WSDOT's New â€œFlexible Bridgeâ€ Technology Designed to Stand Up to Seattle's Next Big Earthquake
SEATTLE, WA By combining memory-retaining metal rods and a bendable concrete composite, a new State Route 99 exit ramp taking shape in Seattle will become the first bridge in the world built to sway with a strong earthquake and return to its original shape.
This pilot project is the first real-world test of 15 years of research inside the Earthquake Engineering Lab at the University of Nevada, Reno - one of the top earthquake engineering laboratories in the U.S.
Modern bridges are designed not to collapse during an earthquake, but this new technology takes that design a step further. In earthquake lab tests, bridge columns built using memory-retaining nickel/titanium rods and a bendable concrete composite proved more flexible. The columns were able to return to their original shape after an earthquake as strong as a magnitude 7.5.
"This is potentially a giant leap forward," said Tom Baker, Bridge and Structures Engineer for the Washington State Department of Transportation. "We design for no-collapse, but in the future, we could be designing for no-damage and be able to keep bridges open to emergency vehicles, commerce and the public after a strong quake."
How it Works
Memory-retaining rebar, called shape memory alloys, and bendable concrete composites are not new materials. However, the combination of the two materials allows the bridge columns to return to their original vertical position after an earthquake.
Titanium eyeglass frames illustrate how memory-retaining rebar works. The frames are built to bend and twist, but always return to their eyeglass shape. Large diameter nickel/titanium rods engineered for bridge construction behave much the same way, but on a much larger scale.
The bendable concrete also moves with the memory-retaining rebar in the column. Unlike standard concrete, this engineered composite contains tiny synthetic fibers that keep concrete damage to a minimum when it moves.
WSDOT has been long-time partner of the University of Nevada-Reno in researching materials to improve construction in earthquake sensitive areas.
The future northbound SR 99 off-ramp to South Dearborn Street was chosen partly because it is a small bridge. The shape metal alloy and cement composite only replace a small portion of the regular steel in the bridge, keeping overall costs reasonable.
These specialized materials are up to 90 times more expensive than standard steel and concrete. The Federal Highway Administration gave the go-ahead for this real-world test, and a federal grant paid for much of the additional costs.
The off-ramp is currently under construction and scheduled for completion in spring 2017. After the new SR 99 tunnel opens, this ramp, just south of the tunnel entrance, will take northbound drivers from SR 99 to Seattle's SODO neighborhood.
WSDOT engineers and researchers plan to monitor the project closely to see how it performs under everyday wear and tear. "We're testing this cutting-edge technology in a real-live lab now," said Baker. "We'll examine how it performs with heavy traffic volumes, rain, freezing and thawing cycles and evaluate where we go from here."