Governor’s Quake Panel Gets Down to Work : Disaster: The goal is to find out why the Nimitz Freeway and the Bay Bridge collapsed, and to help shape the design of new structures.

Gov. George Deukmejian on Tuesday convened his special commission to investigate the collapse of the Nimitz Freeway and the Bay Bridge in the Northern California earthquake, asking the board to determine why the failures occurred and whether they could have been prevented.

“The people of our state are entitled to a comprehensive, thorough and objective examination of the failure of these structures,” Deukmejian told the panel. “Your goal is to find out the facts and to give us recommendations for steps that we should take so that this tragic chapter in our state’s history will not be repeated.”

The Board of Inquiry, as the 11-member panel is officially known, is expected to examine whether the state did all it could to strengthen the elevated portion of the Nimitz Freeway in west Oakland against the forces of earthquakes. The 32-year-old structure was built before a 1971 earthquake in the San Fernando Valley prompted state engineers to rewrite their seismic design standards for bridges.

The board’s report, due by June 1, also will help shape the state’s design of any new freeway structure built to replace the collapsed portion of the Nimitz. And the findings will guide an accelerated program to reinforce older bridges throughout the state, including several in San Francisco that are similar in design and vintage to the Nimitz.

The governor’s panel is chaired by retired Caltech professor George Hausner, a pioneer in earthquake engineering. It also includes professors from UC Berkeley, UC Davis, Caltech and USC, as well as two private engineers and three federal officials.

Top Caltrans officials told the board that they still are not sure exactly what caused the collapse of the Cypress Viaduct on the Nimitz, which killed 42 people. But they said they tend to agree with preliminary reports that blamed the collapse on the failed connections between the double-decked highway’s upper and lower columns.

James Gates, chief of the department’s seismic and structural analysis unit, said it appears that the failure started toward the northern end of the 1 1/4-mile-long structure and moved north and south from there. In almost every case, the column joints cracked and gave way in the same manner.

Gates said the department is analyzing thousands of still photographs taken after the quake and will review California Highway Patrol interviews with scores of witnesses. A team of engineers is still at the site examining pieces of the structure as it is demolished.

As for the Bay Bridge, James Roberts, chief of Caltrans’ structures division, said a 50-foot span of the bridge collapsed after the structure moved 14 inches back and forth during the shaking, finally coming to rest five inches from its original position. The movement sheared two dozen inch-thick bolts, opening a gap through which the top span fell onto the bottom.

The composition of the soil under both the Bay Bridge and the freeway is believed to have played a role in their collapse, because soft soils are known to amplify the ground motion caused by quakes. But the Caltrans engineers and geologists from the state Division of Mines and Geology gave different interpretations of the effect the soil had on the Nimitz disaster.

Gates of Caltrans pointed out that most of the damage was on the portion of the freeway built on pilings sunk 50 to 60 feet through soft soil into hard sand. The collapse ended abruptly very near the point where the soil structure changed, and only minor damage occurred where the pilings were just 10 feet into the harder ground, he said.

But Anthony Shakal, supervising geologist for the Division of Mines and Geology, told the board that the connection between the damage pattern and the soil structure could be a coincidence. While the soft soils under the freeway amplified the ground shaking to a level four to five times that found on solid ground nearby, Shakal said the entire flat coastal area of Oakland experienced the stronger motions, not just the collapsed portion of the freeway.