The collapse of a portion of the upper deck of the San Francisco–Oakland Bay Bridge following the Loma Prieta earthquake in 1989 was a dramatic illustration of the critical need for seismic safety on bridges across California.
By M. Abraham
Now, a group of engineers from the UCLA Henry Samueli School of Engineering and Applied Science are shaking things up here in Los Angeles in the name of earthquake safety.
On Tuesday, Aug. 22, civil and environmental engineering professors Jonathan Stewart and John Wallace and their team of researchers laterally loaded a full-scale $1 million bridge foundation near Los Angeles International Airport (LAX) to the point of failure in a quest to improve engineers’ knowledge about how bridges react in earthquakes.
“These kinds of tests show us how bridges actually behave under realistic conditions, so we can use what we learn to help develop safer future designs,” Stewart said. “Lots of previous tests have been conducted with reduced-scale models, but with those, you’re still essentially guessing at how the real thing will react. Many full-scale tests have also been conducted, but not to the point of failure, which is what did here.”
The concrete bridge foundation, which stands 5 feet above ground and reaches 25 feet into the ground below, is surrounded by 6-foot hydraulic cylinders that have a stroke (or push-and-pull range) of plus or minus 3 feet, and can move about 450,000 pounds each. The cylinders can mimic a small quake or can push the structure to endure “the big one.” The cylinders exert roughly 2.4 million tons of force.
“We’ve built this full-scale bridge foundation and employed sophisticated instruments so we can better understand what happens when we load it to destruction,” Stewart said. “Our team has been working on loading the foundation for some time, and so far we have only moved it about a quarter of an inch. We expect that as we continue to load it with more aggressive
simulated earthquakes that it will give about 4 inches, which is substantial when you’re talking about buildings or bridges reacting to a temblor.”
Structural loads develop due to earthquake shaking and cause stresses, deformations and displacements in structures, which are then analyzed to improve future building designs. Overloading of the structure during an earthquake is common, but the level of damage that results depends on the structural design. Ultimately, Stewart and Wallace hope their earthquake research will help all bridges to be designed more safely and economically.
“Knowing exactly how we need to design bridges to withstand earthquakes takes much of the guesswork out of it, which means we can fine tune how engineers build,” Stewart said. “We can save lives, and secondly, save money.”
The bridge foundation earthquake research has been ongoing for the past five years and is being conducted in conjunction with CalTrans, which funded the project. CalTrans engineers have been on site throughout the project, which ends with this final test. Students and researchers from UCLA Engineering’s George E. Brown, Jr. Network for Earthquake Engineering Simulation, or NEES, also are participants. The site is located at Imperial Highway and Sundale Avenue, near LAX.