The graceful, sweeping curves of the Santa Monica-San Diego Freeway interchange, seemingly a monument to engineering achievement, are now to the earthquake experts in the California Department of Transportation among the most vulnerable structures in the state.
Caltrans engineers, still maintaining even after last month's disaster that California highway bridges are safe from collapse in a major temblor, acknowledge that the vital interchange, used by an average of 292,000 vehicles daily, could sustain substantial damage should an earthquake hit along the Newport-Inglewood Fault.
The interchange--along with dozens of other overpasses, underpasses, connectors, freeway ramps, pedestrian crossings, bridges and railroad passes in the Los Angeles County area--has been designated as a "high-risk" structure and given top priority by Caltrans for retrofitting in the second phase of a seismic safety program in effect since the early 1970s.
"The worst threat gets fixed first," said James E. Roberts, chief of Caltrans structures division. "We wouldn't expect any of them not to hold but we would expect a lot of them to be damaged, which might mean closing some routes. We want to upgrade them so we don't even have to close them. We want to keep the lifelines open."
An earthquake "planning scenario" published last year by the California Department of Conservation says that in those areas where shaking or liquefaction is most intense, even bridges that have been strengthened could be damaged. (The portion of the Nimitz Freeway in Oakland that collapsed had been reinforced in 1977 as part of Caltrans' first phase of retrofitting.)
"The retrofit of a bridge does not guarantee against damage or collapse," the Department of Conservation reported in its scenario.
City and county officials in Los Angeles say they are scrambling to inspect, repair and, in some cases, re-repair nearly 200 bridges under their jurisdiction. While none are as heavily traveled as the freeway corridors maintained by Caltrans, some of the city's bridges are more than 50 years old. A few are architectural treasures and at least one of them is known to be decaying, according to City Engineer Robert S. Horii.
Horii said the city's first priority is to reinspect the 4th Street and 6th Street viaducts, 50-year-old bridges that sluice traffic over the Los Angeles River between East Los Angeles and the city's downtown. The 6th Street Bridge carries 12,000 cars a day, according to a 1977 traffic count which Horii's office concedes is out of date. A 1986 traffic count estimated that the 4th Street bridge carries 27,000 cars a day.
Both structures are long--over half a mile--and about 70 feet high. At least superficially, they bear some resemblance to the portion of the Nimitz Freeway in Oakland that collapsed during the Bay Area earthquake, said Horii.
"Both viaducts are multicolumned structures, the kind of bridges that we are looking at carefully in the light of the Nimitz," he said.
The two viaducts were strengthened after the 1971 Sylmar earthquake, Horii said. But the work that was done then involved reinforcing connections between adjoining decks that cars drive over. The viaducts were among 28 bridges scheduled for decking reinforcement after the Sylmar quake. About half of that bridge work has been completed. The rest is supposed to be done by 1992, Horii said.
Among the bridges left to do is one of the oldest in the city--the North Main Street Bridge which was built over the Los Angeles River north of downtown in 1910. Nominated for the National Register of Historic Places, the bridge was the first of its type--a triple-hinged, reinforced-concrete arched structure--built in the western United States. Today, it carries about 11,000 cars a day and is slowly deteriorating as a result of moisture seeping into the concrete arches, Horii said. He estimated that it will cost nearly $540,000 to repair and said the work is supposed to be done by 1991.
County officials say they plan to inspect 134 bridges of the 1,435 the county maintains.
The most heavily traveled of the 134 bridges are in Burbank at Hollywood Way over Vanowen Street and Hollywood Way over the Southern Pacific railroad tracks. More than 58,000 cars cross those bridges every day.
"We are looking at the ones that are supported by columns," said Jim Noyes, deputy director of public works. Noyes added that none of the bridges are showing signs "that would indicate we need to take immediate action."
Noyes said the county has spent nothing on seismic repair of bridges during the past decade. However, between 1971 and 1975, in the four years after the Sylmar quake, said Noyes, the county reinforced 225 bridges.