Nimitz Is Pressed for Answers : Quake safety: Engineers shake the tottering Oakland structure. What they learn will help shape the retrofitting of freeways around the state.

The double-decked Nimitz Freeway, or at least what is left of it, suffered a magnitude 5 earthquake Thursday afternoon--for two hours.

No, Mother Nature was not wreaking some strange vengeance on the remnants of the freeway that collapsed and killed 42 people during the magnitude 7.1 earthquake that shook the Bay Area on Oct. 17.

Rather, it was a team of civil engineers from UC Berkeley and Caltrans who were shaking an isolated block-long section of the freeway to determine why a mile-long section of it collapsed during the October temblor. By doing so, they hope to learn how to retrofit other freeways in the area--and throughout the state--so that they are less likely to collapse in a future quake.

“There were many tragedies during the Oct. 17 quake,” said UC Berkeley civil engineer Jack Moehle. “But the greatest tragedy would be if we didn’t take time out to learn from them.”

During the next two to three weeks, the research team will gradually increase the amount of force applied to the Nimitz segment each day until they get up to about magnitude 7.5. Eventually, after the supporting columns have been reinforced using techniques that are expected to be applied to other damaged freeways, Moehle and his colleagues will shake it even harder to see whether they can make it collapse despite the reinforcements. If it does collapse, the engineers will know they will have to explore other types of reinforcement.

But it won’t collapse very far. Construction crews have erected a strong supporting structure under the top deck and, if it falls, it will only fall about six inches or so. “The most anyone will hear is a dull thud,” Moehle said.

Television crews and reporters who flocked to the test site here Thursday hoping to see the structure swaying violently were disappointed. A motion sensor on one end of the freeway, looking something like the antenna of a car radio, swayed back and forth, but the movement of the structure itself was not readily apparent.

During the shaking, workers casually walked around on the top deck, monitoring instruments and videotaping the scene, seemingly oblivious to the simulated quake. Others went on with preparations for the ultimate test, in which powerful hydraulic jacks will force the top deck from side to side in an effort to knock it loose from its foundation.

The force for Thursday’s simulated quake, however, was provided by a deceptively simple apparatus: two off-center weights connected to a two-part vertical shaft axis firmly attached to the top deck of the freeway. As the motor-driven shaft rotated the weights in opposite directions, it applied a sideways force to the freeway similar to that produced by a wet blanket in an out-of-balance washing machine, Moehle said.

The maximum effect occurred when the weights were rotated at what is called the resonant frequency of the structure, about 2.2 cycles per second. At that frequency, each rotation of the weights reinforced the shaking of the structure in the same way that small pushes applied to a swing cause it to go higher and higher.

To increase the size of the simulated quake in the coming weeks, the workers will simply add more weights to the shaft.

Researchers have offered a number of explanations for why the upper deck of the Nimitz collapsed unexpectedly during the earthquake. The growing consensus seems to be that the columns supporting the upper deck were not anchored to the lower roadway firmly enough.

As the earthquake shook the freeway structure, those columns apparently broke free from the lower deck, straddling that deck and allowing the upper deck to fall straight down.

Furthermore, by a quirk of fate, the shaking of the ground in the Oakland area apparently matched the resonant frequency of the structure. That and loose soil under some sections magnified the effects of the quake.

After the preliminary studies now under way are completed, the engineering team will attempt to reinforce the points where the columns are anchored. Moehle noted, however, that they have not yet decided what might be the most effective way to do so.

Proposals include jacketing the column with an additional layer of cement, attaching steel girders to it and tying together the columns on opposite sides of the deck with steel cables.

After all the tests are concluded, Moehle said, engineers will be able to be more confident that repairs being made on other freeways “will really do the job.”