SACRAMENTO — Caltrans never conducted a complete seismic study of the double-decked Nimitz Freeway, even though computer technology available for more than a decade could have uncovered flaws that contributed to its collapse, a top department official acknowledged Thursday.
State engineers conducted a partial seismic study of the Cypress viaduct in 1977 as part of a program to strengthen the freeway structure in the event of an earthquake. But they never examined the design of the expressway's columns, which gave way in the Oct. 17 earthquake, said James E. Roberts, chief of the department's structures division.
"We were not looking at columns at the time," Roberts said. "That was not our high priority."
Last week, Roberts said the 1977 analysis had shown that the freeway would withstand a major earthquake--but that state officials could not find a copy of the study.
In an interview Thursday, however, he said the missing seismic analysis only examined how to strengthen the roadbed portion of the Cypress structure and was not a comprehensive earthquake study.
Roberts' disclosure is significant because Caltrans officials have repeatedly said they had believed the Nimitz, with its two-level, double-column design, would stand up in an earthquake.
Forty-one people were killed in the collapsed section of the Nimitz Freeway in Oakland.
James H. Gates, a Caltrans structural mechanics engineer who headed the 1977 study, lamented in an interview this week that he and his staff did not discover the danger in the columns when they conducted their analysis.
"The disturbing thing from my point of view is that I didn't spot that problem," Gates said. "The disturbing thing from the point of view of the guys working for me is that they didn't spot the problem.
"We've been trying since 1986 to identify those vulnerable structures. We don't have any excuse. We just weren't able to do it. When you're looking at 15,000 bridges, it's like finding a needle in a haystack," he said.
The first warning the state received that such columns might be unsafe came about 2 1/2 months before the earthquake struck, when a committee of four engineers working as consultants to the department alerted the state that three freeways in San Francisco with a similar design were vulnerable to quakes.
Like the Nimitz, these freeways were supported by columns that were fastened to the roadway with "hinges," which were designed to provide flexibility. In the case of the Nimitz, however, the columns were not strong enough to withstand the 7.1 earthquake that rocked the Bay Area. The three San Francisco freeways were damaged but did not collapse in the quake.
The 1977 analysis of the Cypress structure was conducted as part of the department's earthquake retrofit program, in which older bridges and highways were upgraded to meet new earthquake standards.
For a freeway such as the Nimitz, that meant tying together sections of the roadway with steel cables so they would not come apart in a quake. But strengthening the roadbed may have put even greater stress on the columns and contributed to the collapse of the structure, state engineers have said.
Roberts said a computer program developed in 1978 made seismic analysis much easier and could have helped spot the flaw in the Nimitz structure. But by then, he said, department engineers had moved on to other work.
Because of the quake, the department will now conduct a complete seismic study of the San Francisco-Oakland Bay Bridge at a cost of several hundred thousand dollars, he said. One section of the bridge collapsed in the quake and is undergoing emergency repairs that are scheduled to be completed by Nov. 17.
Roberts said it is unlikely that the 1977 analysis will ever be found, because such records are frequently destroyed after four years. Rather than a formal study, he said, the seismic analysis was a series of calculations for strengthening the roadway.
Gates, in a memo to Roberts earlier this week, said it is not possible to "guarantee" that there will not be fatalities on highway bridges in the event of another major earthquake. But he said the program to upgrade structures had been effective in reducing casualties in quakes during the last decade.
"We have tried to the best of our ability to anticipate the potential damage from future events and feel badly that we did not foresee the collapse at Cypress Avenue," he said in the memo.
Times staff writer Virginia Ellis contributed to this story.