Seeking Better Ways to Test Quake Safety
California has spent billions retrofitting freeway overpasses and public buildings to make them safer in earthquakes.
But in the absence of a real quake to test their strength, it has been impossible to say for certain how the structures would hold up. Some older regulations thought to protect structures against earthquakes have been proved by real temblors to be inadequate.
Scientists are trying to change that by building elaborate models of freeways and office towers and testing how they would respond to extreme shaking.
UCLA researchers spent $1 million building the foundation of the freeway overpass near Los Angeles International Airport and on Tuesday put it to the test.
They gave it a gigantic shove, pressing 2 million pounds of force against the foundation to see how far it would move.
Scientists were surprised by the results: The makeshift structure -- which looks like a giant block of concrete with metal rods running through it -- held up better than expected.
The data showing that the foundation slipped about an inch and a half on impact will be factored into future computer models of how bridges behave in earthquakes.
Earlier tests at UC San Diego used a massive shake table to jiggle life-size or near-size models of every part of the new San Francisco-Oakland Bay Bridge to make sure that it will hold up during the next big temblor.
One of the most elaborate experiments came last fall, when UC San Diego researchers built a seven-story building and tried to knock it down by replicating the shaking of the 1994 Northridge earthquake.
The work is important, said Tom Jordan, director of the Southern California Earthquake Center at USC, because otherwise the complex computer models used to develop building codes, insurance premiums and even architectural designs would not be grounded in real data.
The problem, Jordan said, is that there is a huge variation in how the soil and structures on it will react to any given earthquake. Simple mathematical models -- in which, say, it is assumed that a building will move twice as far if it is hit with twice as much force -- just don’t work.
“Nobody is yet comfortable modeling these things on computers to predict real behavior,” Jordan said. “We’re always skeptical of simulations because materials behave in a very complex way -- especially when subjected to very strong forces.”
Sophisticated as they are, the computer models used to predict how a structure will fare in a quake are mostly theoretical -- which means that when real shaking occurs, a building believed to be safe could fall down, or one thought to be vulnerable might hold up well.
To develop better data, the National Science Foundation has set up a network of 15 universities and researchers throughout the country who are charged with building real-sized models and subjecting them to earthquake-like forces.
Called the Network for Earthquake Engineering Simulation, the agency promises on its website to “revolutionize earthquake engineering research and education.”
The goal, according to Jordan, is to combine the engineering-based results of tests on buildings and infrastructure with reality-based tests of how soil and rock respond to shaking, leading to improved understanding of how the ground and the structures on it will behave.
Then, he said, scientists can run simulations of various types of quakes -- those with fast seismic waves and slow ones, those that move the earth vertically or horizontally -- and wind up with a better idea of what that means for human beings and the infrastructure we rely on.
“We want to put it all together,” Jordan said.
In the UCLA test, engineering professor John Wallace was surprised to find that the partial freeway foundation he had built to California Department of Transportation construction standards held up better than expected. Even though it moved, and the underground portions probably cracked, it was not shoved out of plumb to the point where a bridge built on top of it would have collapsed.
“Things were starting to crack up and get damaged but it was still taking more force,” Wallace said in an interview Tuesday. “We could have kept testing it but we met the limit of our test equipment.”
The researchers are considering trying again and applying greater pressure to the foundation, Wallace said.
But he cautioned that the data represent only a single effort to move one piece of a complex overpass. UCLA scientists don’t really know what magnitude earthquake the 2 million pounds of force they applied would compare to, or whether the bridge would have collapsed due to problems with another part rather than the foundation.
Frieder Seible, dean of the Jacobs School of Engineering at UC San Diego, said such tests would never be able to provide broad information on how a particular type of bridge or building would survive a big quake.
But, he said, the information can be used to fine-tune current computer models by providing real data on how a structure responded to a particular type of movement.
From there, he said, scientists could use mathematics and physics to extrapolate how the structure might move in a variety of situations, including various types of earthquakes.
“From a statistical point of view, what we’re doing is completely useless,” said Seible, an expert on the seismic safety of bridges. “But we can use it to calibrate our analytical tools.”
Wallace’s discovery that the Caltrans foundation moved less than anticipated is precisely the kind of information that such studies are intended to turn up, he said.
“That’s exactly the kind of result which we are looking for,” Seible said.