Coming to the south portal: A “flexible bridge” to stand up to Seattle’s next big earthquake
When the SR 99 tunnel opens to traffic, drivers on northbound SR 99 will use a new off-ramp near the stadiums to reach downtown Seattle. In addition to linking travelers to their destination, engineers and researchers hope the new ramp will provide a link to something else: earthquake-resistant bridges.
By combining memory-retaining metal rods and a bendable concrete composite, the future off-ramp will become the first bridge in the world built to sway with a strong earthquake and return to its original shape.
The video below explains how this exciting new technology works, and this folio provides a detailed technical description of the materials being used.
Years in the making
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.
WSDOT hopes the new technology will represent a giant leap forward in bridge technology. Engineers design structures for no-collapse, but in the future, they could be designing for no-damage. This would enable them 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 a 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. WSDOT engineers and researchers plan to monitor the project closely to see how it performs under everyday wear and tear.
And while no one wants an earthquake, this bridge will be important to study when the next quake hits.