It was an engineer's paradise: modeling earthquake damage by pushing on a highway overpass with the force of hundreds of thousands of pounds.
Listening to the snapping and cracking of concrete and carbon, University of Utah professors and students worked under the old South Temple I-15 overpass Saturday, ever so patiently observing and measuring data in the first-ever experiment of its kind.They showed that a new fabric-like carbon-fiber wrapping can give a bridge the support it needs to survive the kind of damage that is predicted from an earthquake along the Wasatch Fault, said Larry Reaveley, chair of the U.'s civil engineering department.
"This is like Christmas," Reaveley said. "This is better than Christmas."
For the past two years, U. professor Chris Pantelides and his colleagues have been working to show that the carbon-fiber wrap developed in California can extend the life of Utah's older I-80 overpasses before and after earthquakes.
In the second of three tests, they bolted a 25-foot hydraulic jack to the overpass, the only remnant left of the former northbound I-15 lanes at the South Temple junction.
After wrapping three columns and their connecting beam with yards of black carbon-fiber material, they let the hydraulic jack slowly push and pull against the side of the overpass, moving about ten inches in each direction over the six-hour experiment.
The group had to decide what to do when the bridge did not fail at 316,000 pounds late Saturday afternoon, about 16,000 pounds more than the jack was designed to support. They brainstormed with their calculators and paper atop car hoods, scribbling the game plan on the back of a napkin.
They would go until 397,000 pounds, then stop the experiment, they decided. But a later huddle decided that they would keep going to 490,000 pounds of pressure.
Each push-pull cycle took several minutes as the steel piston slowly strained against the bridge and then pulled back.
The jack would simulate the damage that a quake about 7.5 magnitude on the Richter scale would have caused, said Evert Lawton, U. associate professor of civil and environmental engineering.
Two weeks ago, they bolted the jack to the southern side of the overpass but did not wrap those three columns and beam. At about 350,000 pounds, the overpass failed, reaching the maximum pressure that it could be pushed.
Compared to the unwrapped southern side, which yielded at about five and a half inches in each direction, the wrapped northern side was still going at 350,000 pounds, moving about two inches in each direction, a 40 percent increase in strength.
"We are one of the first in the world to do this (experiment) on a natural bridge," Pantiledes said Friday. "We are hoping to have a good impact so that the Utah Department of Transportation and others involved can retrofit I-80 and the freeways."
It is an inexpensive method to prepare for quakes, especially since the state will have difficulty replacing all of the structures as they age, the researchers agreed.
"This is one alternative technology that is fairly cheap," said Steve Bartlett, UDOT research project manager. "This is a way we can get more life out of a structure and strengthen it for earthquakes."
The overpass, about 30 years old, showed noticeable deterioration from the winter salt and freeze-thaw cycles. About 200 measuring instruments were attached to the bridge and foundation, taking data for the experiment, part of a half-million dollar project.
The Wasatch fault runs along the the western base of the Wasatch mountains, from Fayette in central Utah to Malad, Idaho, about 220 miles to the north. A major sized-quake at about a magnitude 7 occurs along the fault in the Salt Lake Valley about every 1,400 years, said Walter Arabasz, director of the university's Seismograph Stations.
The last major quake was about 1,230 years ago, he said.