Engineers have demonstrated that, even after a strong earthquake or hurricane, bridges built with innovative materials and designs are not only still standing, but usable in the aftermath of a disaster. In a large-scale experiment that ended Feb. 6 at the University 's world-renowned Earthquake Engineering Laboratory, a bridge withstood multiple simulated earthquakes and showed little signs of stress. The bridge design is the culmination of years of testing and simulations.
"We have solved the problem of survivability, we can keep a bridge usable after a strong earthquake," Saiid Saiidi, civil engineering professor, said. "With these techniques and materials, we will usher in a new era of super earthquake-resilient structures."
The 50-ton, 70-foot-long higher seismic-performing bridge, designed of pre-cast concrete columns and beams, was pre-cast and then built atop three 14- by 14-foot, 50-ton-capacity hydraulically driven shake tables. It was shaken Feb. 6 in a simulated earthquake, mimicking the large ground motions of the deadly and damaging 1994 Northridge, Calif. earthquake. Researchers used 230 sensors and gauges to monitor the stresses on the bridge and its components.
"It had an incredible 9 percent drift with little or no damage," Saiidi said. "I'm excited to see the results and pleased with how well the bridge performed under extreme conditions. We subjected this bridge to a series of earthquakes, took it apart, and reassembled it before the final experiment. There's a lot of data analysis ahead of us, but the initial result shows success."
The bridge moved more than six inches off center at the base and returned to its original position, as designed, in an upright and stable position. Using the computer-controlled hydraulics, the lab can increase the intensity of the recorded earthquake. Saiidi turned the dial up to 250 percent of the design parameters and still had excellent results.
"This is a highly impulsive earthquake that we simulate," he said. "It is within 10 miles of the earthquake fault that shook Northridge and the Los Angeles area in 1994. This earthquake tends to push the bridge to one side causing a permanent tilt. The innovative materials we use help bring the bridge back to upright position. The simulated earthquake is 10 seconds long."
This bridge combines shape memory alloys, such as nickel-titanium and copper-aluminum bars, with rubber and carbon fiber shells around the columns and includes special fibers in the concrete to ensure that it remains operational even after devastating earthquakes.
The novel materials and techniques used in this experiment are the culmination of nearly 12 years of testing and computer simulations, with six large-scale bridge models, some as long as 110 feet and weighing 100 tons. The bridge design also has the unique feature that allows it to be disassembled and reused instead of heading for the dump when it is obsolete.
The shape memory alloys, in contrast to steel rebar used in conventional construction, are super-elastic. They can be distorted about 20 times as much as the steel components they replace before they reach their elastic limits. They are expensive, but the shake table experiments show where they can best be deployed to keep costs down while keeping bridges up. Saiidi has introduced the copper-aluminum manganese alloy, which is more easily machined, as an alternative to the higher cost titanium.
In the first phase of the project, with three different quarter-scale modular bridge columns, the models were disassembled, inspected and reassembled six times, once for each test. All models exhibited minimal or no damage despite drifts exceeding six percent.
These accelerated bridge construction method experiments, known as ABC, are also part of the University's national Tier-1 Transportation Center project in the College of Engineering led by the Florida International University which, along with Iowa State University, will study other aspects of bridge technology.
Through the Florida grant, this project will be expanded and integrated with the ultimate goal of widespread implementation of the technology in areas with high potential for seismic events, hurricanes and storms, among other disasters, that could affect bridges.
After a strong earthquake or hurricane, a properly engineered conventional bridge may still be standing, and might be usable with extensive, expensive and time-consuming, lengthy repairs, but that is changing with both the new materials and design coupled with the ABC methods that have been evolving over the past few years.
The first real-life application of Saiidi's innovative materials and novel techniques is the construction of a showcase bridge in downtown Seattle. The 400-foot-long bridge, part of the tunnel project that bypasses the downtown area, will use shape memory alloys and spliced connections pioneered in past experiments conducted by Saiidi.
He has been working closely with bridge engineers in the Washington State Department of Transportation in the design of the elevated on-ramps at the ends of the tunnel on their State Route 99. They plan to break ground on the project in March. This is the first time any shape memory alloys have been implemented in any bridge project anywhere in the United States.
"We are the first state to adopt this new method for bridge construction," Bijan Khaleghi, state bridge design engineer in the Bridge and Structures Office, said. "This is a heavily used highway. This is a good approach, especially for structures in areas with high seismic hazard potential."
Khaleghi said he and his agency have collaborated with Saiidi through his work with the National Science Foundation, the Federal Highway Administration and the Transportation Research Board.
"Saiid is a great researcher and we always follow his work," he said. "We're excited to adopt his work for use in Washington."
The current study is funded by the National Science Foundation Partnership for Innovation Program. Four small businesses, two from northern Nevada and two from California, and an advanced materials company from Japan are cooperating with Saiidi on this seismic safety project.
The University's new Earthquake Engineering Laboratory combined with the Large-Scale Structures Laboratory comprise the biggest, most versatile large-scale structures, earthquake/seismic engineering facility in the United States, according to National Institute of Standards and Technology, and possibly the largest University-based facility of its kind in the world.