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SR 99 project
A view north toward downtown Seattle shows the future northbound SR 99 off-ramp bridge to South Dearborn Street. The new ramp is under construction just west of Seattle’s stadiums. (Via WSDOT Flickr)

The potential collapse of the Alaskan Way Viaduct during a major Seattle earthquake is one of the main reasons why the aging elevated roadway is being replaced by a new SR 99 tunnel. Above ground, an exit ramp bridge that is part of the new highway could still be shaken by an earthquake — but technology being employed in its construction could keep it from becoming impassable.

According to the Washington State Department of Transportation, the bridge being built with memory-retaining metal rods and a bendable concrete composite will be the first in the world to sway during a strong earthquake and return to its original shape.

While the hope has always been that bridges can stay standing rather than collapsing during an earthquake, the new design aims to achieve a “no-damage” standard so that bridges can remain open after a disaster to allow emergency vehicles and eventually the public and commerce to keep flowing.

SR 99 project
(Via WSDOT Flickr)

WSDOT says the project in Seattle — which will take northbound drivers into the city at South Dearborn Street — is the first real-world test after 15 years of research by the Earthquake Engineering Lab at the University of Nevada, Reno. Bridge columns subjected to a magnitude 7.5 earthquake test return to their original shape because they are built using memory-retaining nickel/titanium rods and a bendable concrete composite.

A new video from WSDOT calls it a “small bridge with a big promise” and Tom Baker, a bridge and structures engineer for WSDOT, says the ability to get out of an earthquake with no damage would be a “giant leap forward” if it comes true.

The specialized materials are up to 90 times more expensive than standard steel and concrete, WSDOT says, adding that the Federal Highway Administration approved the construction and a federal grant paid for much of the additional costs.

The off-ramp is scheduled to be completed next spring. The tunnel itself, currently being dug by the boring machine Bertha, is looking at a possible spring breakthrough at the northern portal, after its nearly 2-mile journey.

 

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