EARTHQUAKE: THE LONG ROAD BACK : State Says No Retrofit Could Have Saved Road : Damage: Officials recant claim that I-5 and Antelope Valley Freeway interchange might have survived quake.

Reversing initial assessments, California transportation officials acknowledged Monday that even if the interchange of Interstate 5 and the Antelope Valley Freeway (California 14) had been fully retrofitted, it probably would not have withstood the shaking it took from the Northridge earthquake.

James Roberts, Caltrans’ chief bridge engineer, said it is doubtful that any retrofitting plan could have strengthened the structures at the interchange sufficiently to hold up under the kind of horizontal movement that occurred during the early morning temblor.

“I don’t think you could retrofit it,” said Roberts. “You’d have to at least take a chunk of the span out and rebuild it entirely.”

He said new data gathered at the site of the interchange collapse indicates that during the quake, the extent of lateral swaying may have been as much as 10 feet, a figure that is three or four times higher than the greatest movement recorded at similar structures in previous California quakes.

He said the retrofit plan being formulated for the interchange before the earthquake anticipated movement of only two or three feet.

Roberts’ comments came as the Assembly Transportation Committee began a hearing to scrutinize the department’s seismic retrofitting program and its plans for rebuilding bridges that collapsed in the Northridge earthquake.

Caltrans Director James Van Loben Sels told lawmakers that with round-the-clock construction work, the department expects to have some of the bridges rebuilt much earlier than initially expected.

Shortly after the quake, the department estimated that the replacement bridges would be ready in about a year, but now Van Loben Sels said some can be completed in six months.

Targeted for that six-month timetable, he said, will be bridges to replace collapsed structures at the Golden State Freeway and Gavin Canyon interchange. He said new bridges on the Santa Monica Freeway will probably be ready in nine months.

It will take as long as a year, he said, to repair the Golden State Freeway and Antelope Valley Freeway interchange because of problems with unstable soils. Likewise, structures on the Simi Valley Freeway will take about a year but in the meantime will be replaced by temporary bridges, Van Loben Sels said.

“We will require our contractors to work 24 hours a day, seven days a week, to get this finished,” he said.

Despite the optimistic rebuilding forecasts, lawmakers at the hearing grilled Caltrans officials on the causes of the freeway failures.

Roberts conceded that engineers were unprepared for the strength of the quake. And he acknowledged that retrofitting alone probably could not have worked.

Had engineers known that the higher forces could strike the bridges, he said they probably would have opted to tear down and rebuild, rather than retrofit, some of the structures.

However, Roberts said he still believes that the state’s retrofitting technology is sound and has proven that it can protect large highway structures from collapse in a major temblor. The exception, he said, is the combination of a strong quake occurring directly beneath a roadway structure.

Most of the bridges that collapsed in the Jan. 17 temblor, in fact, he said, are in this category. They include structures on the Antelope Valley Freeway, at the I-5 and Gavin Canyon interchange and the I-5 and California 14 interchange.

“You literally had a fault zinging right through the middle of it that we didn’t know about,” he said.

As far as engineers know, he said, those are the only bridges in the state that face that kind of danger.

With new construction and design techniques, he said bridges can be built today to withstand those kind of forces.