Civil engineers at the University of Washington are testing a novel concrete framing system that may be cheaper and more earthquake-resistant than existing approaches for framing large buildings. The system employs concrete columns and beams reinforced with steel cables stretched like rubber bands along with conventional steel rebar that enable a building to ride out an earthquake with minimal damage.
"It works like a car's suspension system," says John Stanton, professor of civil and environmental engineering, who was part of a group that conceived the new system. "The stretched steel cables act like shock absorbers to pull a building back to its original position after an earthquake, and the steel rebar does the job of dampers to keep the building from continuing to vibrate. It's the combination of the two that makes the system work better than anything else out there."
Stanton, working with fellow civil engineering professor Greg MacRae and several students, is conducting three tests for Charles Pankow Builders, Ltd. The California-based firm hopes to use the new system this June in a 40-plus story building in San Francisco. The tests are required by the American Concrete Institute because the use of stretched steel cable, or post-tensioning, in concrete framing currently is not approved by the Uniform Building Code.
The first test, conducted Feb. 2, placed an interior column-beam joint under weight similar to that of a large building and tugged it side to side to simulate the effects of ground motion during an earthquake. Similar tests will take place Feb. 26 and in late March on an end joint and a corner joint, respectively, which undergo different stresses than an interior joint during an earthquake. If all three tests satisfy the American Concrete Institute criteria, as Stanton expects, Charles Pankow Builders will be approved for using the new concrete framing system in all configurations comparable to those of the tests. Other builders wanting to use the system will have to undertake similar evaluations until enough tests have been done to satisfy the professional industry.
"Since the system violates so many parts of the existing building code, there's a considerable amount of resistance to it among engineers at this point," Stanton says. "Ironically, it's the use of un-bonded steel cable post-tensioning that is forbidden by the building code but which is largely responsible for the superior performance of our system."
The steel cable is threaded through a hollow duct in the center of precast concrete columns and beams. During construction, the cable is anchored at one end of the building then stretched through a series of joints to another anchor. It's this stretched cable, or post-tensioning, which allows the building to snap back to its original position after an earthquake, Stanton explains. Traditional concrete beams and columns are reinforced with only steel rebar, which usually is bent by significant ground motion and leads to permanent structural damage.
Another advantage of the new system is that it allows builders to use precast concrete beams and columns rather than materials cast at the construction site. This saves time because the building frame can be erected more quickly, according to Stanton. It also can save money, he says, because precast materials have a better finish than materials cast on site and don't require additional glazing or cladding when used on the outside of a building.
"You get economy and safety at the same time, so there's not the usual tradeoff," says Stanton. "Once these advantages are understood more fully, I expect this system to be accepted readily by builders in seismically active regions."
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