An article appearing in the February 18, 2000 issue of the
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by Monte Basgall
Henry Gavin's shaking table hums loudly in the Hudson Engineering Building's basement as the table's veins fill with hydraulic fluid. He keys a few numbers into the device's computer controller, and with an sharp crack, the hydraulics jolt the table to mimic at one-fifth-scale the wallop of the 1994 Northridge, Calif., earthquake, which killed 57 in the Los Angeles area.
Gavin, an assistant professor of civil and environmental engineering at the Pratt School of Engineering, has a multitude of such quake reenactments stored in his computer, drawn from seismic data taken during the actual earthquakes. His important purpose in mounting these elaborate recreations is to test the mettle of his experimental new breed of shake suppression devices for buildings.
His quake-damping systems, which could make the difference between building failure and survival, depend on "electrorheological" (ER) and "magnetorheological" (MR) materials substances that can change abruptly from free-flowing liquids to spongy or solid consistencies when subjected to electrical or magnetic fields. In full-scale shake-suppression systems, the materials would be stored in basement chambers that would electrically or magnetically activate themselves within milliseconds after a temblor strikes.
Like water frozen in mid-slosh, or fruit juice flash-thickened into a sudden gel, these fluids could be engineered to change their viscosity just enough to absorb much of the building-damaging energy as it strikes.
At least that's his hope, said Gavin, who obtained the shaking table with a research equipment grant from the National Science Foundation. He also won a highly competitive Career Award from the NSF to broaden his investigations of ER and MR materials for vibration suppression in structures during earthquakes.
He has since received additional related support from the U.S. scientific funding agency and its Japanese counterpart for a joint program that will allow him to test his evolving experimental devices in 2001 on very large scale machines in Japan.
ER and MR materials both consist of suspensions of microscopic particles in oils. These particles roughly a 15th of the diameter of a human hair in diameter are normally randomly oriented in the oil. However, when exposed to strong electrical or magnetic fields, they become oriented in a particular direction, a phenomenon known as "polarization." Once polarized, the materials quickly align themselves in the fields into fiber-like structures that mesh together and solidify.
Gavin is designing devices in which ER or MR materials could sit inactive for years in a liquid resting state, then transform back and forth between solid and liquid during an earthquake to suppress the building's vibrations. The suppression systems would be mounted within the lowest floors of buildings in proximity to rubber bearing pads and other compliant structures engineers already are installing to passively absorb shocks in earthquake-resistant structures.
"Passive structures like rubber pads are designed to disconnect the building from the ground, so that the structure is isolated from the trembling earth," Gavin said. "However, if we get too much deformation or deflection between the building and the ground, then these bearing pads can rip apart."
To prevent such structural ruptures, we want to try to limit that displacement," he said. "But as you try to limit the displacement, the building is required to carry more of the earthquake's force."
His research seeks to retain the best of both worlds, he said. Buildings still would be constructed with passive measures like pads that could tolerate moderate deflections. But a parallel system employing ER or MR materials would also absorb part of a quake energy using its controllable damping and stiffening properties.
The fluids would be stored in a container, typically the kinds of hydraulic cylinders used as shock absorbers. The sudden force changes from an earthquake would pressurize the fluid, forcing it to flow through narrow spaces.
Simultaneously, sensors would react to the earthquake vibrations by activating electrical or magnetic fields in those spaces. And the viscosity of the ER and MR materials could be adjusted by changing the strengths of the fields in response to the intensity and frequency of the shaking.
"There are challenges in implementing these materials," he said. "When they're in their so-called solid state, it doesn't take near as much stress to deform them as it would steel or aluminum or concrete. In other words, they are relatively weak.
"There are significant drawbacks in both of these materials. It's relatively simple to make mixtures that exhibit electrorheological characteristics, but to make materials that are really useful requires quite a bit of chemistry.
"I think we're at the very beginning of developing something that will probably be in much wide spread use after some very fundamental and persistent materials science issues are resolved. This general area is in a very young stage now."
For now, Gavin and his earthquake control research team, currently including one senior undergraduate and three graduate students, continue to test out their experimental ideas on his basement shaking table.
"What we see in the news are the worst examples of disaster, and there are many structures that are partially damaged that don't make as good a news story, but could be significantly helped by this kind of research."