Based on pioneering research, an offramp being built near the new Highway 99 tunnel in Sodo is designed to be usable as soon as the ground stops shaking — a standard few U.S. bridges can meet.
A new offramp rising near Seattle’s rickety Alaskan Way Viaduct could represent the future of bridge-building in earthquake country.
The bridge is the first in the world with a new type of column that flexes when the ground shakes, then snaps back to its original position. The goal is a structure that not only survives a quake without collapsing but also sustains so little damage that it can be used immediately.
“That’s a game changer,” said Tom Baker, state bridge engineer for the Washington State Department of Transportation. “If this performs the way we expect, it will be a big leap forward in structural design.”
The project is based on research by Saiid Saiidi, an engineering professor at the University of Nevada, Reno (UNR), who has spent more than three decades looking for ways to make bridges more earthquake-resistant.
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Construction methods have improved significantly since the double-decker viaduct on Seattle’s waterfront was completed in 1953. But most bridges in the U.S. are built with a limited seismic objective in mind: Don’t fall down.
“The state of the art is to design bridges so that under strong shaking from an earthquake, they do not collapse, so people don’t get killed,” Saiidi said. If a bridge is impassable for months or so damaged it has to be torn down, it’s considered an engineering success as long as it didn’t topple and kill anyone.
But impassable bridges will hinder emergency response and lifesaving assistance after a quake, and delay the region’s economic recovery.
“Our transportation network is the linchpin of our response,” said Robert Ezelle, director of the Washington Emergency Management Division. “If we can’t get our transportation networks open, we don’t have the ability to move critical supplies like food, water, medical aid and fuel.”
The Federal Emergency Management Agency estimates as many as 1,000 bridges in Washington and Oregon could collapse or be severely damaged in a magnitude 9 earthquake on the offshore fault called the Cascadia Subduction Zone, leaving many communities isolated and inaccessible.
Saiidi is among a growing number of engineers who argue that critical structures like bridges can and should be designed to remain usable after an earthquake.
“I’m saying: What if we take this to a higher level, meaning that not only do we prevent collapse, but we make the bridge so that it doesn’t have to be closed to traffic?” he said.
Bendable bars, concrete
His approach relies on reinforcing bars crafted from an unusual metal that bends, then springs back, coupled with fiber-reinforced concrete that can also bend without cracking.
“That’s what makes this bridge so unique,” Saiidi said.
Rods of nickel-titanium “shape-memory alloy” were coupled with conventional rebar in the tops of the two columns that support the Highway 99 offramp near Safeco and Century Link fields.
The alloy is most commonly used for orthodontic braces, twistable glasses frames and arterial stents that can be compressed for insertion, then spring apart to open clogged blood vessels.
Saiidi and his colleagues built a 110-foot model of a similar bridge and rattled it on the shake table at UNR’s Earthquake Engineering Laboratory. Subjected to mock earthquakes that buckled ordinary rebar and caused concrete to crumble, columns built with the new technology were barely damaged.
Saiidi said the tests and modeling give him confidence that the bridge will be able to withstand the level of shaking expected in Seattle from a Cascadia megaquake or a quake on the Seattle fault.
The resilience comes at a price. The shape-memory rods cost 90 times more than conventional rebar, and the bendable concrete is four times more expensive than the ordinary version. But the materials are only used in the tops of the columns — the most quake-vulnerable spot — so the innovations added only about 5 percent to the overall cost, Saiidi said.
That premium, along with the cost of testing and design, was covered largely by a $400,000 grant from the Federal Highway Administration.
The use of high-tech materials is only one approach to resilient bridge design — and some of the others are less costly.
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University of Washington engineering professor John Stanton and his colleagues developed a design that incorporates high-strength cables with rebar to create columns that snap back upright after being shoved sideways by an earthquake. The cost is low, because the approach relies on conventional materials and prefabricated components.
“In my opinion, it’s a better way to go than these shape-memory alloy bars, because they are wickedly expensive and extremely hard to work with,” Stanton said.
California’s Department of Transportation has two bridges designed to be passable within days of a major quake, said Mark Mahan, Caltrans chief of earthquake engineering. The most famous is the new Oakland Bay Bridge. The bridge’s single tower has two legs, designed to move independently in a quake, and a pistonlike structure that prevents sections of bridge deck from crashing into each other, Mahan explained.
The Benecia-Martinez Bridge, northeast of San Francisco, was built with lightweight reinforced concrete and tall, pliant columns anchored in bedrock to minimize damage so emergency vehicles will be able to get across soon after a quake.
Bridges in Japan and parts of Europe are commonly designed to ride out quakes with little damage, often through the use of flexible bearings or pads. But the idea of building bridges to a higher standard has yet to catch on in the U.S. — even though it makes economic sense, Saiidi said.
“You could come out way ahead by spending a little more in the beginning, knowing that if you have a strong earthquake, you don’t have to repair the bridge and close it to traffic,” he said.
The 400-foot, two-lane bridge where Saiidi’s approach is getting its first real-world test is part of the $3.1 billion project to dismantle the seismically vulnerable viaduct and replace it with a tunnel, interchanges and waterfront boulevard. The tunnel itself is designed to withstand the strongest shaking likely to occur in a 2,500-year period, Baker said.
Located near the south entrance of the tunnel, the ramp will connect with South Dearborn Street and the Sodo area. It’s a good choice for the seismic project, because it’s big enough to provide a valid test of the technology — but not so crucial that unanticipated flaws would be disastrous, Baker said.
“The proof will be in the pudding,” he added, ”on the actual performance in an earthquake.”
While it’s possible to build new bridges that will remain usable after a quake, it’s much harder — and prohibitively expensive — to retrofit older bridges to such a high standard, Mahan said.
Old bridges across the Northwest remain highly vulnerable to earthquakes.
WSDOT has spent about $200 million since 1991 to strengthen more than 400 bridges so they won’t collapse — but the agency expects many of those bridges will be severely damaged in a major quake. Almost 500 bridges have yet to be upgraded.
The state also has yet to complete a so-called lifeline route between Joint Base Lewis-McChord and Paine Field in Everett, along portions of Interstate 5, Interstate 405 and Highway 99. Twenty-seven bridges and multiple ramps and overcrossings still need to be retrofitted to create a corridor that could be opened to emergency traffic within three to seven days of a quake.