A rendering of the flexible bridge that Seattle hopes will sustain little to no damage during an earthquake. (WSDOT)
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A Flexible Bridge Bends the Rules of Earthquake Engineering
A new design in Seattle promises to stand strong after being rattled in a violent shakeup.
A good bridge has to endure a lot from Mother Nature. In earthquake-prone parts of the globe, that’s an especially tall task: even if they survive a temblor without collapsing, the damages sustained can still seriously hamper relief efforts by blocking emergency aid and causing traffic mayhem when the bridges have to be shut down for repairs.
What if a bridge could ride out an earthquake without cracking or suffering severe damage? It would have to be flexible, says Saiid Saiidi at the University of Nevada, Reno. And no, not flexible like the doomed Tacoma Narrows Bridge, but one that snaps back in shape, enduring the force of a strong quake.
For nearly 16 years, Saiidi has kept busy inside the university’s Earthquake Engineering Lab, testing a new kind of bridge that uses materials outside the typical steel and concrete. His tests have shown his design can withstand, at the very least, 7.5 magnitude shakeups. Now, one city is about to be the first to bring his idea to the real world.
Located above a fault zone, Seattle has long been bracing itself for—among others—a 7.2 magnitude earthquake that could level much of the city. Experts believe it could happen anytime; the last huge earthquake struck the region roughly 1,000 years ago. That’s a large part of why the city is building a tunnel to replace the old, vulnerable elevated highway known as the Alaskan Way Viaduct. This bridge will sit atop the tunnel, serving as an exit ramp from State Route 99 into the heart of the city. And it has been designed to not only prevent a full-on collapse, but also to sustain little to no damage as it moves.
The smooth, vertical rods in the center section of this column reinforcement is constructed of a metal alloy can will return to its original shape after being bent. (WSDOT)
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Most of the bridge is constructed with steel and concrete—nothing new there. But inside the columns, at upper joints of the bridge, are rods made with a metal alloy of nickel and titanium that automatically returns to its intended shape after being manipulated by outside forces.
“That’s where energy [from the quake] is dissipated to prevent the bridge from being overloaded and collapsing,” says Saiidi. “But there’s a price to be paid for that; the steel bars [in older bridges] stretch too much.” With this special alloy, the bars would still bend as they shake, but they’d return to their original shape, preventing a permanent tilt in the columns that render a bridge unusable.
There’s still the issue of the concrete that reinforces these bars. That’s where Saiid is using a special concrete composite filled with fibers that allows it to bend without cracking and falling apart. Saiidi says as long as the columns are secure, the span of the bridge—and the surface of the road—should be able withstand a quake as well.
Together, the materials are 90 times more expensive than the traditional stuff. But Saiidi says because they’re being strategically placed in the critical areas rather than the entire bridge, implementing this technology will cost only 5 to 10 percent more than a traditional bridge. Plus, he’s currently working on using copper in the alloy to bring the cost down even further. Either way, it’s less expensive than shutting a bridge down for repairs and diverting traffic elsewhere, and far cheaper than replacing a fallen structure. He believes that the cost shouldn’t be a barrier for other earthquake-prone cities to consider his design.
Once fully installed, the state transportation department will monitor everything from its durability and maintenance needs. But the first, real-world test will come when an earthquake strikes. Aside from the 7.2 magnitude earthquake that Seattle is expecting, the city is also threatened by the infamous “Big One” that reportedly has a 1 in 3 chance of striking the coastal northwest with a magnitude of 9.0 in the next 50 years. That’s leaving many West Coast states and cities scrambling to upgrade their aging infrastructure. Many are currently retrofitting their bridges, strengthening the foundation, replacing old support towers, and adding new bearings.
Yet despite $14 million in retrofitting bridges in California, for example, nearly 200 still need major seismic upgrades, according to an investigation by NBC News. On top of that, SFGate reports that after Californians spent $6.4 billion to replace part of the San Francisco Bay Bridge, a “plague” of problems render it unlikely to withstand the Big One. Half of Oregon’s 1,232 bridges have been deemed “seismically vulnerable” by the state transportation department. And in Washington, some 470 bridges still need to be retrofitted, according to the latest data from WSDOT.
Saiidi’s new design represents the start of a changing philosophy among bridge engineers. “In the past 20 years we set out to design bridges that will not collapse, but we also understand that there is going to be damage, and we take that as an acceptable situation,” he says. “Now we are trying take it one step further, not only to prevent collapse by also to minimize damage.” Or, in this case, to prevent damage altogether—at least that’s the hope. He adds that he’s also currently working on developing new guidelines.
He’s not the only working on making bridges indestructible. Whereas Saiidi is finding innovation in the material, a recent paper from University of Warwick in the U.K. lays out a way of building bridges and other structures using a mathematical model that mimics simple patterns found in nature. In bridges, that means that building an arch that follows the way leaves or shells are curved might enable it to withstand more pressure without generating a complex network of stress.
But the reality of all bridges sustaining no damages during seismic events remains uncertain, as Saiidi says there’s still much to learn about predicting earthquakes and gauging their damage. “Keep in mind that the whole bridge design is an evolutionary process,” he tells CityLab. “It’s going to take time before we begin to implement this [new thinking].”
source: City Lab