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Seattle Builds ‘Earthquake-Proof’ Bridge

Wednesday, November 23, 2016

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A bridge being built in Seattle will be the first large-scale application of a new set of structural materials that will be “earthquake-proof,” bending and flexing in the event of a quake, then returning to form, so it remains usable.

The bridge, part of the massive SR99 project (which also includes the 9,270-foot-long tunnel being bored by “Big Bertha”), will utilize rebar made of a nickel-titanium shape-memory alloy that returns to its original form after being stressed. This rebar will be used in the top five feet of the columns holding up the deck.

Seattle bridge off-ramp under construction
Bridge images: WSDOT, CC BY-NC-ND 2.0, via Flickr

The new SR99 off-ramp bridge will utilize rebar made of a nickel-titanium shape-memory alloy that returns to its original form after being stressed.

The concrete used in the same section of the bridge will be made from a low-crack concrete made with polyvinyl fibers in the mix.

Decades of Research

The engineered composite that should allow the bridge to make it through a quake unscathed is a product of the research of Dr. Saiid Saiidi, of the University of Nevada, Reno, where, for more than 30 years, the Center for Civil Engineering Earthquake Research has been developing materials and plans to help structures survive seismic activity.

Saiid Saiidi
University of Nevada, Reno

The engineered composite is a product of the research of Dr. Saiid Saiidi, of the University of Nevada, Reno.

Saiidi was the founder of the American Concrete Institute’s Committee on Earthquake Resistant Concrete Bridges, and is a fellow of the ACI and the American Society of Civil Engineers. The Seattle SR99 bridge is the first major real-world example of this particular development in practice.

While many shape-memory alloys stretch and return to form with heating, the nickel-titanium alloy is “superelastic,” stretching and rebounding on its own. According to engineers from Saiidi’s lab at the University of Nevada, its elasticity is 10 to 30 times that of steel. 

According to Seattle TV news station KING-5, a 5-foot bar of the nickel-titanium alloy can stretch 5 or 6 inches and then return to its original form. “With this bar, it bends out, then comes back,” Saiidi told the station. Similar spans tested in the lab were able to bounce back from the equivalent of a 7.5-magnitude earthquake successfully.

Preventing Damage, Collapse

Earthquake engineering of bridges took center stage in the U.S. after 1989’s Loma Prieta earthquake, in which portions of San Francisco’s Bay Bridge fell in, and one motorist was killed. Traditional earthquake engineering in the past has largely sought to limit damage and prevent complete collapse of structures, but failures still require a great deal of rebuilding, and limit travel and emergency response in the immediate aftermath of a quake.

The idea behind the new Seattle bridge is that the span will quickly return to form, be subject to a post-quake inspection, then will immediately be returned to service, allowing emergency responders to use it during post-quake deployment.

SR99 offramp bridge under construction

The earthquake-proof bridge will support the northbound off-ramp from SR99, taking motorists from the highway toward downtown Seattle and the city's sports stadiums.

“The place can function. We don’t have to shut down traffic, we don’t have to have detours, traffic congestion,” Saiidi told KING-5.

Because the structure is the first of its kind, it will be subject to close scrutiny by officials, who will monitor the concrete to ensure it stands up to weathering as expected.

About the SR99 Project

The location of the SR99 project, known as the Alaskan Way Viaduct Replacement, is in an area that’s vulnerable to seismic activity; the replacement project began because of damage from the 2001 Nisqually quake, a 6.8-magnitude temblor with its epicenter near Olympia. That was the most recent major quake in the area.

The earthquake-proof bridge will support the northbound off-ramp from SR99, taking motorists from the highway toward downtown Seattle and the city's sports stadiums.

The project includes the boring of the lengthy tunnel by the “Big Bertha” borer, which has faced significant delays since starting its mission in 2013. As of Tuesday (Nov. 22), Big Bertha was 5,774 into the 9,270-foot dig. That means the borer is at the deepest point of the dig, about 265 feet below the surface.

The dig is scheduled to be complete in June 2017.

   

Tagged categories: American Concrete Institute; Asia Pacific; Bridges; Colleges and Universities; concrete; EMEA (Europe, Middle East and Africa); Engineers; Infrastructure; Latin America; North America; Program/Project Management; Research; Roads/Highways

Comment from John Fauth, (11/23/2016, 8:37 AM)

How closely related are "earthquake proof" and "unsinkable"?


Comment from M. Halliwell, (11/23/2016, 11:19 AM)

John, I couldn't agree more... it is more accurate to call this one "earthquake resistant", assuming everything works as designed, not "earthquake proof". It's like stainless steel (no, it doesn't mean it won't stain or rust, it just does it less) or a bullet proof vest (which are rated to prevent penetration by a projectile to a certain level of energy...I've seen what a .30-30 will do to a "bullet proof" vest made to stop pistol bullets). Still, nice to see innovations like this being integrated into new construction...hopefully it does mean better survivability and access in the event of an earthquake.


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