Engineers Plan Quake Test for Nimitz Freeway

Civil engineers at UC Berkeley have reached an agreement with Caltrans that will allow them to simulate the recent 7.1-magnitude earthquake on a block-long segment of the double-decked Nimitz Freeway in Oakland.

A one-mile section in the Cypress structure portion of the freeway collapsed onto the lower roadway during the Oct. 17 earthquake, killing 41 people. The rest of the structure is scheduled to be torn down by the end of the year.

But before that happens, the researchers will use a large rotating weight and powerful hydraulic jacks on a piece of the still-standing portion of the roadway to reproduce the shaking encountered during the temblor in an effort to learn why the Cypress structure failed.

In a second phase of the project, Caltrans engineers will reinforce the concrete columns of the structure by jacketing them with steel reinforcements. The engineers will then attempt to shake the bridge to the point of destruction.

“We’re going to put instruments on the freeway and study the hell out of it,” said James E. Roberts, chief of Caltrans’ structural division.

The block-long segment to be tested will first be cut away from the rest of the freeway to prevent the type of domino effect that is believed to have occurred during the quake. “There’s no way we are going to bring down the rest of the Cypress structure,” said UC Berkeley civil engineer Jack Moehle, who is leading the effort.

And the test will probably not be very dramatic, he added. “We don’t want to stir up a lot of dust, we don’t want to make a lot of noise, we don’t want an unpleasant reminder of the quake,” Moehle said. Because the upper deck will be shored up with timbers, he noted, the farthest it could possibly drop will be six inches. “There’ll be a small thud, at best,” he said.

Researchers have offered a number of potential explanations for why the upper deck of the Cypress structure collapsed onto the lower roadway, and most believe a combination of factors contributed.

The composition of the soil under the structure probably amplified the wave motions caused by the earthquake. Those wave motions probably also occurred at what is called the resonant frequency of the roadway, which caused further enhancement of the shaking in much the same way that a small push at the top of a swing’s arc will send the swing much higher.

But the key factor in the structure’s collapse, Moehle reported two weeks ago, was that the columns supporting the upper roadway were not adequately attached to the lower roadway. When the columns broke, they straddled the lower roadway and allowed the top to collapse.

The new tests are designed to determine the role of each of these factors in the failure.

In the first phase of the project, the researchers will mount off-center weights on a vertical shaft attached to the upper roadway and rotate them to induce vibration. Separately, they will repeatedly push sideways on the roadway with 500-ton hydraulic jacks. Measurements obtained during this test will be used to construct computer models that can be used to study the safety of other bridges.

Caltrans engineers will then reinforce the columns and the tests will be repeated with more powerful thrusts applied. Neither phase should collapse the structure, Moehle said.

“We only have one shot at this, so we’ll take care not to collapse the unstrengthened structure,” he said. “And once we strengthen it, I don’t think we’ll be able to collapse it.”

The tests will be invaluable, he added, “because for the first time we will be able to study the safety of retrofits (reinforcements)” using actual structures.

The studies are expected to begin in about a month and be completed by Christmas.