Quake Lab Tests Building Designs
Scientists use 'shake table' at US earthquake engineering center to study structural stress
EVERY week or so, scientists here in Buffalo, N.Y., strap a scale-model building or bridge onto a movable concrete slab and shake it to smithereens. Or try to, anyway.
The researchers in this earthquake laboratory at the State University of New York (SUNY) are studying how to make structures more quake-proof. The more structures they can test in a simulated earthquake, the more they learn about what works.
Mention earthquakes and people think of California, Japan, Mexico, or Armenia - not Buffalo. Indeed, when the National Center for Earthquake Engineering Research (NCEER) was established here in 1986, it raised eyebrows among seismologists.
But the center is helping to dramatize the global nature of the challenge posed by earthquakes. "This is a worldwide problem," says Ian Buckle, the center's deputy director. "The eastern United States is not immune."
Earthquakes occur along weak zones in the earth's crust called faults. California's San Andreas fault, for example, lies at the juncture of two continent-sized geologic plates, which are locked together under great stress. Periodically, the pressure becomes so great that parts of those geologic plates break loose, causing the surface to shake. More than 90 percent of the world's seismic events occur along such plate boundaries.
But earthquakes sometimes occur in the middle of plates, where the earth's forces have stretched and weakened the crust. Scientists don't know where all these weak points lie.
"In California, they have their faults and they are visible; in the East, we have our weaknesses and they are hidden," quips Peter Gergely, a Cornell University engineer and member of the NCEER executive committee.
Quakes in the East are much rarer than in California (by one estimate, 10 times rarer). But they can be equally devastating. In 1811 and 1812, three earthquakes measuring an estimated 8.0 to 8.3 on the Richter scale hit the area around New Madrid, Mo. (The Richter scale in essence is a measure of the energy released at the focus of an earthquake. The highest recorded value was at point 9.5 on the scale in Chile in 1960.) The quakes were so powerful that they created lakes, blew tons of underground sand o
nto the surface, cracked pavement in Washington, D.C., and woke up Thomas Jefferson at his Monticello home in Virginia.
The eastern US is vulnerable to earthquakes for several reasons. It is more densely populated. It has lots of soft and sandy soils, which can shake like Jell-O or turn into a liquid-like substance during a quake, exaggerating the destruction.
Most important, the region is completely unprepared. Virtually none of its buildings are designed to withstand a major seismic event. "It's the classic problem of low probability and high consequence," says Arch Johnston, director of the Center for Earthquake Research and Information at Memphis State University.
NCEER is trying to change this. By working with a variety of eastern universities and earthquake centers, the organization hopes to improve not only how structures are built but also how communities respond to a major seismic event.
The focus here is engineering.
Inside the giant lab (which looks a little as if it was hit by a small earthquake), a large three-story model building about 20-feet tall stands in the center. Red and blue lines highlight the fractures received from a simulated earthquake. To the side is the huge and moveable concrete slab, called a "shake table," which can be programmed to replicate hundreds of different earthquakes from around the world.
"It feels like a good roller-coaster ride," explains doctoral student Joe Bracci.
These quakes vary not only in strength but in frequency. Most of them cause rapid oscillations, destroying low- and medium-high buildings much more readily than skyscrapers, which swing at a slower pace. But the oscillations of the Mexico City earthquake in 1985 were unusually slow, which damaged several tall buildings and caused at least one 21-story tower to collapse.
Buildings aren't the only challenge for engineers. Bridges and pipelines are also vulnerable. Bridges have to survive a quake in order for emergency vehicles to get around. (See related story Page 12.) Oil pipelines not properly protected could cause a major spill. Water pipelines are also crucial to firefighters. By one estimate, a major New York City quake could cause up to 150 fires to start simultaneously.
For years, states east of the Rocky Mountains ignored the threat. The region hasn't had a major earthquake in the 20th century (although the 19th century was very active seismologically). Now, thanks to the work by NCEER and other centers and several attention-getting earthquakes, eastern states and cities are beginning to take action.
SEISMOLOGISTS and earthquake engineers are excited about New York City, which could approve new earthquake provisions in its building code early next year."If a large city like that will take a stand and say: 'We have to do this,' then we expect other cities to sit up and take notice," says Dr. Buckle of NCEER. One engineering consultant says the city's action is leading similar movements in Europe and Australia.
Other urban areas have taken action or are expected to do so: Shelby County, Tenn., (Memphis); Charleston, S.C.; and St. Louis. States such as Connecticut, New Jersey, New York, and Tennessee are moving to catch up with Massachusetts, which has had earthquake provisions in its building code for years.
The real challenge for engineers, however, is not new buildings but old ones. Making a new building quake-resistant adds only about 2 to 4 percent to the cost, Dr. Gergely estimates. But retrofitting existing structures can cost anywhere from 30 percent to 50 percent of the building's value, he says.
"It's a question of minimizing the cost and maximizing the performance," says George Lee, a member of NCEER's executive committee.
That challenge should keep the scientists busy at Buffalo's shake table for years to come.