titanium rebar cages. The bottom two-thirds was filled with conventional concrete. A few days later, the top third was filed with flexible concrete.
WSDOT has made a video showing how the columns are made and posted it on its YouTube channel. If you’re curious, you can find it at http://bit.ly/2g2YPRd.
Korky Koroluk is an Ottawa-based freelance writer. Send comments to email@example.com.