Oct. 7, 1999 Recent devastating earthquakes in Turkey, Greece and Taiwan point out the need for structural controls in buildings that could reduce the impact seismic events have on structures. Such controls could save lives and millions of dollars from building and infrastructure damage.
A professor of civil engineering at Washington University in St. Louis has tested a new device on a model building set atop an earthquake-simulating "shake table" that shows promise in minimizing damage in earthquakes.
Shirley Dyke, Ph.D., assistant professor of civil engineering in Washington University's School of Engineering and Applied Science, tested an earthquake-damping device called a magnetorheological (MR) damper on a six-foot tall metal structure. The test was performed in the university's Structural Control and Earthquake Engineering Laboratory, which she directs. Using data from a standard 1940 El Centro, Calif., earthquake, she applied the force parameters of that quake to reproduce the effects on the model. She compared the test results of the structure without the MR dampers, and then with the dampers.
Measuring responses on every floor of the building, Dyke found that the MR dampers reduce the peak acceleration by 50 percent.
"That's very encouraging," says Dyke. "Our purpose was to demonstrate that these devices are effective for response reduction, and we showed it clearly at this scale. Our idea is to start with small-scale tables and then to try it with a large shake table. Now, we're also working on defining which types of structures would benefit from MR dampers, and we eventually will design guidelines for them."
Dyke presented a paper on the shake table research at the IEEE International Conference on Control Applications, held Aug. 23-27, 1999, in Kona, Hawaii. Her work is sponsored by the National Science Foundation.
From liquid to solid
The MR damper is a device that acts like a shock absorber on a structure. Three horizontal metal plates are sandwiched together, with the outer two plates connected to one end of the building, the middle one connected to the other end.
When the shake table's hydraulic system moves the building, the middle plate slides back and forth between the two outer plates. Sensors are attached to the building's floors to measure the swaying when the shaking occurs. This data is relayed immediately to a computer that calculates where to turn the power on and put the dampers to work to lessen the shaking.
A MR fluid coats the middle plates and turns into a solid when a small electrical current is applied from a battery. This makes the three plates stick together, reducing the shaking. The liquid turns into a solid because iron particles in the fluid join together from a magnetic field created by the electrical current.
The whole process happens in fractions of a second, and when the current is removed, the solid turns back to liquid. The process thus dampens the vibration so that it cannot accelerate through and up the floors of a building or structure.
"The device holds great promise in offering the best possibility of implementation and acceptance by the engineering profession both for new construction and retrofit applications," says Dyke.
The damper in the model building fits in the palm of the hand. In a real building, the damper would be about the size of a cardboard packing box and would be inserted inside the wall of a structure.
The concept of the fluid was developed in the 1940s, but it wasn't until the late 1980s that researchers began seeing applications for it, Dyke says. Lord Corp. of Cary, N.C., developed the fluid for a device that would help dampen the vibrations long-distance truck drivers are exposed to. Lord Corp. also supplies materials for Dyke's research. Other current applications are in exercise equipment and washing machines.
The MR damping technology is inexpensive, relatively straightforward and will operate on just 20 to 50 watts of power -- the amount used by a household light bulb. And if electrical power is cut in a major quake, the device would still work because it's powered by a battery.
"We've demonstrated that the dampers work and now want to investigate more thoroughly to the point of developing design guidelines so that someone developing a 20-story structure will know how to best implement this technology to minimize damage during an earthquake," Dyke says.
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