Tests Prove Reinforcing Could Have Saved Nimitz

Engineering tests on a still-standing segment of the Nimitz Freeway in Oakland have confirmed that a relatively simple strengthening of a mile-long elevated section could have prevented its collapse in the Oct. 17 Bay Area earthquake, Caltrans officials said Tuesday.

The technique, which involves attaching steel reinforcement to the double-decked structure’s concrete columns, was never applied to the Nimitz because engineers considered the structure to be one of the safest in the state until the 7.1-magnitude earthquake turned it into a stack of concrete pancakes, killing 42 people.

But now California Department of Transportation engineers are preparing plans to use the retrofit technique on five Bay Area freeway segments similar to the Nimitz which remain closed because of fears that they, too, might collapse in a major quake. Information gleaned from the testing also may be applied to other freeways throughout the state.

“We now have physical proof that the retrofitting concept works,” James Roberts, chief of Caltrans’ structures division, said in a statement released by the department. “We can move forward confidently to apply this seismic strengthening to the other structures.”

The test results and recent comments by Caltrans officials appear to undercut what the department said in the immediate aftermath of the disaster: that the Nimitz Freeway columns were never strengthened because the theoretical technology to do so did not exist. In October, Caltrans officials said repeatedly that any such retrofitting was expected to take years to evolve out of research under way at UC San Diego.

That claim was disputed by engineers in the private sector and at universities, who said the technology existed to strengthen the Nimitz but the state had never thought of applying it. And now, after the quake emboldened Caltrans engineers and prompted state officials to provide more funds for research, UC Berkeley professors have applied existing technology developed by Caltrans to confirm that the structure could have been saved.

Three different retrofit schemes were designed to strengthen the joints connecting the upper and lower portions of the concrete columns. Engineers agreed that the joints were the weak link that caused the columns to break apart and fall, leaving no support for the massive upper deck, which fell upon the lower lanes.

In the testing, engineers attached steel bars, plates and beams to the columns and then subjected the structure to the kind of vibrations and movement that could be expected in a major quake. They used hydraulic jacks to apply more than 4 million pounds of lateral force to the structure, moving it more than 10 inches. The force applied in the test was greater than that caused by the earthquake, Roberts said.

“The structure did suffer some cracking, but did not collapse under the extreme forces,” Roberts said in the prepared statement.

Development of this kind of retrofit was not planned before the Bay Area earthquake, when Caltrans was focusing its effort on elevated structures supported by a single line of columns. But even if the research had been envisioned, Caltrans spokesman Jim Drago said, it would have taken much longer to complete without the availability of an existing structure to use for the testing. Without a chunk of the Nimitz to act as a guinea pig, engineers would have had to test their theories on a model.

Officials said plans for retrofitting the closed Bay Area freeways should be finished by the end of the month, when they will know how much it will cost. The work is expected to be done this spring.

Caltrans officials said the Nimitz Freeway testing was the biggest experiment of its kind ever conducted. In addition to confirming the retrofit technique, UC engineers were able to collect information that they will use to develop advanced mathematical models for further theoretical studies on elevated structures statewide.

“The knowledge we have gained from these tests may be applied to other bridges across the state built before 1971,” Roberts said. Structures built after 1971 have more advanced designs developed after a major earthquake in the San Fernando Valley.