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WSDOT finishes earthquake-flexible Highway 99 bridge

When the tunnel opens, the bridge will carry cars downtown


The just-completed South Dearborn Street Off-Ramp Bridge won’t carry cars for a long while—not until the State Route 99 tunnel opens to traffic, likely in 2019. Since the tunnel won’t have any exits downtown, the bridge will eventually take cars away from the tunnel and into downtown starting around the stadiums.

But it’s notable less for its eventual day-to-day traffic as much as its unique, first-of-its-kind engineering features that keep it earthquake-ready.

The bridge’s columns combine shape-memory alloy (SMA) rebar, which returns to its original shape after a disruption, and engineered cementitious composite (ECC), a concrete-like substance that, unlike concrete, contains small fibers that keep it from cracking. The two materials working together minimize damage in the event of an earthquake.

It’s the first bridge in the world to use both SMA and ECC, according to the Washington State Department of Transportation (WSDOT).

Back in November, WSDOT posted a three-minute documentary on the technology. Watch it below.

Earthquake resistance is especially key given what this and the other aspects of the Viaduct Replacement Project are replacing: the Alaskan Way Viaduct, so heavily-damaged in the 6.8-magnitude Nisqually earthquake in 2001 that it requires semi-annual closures for inspection.

The tunnel itself will be built to withstand up to 9-magnitude quakes.