The engineering behind the new earthquake-proof bridge near Seattle’s SR99 tunnel project

Updated Dec 2, 2016

The bridge being built near the south entrance of the tunnel for the State Route 99 project under Seattle is designed to be earthquake-proof, King 5 reports.

If that project sounds familiar that’s because it’s the tunnel being dug by Bertha, the largest tunnel boring machine in the world and a piece of equipment whose exploits we’ve kept a steady document of here.

The bridge is made of the same materials as other concrete bridges across the nation—concrete, rebar and pre-stressed concrete beams holding up the road deck—but the connective tissue, the joints, are designed to be flexible. Reinforcing bars made from a special metal alloy—a mix of nickel and titanium—will flex and snap back to their original position when the earthquake movement and stress are over.

“Once you let it go, it goes back to its original place, as though nothing has happened. It’s what we call engineered, cementitious composite,” Saiid Saiidi, a professor of civil engineering at the University of Nevada, Reno, who works in the Department of Civil and Environmental Engineering, told the news agency, adding that this bridge is the first full test of his idea.

A special concrete, made in small batches, is filled with millions of poly-vinyl fibers that are coated to bind with the concrete around it and limit cracking. Then, the top 5 feet of the columns that support the beams and the bridge deck are built from the combination of low-crack concrete and shape-holding metal alloy.

“It does not crack easily, and it’s able to develop tiny cracks vs. large cracks,” Saiidi told the news agency. He says the material is expensive and, at current prices, would add 5 to 7 percent to the cost of the bridge, but the price  is expected to drop if the materials become widely used. However, he adds that the costs of damages to a bridge from an earthquake can go well beyond the extra price paid for a flexible bridge.

“The place can function. We don’t have to shut down traffic, we don’t have to have detours, traffic congestion,” Saiidi explained, adding that the new bridge, if widely adopted, would not only show the ability to survive a quake, but would allow the economy and society to recover far more quickly.

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The Washington State Department of Transportation (WSDOT) plans to watch the bridge and examine it to see if the concrete holds up to wear and tear and the weather as predicted, but the real test will come when the next earthquake strikes.

“The vision is really quite exciting,” Tom Baker, head of bridges and structures for WSDOT told the news agency. “And for people involved with engineering, (it will) change the paradigm on earthquakes.”