New, Safer Designs Reduced L.A. Damage, Engineers Say
ALMOST every big earthquake reveals something new. The Jan. 17 temblor in suburban Los Angeles was no exception.
Preliminary indications are that seismologists may have uncovered a new fault in the fissure-ridden southern California crust. Engineers, meanwhile, have gotten a first-hand look at how their buildings and bridges held up. Their initial conclusion: not too bad under the circumstances.
``It's the sort of damage you'd expect for an earthquake of this size in a populated area,'' says Ian Buckle, deputy director at the National Center for Earthquake Engineering Research at the University of Buffalo. At least 30 fatalities were blamed on the moderate quake.
``Most definitely, it's not the `Big One,''' adds Larry Braile, a seismologist at Purdue University. The quake measured 6.6 on the Richter scale - much less powerful than the 8.0-plus temblor that seismologists have long predicted for the region.
``I'm in no way minimizing the disaster,'' says Jim Drago, press secretary for the California Department of Transportation (Caltrans). ``But we're very encouraged by what we've seen initially.... We're on the right track of providing additional safety when we have a major seismic event.''
Caltrans officials are encouraged because the bridges they retrofitted survived, apparently intact. Of the six areas in the San Fernando Valley where bridge spans collapsed, four were pre-1971 structures that had not been retrofitted. The other two were constructed in 1976, when a stricter building code was in effect. But these were not designed to take such shocks because engineers were unaware that a fault existed in the area.
``That was an unmapped earthquake fault that ruptured,'' Mr. Drago says. So engineers will now reexamine some 200 bridges in the vicinity to see if they're correctly engineered for more seismic activity along this fault.
``We're dealing with fairly large forces here,'' says Anne Meltzer, professor of earth and environmental sciences at Lehigh University. ``You can't necessarily predict all the stresses involved.''
Building a structure that is absolutely earthquake-proof is often economically prohibitive, says Leroy Emkin, founder of the Computer-Aided Structural Engineering Center at the Georgia Institute of Technology. ``The bottom line is: Society determines how much safety it's willing to pay for.''
The biggest problem is usually not new structures. These are generally built to the latest and strictest codes. The old buildings and bridges usually cause the most damage. ``It's an extremely hard problem to deal with because of the costs involved,'' says Robert Crosson, a seismology specialist at the University of Washington. How much should one pay to retrofit an older building against an earthquake that may not happen? And who pays - the current owner or taxpayers?
Ironically, it was the 1971 Sylmar earthquake, which destroyed several highway bridges very near Monday's temblor, that caused Caltrans to embark on its 20-year retrofit program. ``It was really the 1971 earthquake ... that really was a wake-up call,'' says Robert Whitman, professor emeritus of civil and environmental engineering at the Massachusetts Institute of Technology.
By 1990, Caltrans had retrofitted 1,262 bridges at a cost of $54 million. And the department is still not finished. Two other quakes showed engineers that another 900 bridges might be at risk. The bridge spans that collapsed Monday along Interstates 5 and 10 were due to be retrofitted this year, Drago says.
One method of retrofitting is to fix steel jackets around bridge columns, strengthening the joints while allowing them to flex at the same time. Bridges in the San Fernando Valley that were retrofitted this way survived Monday's temblor.
``Is there a better way to do it?'' asks Dr. Buckle in Buffalo. ``Some of us believe there is.''
Instead of tying down the structure more securely, a contrary technique called ``base isolation'' is being worked on by Buckle's lab and several California labs. This theory allows the structure to move freely so as to alleviate stress during violent shaking. Since 1986, researchers have built some 50 ``base-isolation'' structures, including three bridges in California. But none of them has been located close enough to a major earthquake to provide convincing evidence, Buckle concedes.
``It's certainly a technique that we're interested in,'' Drago says. But in the hard-knocks school of earthquake engineering, it will probably take a major earthquake to prove its value.