Nimitz Collapse Foreseeable, Report Says


Caltrans would have known that the Nimitz Freeway was likely to collapse in a major earthquake if the department had performed a comprehensive seismic study of the double-decked highway, an independent investigation of the disaster concluded Thursday.

But with funds short and priorities placed elsewhere, the state transportation department never ordered such a study, dooming the Nimitz to collapse in the 7.1-magnitude earthquake that shook the San Francisco Bay Area on Oct. 17, according to the board of inquiry appointed by Gov. George Deukmejian.

Similar research also would have revealed that the Bay Bridge was vulnerable to collapse but might not have found the exact point at which that bridge gave way, the panel said.

The Nimitz collapse killed 42 people, and the 1.5-milelong section that fell has since been demolished. One motorist died in the Bay Bridge failure when a span of the upper deck fell on the lower lanes.

The 264-page report, presented to Deukmejian on Thursday, urged the governor and other state leaders to speed repairs on freeways and public buildings that could collapse in the next major temblor.

"The Loma Prieta earthquake should be considered a clear and powerful warning to the people of California," said the board's chairman, George Housner, a retired Caltech professor.

"We have made progress during the last two decades in reducing earthquake risks," Housner told Deukmejian, "but much more could have been done and awaits doing. More aggressive efforts are needed if the disaster potential of future earthquakes is to be minimized and the public safety adequately protected."

The report specifically recommended that thorough engineering studies be done on the state's major bridges and that seismic safety be a "paramount concern" in the design of all state-owned buildings and structures. The panel also said all private buildings in the state should be subject to seismic safety standards for construction.

Deukmejian said he had "no doubt" that the recommendations in the report would be implemented and that they would "save lives of the people in our state" in a future quake.

The panel of engineers spent six months examining the effects of the October quake, focusing, at Deukmejian's request, on the collapse of the Nimitz and the damage to the Bay Bridge.

Concurring with earlier analyses of the failures, the board said the Nimitz collapsed primarily because the joints connecting the upper and lower columns were too weak. The Bay Bridge failed because 40 one-inch thick anchor bolts were sheared by the force of the quake, causing the bridge structure to separate and a single span to fall.

The engineers said the soft ground under the two structures amplified ground motions from the quake, which was centered more than 50 miles away. But they concluded that the motions were no greater than might have been predicted for a nearby quake of the same intensity.

The report, however, went beyond a mere technical evaluation of the damage to suggest that the failures also were due to Caltrans being too focused on fighting "the last earthquake." The department failed to use state-of-the-art technology to anticipate how future quakes might produce damage not seen in earlier temblors, the report said.

Although the report lauded the effort the transportation agency undertook to make bridges safe after the 1971 San Fernando quake, it said too little was spent on the program. What work was done took too long and may have been, in some cases, misdirected, the report said.

"The retrofit program was not given a particularly high priority by Caltrans," the report said. "A good case could be made that the average of $3.5 million spent per year (1972 to 1987) to retrofit obviously hazardous bridges was too small a portion of the average annual budget for new construction."

The 17-year program consisted primarily of tying bridge decks together with steel cables to prevent them from slipping off their moorings during strong shaking. This technique was developed to prevent a recurrence of the collapse of the interchange of I-5 and I-210 in the San Fernando quake. It was not until after the 1987 Whittier quake that Caltrans began to focus on the vulnerability of columns supporting its elevated structures.

The report acknowledged that the "restrainer" program probably prevented some bridge decks from falling in later quakes, including the Bay Area temblor. But it concluded that the technique was used on too many bridges, including the massive Nimitz, where it should not have been expected to do much good.

"It may not have been wise to expect joint restrainers to prevent collapse for each of the great number of bridge types on which they were installed," the report said.

Instead, Caltrans could have undertaken a comprehensive engineering study of the Nimitz structure, which was built in the 1950s to seismic standards that were obsolete 20 years later.

Such an analysis, the board concluded, "would have predicted the failure of the (Nimitz) under a ground motion equivalent to that experienced in the Loma Prieta earthquake of Oct. 17, 1989, or greater."

Similar research, Housner said, "would have found that the Bay Bridge posed seismic risks," although the precise point of failure may not have been revealed.

"Earthquake engineering research has been too little, too late," Housner said. In a question-and-answer session with reporters, he acknowledged that the Caltrans approach lacked "imagination."

Caltrans Director Robert Best said he agreed with the panel's recommendations, which he predicted would produce a "momentous change" in the way the state views earthquake hazards.

"We need to change our approach from looking at failures that occurred and repairing them to predicting what kind of failures might occur in the future and preparing for them," Best said.

The board said its recommendations could apply to all public structures, not just freeways. Many public buildings now standing could collapse in a major quake comparable to the 1906 temblor that shook San Francisco, the report said.

"Seismic deficiencies in particular types of structures can be determined by engineering investigations with reasonable confidence without waiting for an earthquake to demonstrate the structure's shortcomings," the report said.


The Oct. 17 Bay Area earthquake prompted Caltrans to give new vigor to the effort to strengthen bridges against seismic forces. Before the quake, the state had spent $54 million to shore up 1,200 elevated structures over a 17-year period. Another $64 million was budgeted to strengthen the columns of about 400 more bridges. But since the quake, Caltrans has said an additional $500 million must be spent on research and repairs between 1990 and 1994.

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