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EARTHQUAKE: THE LONG ROAD BACK : Lessons from the Past Hold Up Well : Caltrans Gets Confirmation of Its Quake-Proofing Remedies as Most Retrofitted Overpasses Withstand 6.6 Jolt


Engineers combing through collapsed freeways in the wake of Monday's Northridge earthquake say they knew how to prevent the destruction but simply ran out of time before disaster struck.

Although the last three major quakes--Sylmar, Whittier Narrows and Loma Prieta--all revealed significant deficiencies in freeway overpass construction, they also taught Caltrans and industrial engineers important lessons about quake-proofing vital arteries, and those lessons seem to have been learned well, experts said this week.

All of the 122 Southern California overpasses that had been retrofitted survived the magnitude 6.6 temblor and, with one exception, the 11 that collapsed were on the list for future retrofitting.

The exception was the Mission/Gothic interchange on the Simi Valley Freeway (California 118), close to the quake's epicenter. "If Caltrans had known it was sitting on an active fault, it would have been on the list too," USC engineer James Moore said.

The overpass collapses were traced to well-known mechanisms and "the retrofit remedies that have been and are being installed could almost certainly have saved all of these bridges," said structural engineer Nigel Priestly of UC San Diego. All the bridges in Los Angeles were scheduled to have been retrofitted by the end of next year.

Nearly all of the bridge failures in Monday's temblor could be traced to fracturing of the shorter columns supporting the roadbeds. Studies of collapses in previous quakes have shown that the rigidity and brittleness of the shortest columns limited their ability to flex like the taller pillars on the same spans. That appears to be the cause of the failures in Monday's quake as well.

Scientists say that fracturing can be prevented by jacketing the columns in steel. This has been the major thrust of retrofitting in the last five years.

The retrofitting now being done is based on knowledge gained during the three previous large quakes. The 1971 magnitude 6.4 Sylmar quake showed engineers that bridge spans could slide off their supporting columns if the spans were not physically joined together. Since the temblor Caltrans has installed hinge joints--a series of steel cables that hold the spans together--on all bridges in the state, a task that was completed in 1989. Those have worked successfully.

In the 5.9 Whittier Narrows quake in 1987, engineers were stunned when the supporting column on a ramp connecting Interstate 5 and Interstate 605 burst under compression from the quake. That triggered research on so-called single-column bridges, most of them on- and off-ramps, in which each span is supported by a single column in the center. That research led to the technique of jacketing the columns in steel and concrete for extra strength.

Close on the heels of Whittier Narrows, the magnitude 7.1 Loma Prieta temblor in 1989 put the spotlight on overpasses with supporting columns of different lengths.

The relationship between the two types of columns is roughly similar to that of the reed and the oak tree, Priestly said.

In a windstorm, the reed escapes damage by bending with the wind. It can be bent nearly parallel to the ground without being damaged because of its flexibility, thereby riding out the fiercest storms. But the oak tree resists bending, and thus absorbs the full force of the wind, eventually shattering.

A similar situation occurs on freeway overpasses. Tall pillars bend with the sideways movement caused by a quake, but shorter, more brittle ones resist bending. Because all the pillars are linked by the bridge, however, the bending of the tall pillars causes most of the force to be transferred to the short pillars, which are not designed to absorb such forces.

Therefore, they shatter, losing their load-bearing ability and causing the overpasses to fall. That seems to have happened at nearly every overpass that failed in Monday's temblor, Priestly said, including the interchange of Interstate 5 and the Antelope Valley Freeway (California 14) in Sylmar.

Some geologists have speculated that unusual vertical acceleration of the ground at that interchange caused the pillars to punch through the overpass, "but that is not our reading of what happened there," Priestly said. "The short column adjacent to the south abutment attracted a very high shear force. It had little transverse (sideways) reinforcement, so it failed, lost its load-bearing capacity and dragged the rest of the structure down."

The solution to the problem is simple, experts say, and is virtually the same solution as that for single-column bridges. After the 1987 Whittier quake, engineers began jacketing the columns in steel-reinforced concrete to bolster them against cracking. By preventing large chunks from shearing off, the jacketing renders the short columns more flexible, allowing them to bend as much as five inches, compared to a maximum of about one inch before retrofitting.

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