EARTHQUAKE / LIFELINES OF L.A. : THE BRIDGES : Funding for Vincent Thomas Span Work Sparks Fight

With twin shimmering green towers thrusting 365 feet into the sky over Los Angeles Harbor, the Vincent Thomas Bridge is by far the tallest structure in the South Bay. Now that the destructive Northridge earthquake has focused renewed attention on elevated roadways, the busy bridge linking San Pedro to Terminal Island also looms large in the worries of motorists, engineers and legislators.

The bridge sustained no apparent damage though it was bent, plucked, twisted and shaken by forces far more powerful than had been expected. At some points along the 6,060-foot welded-steel suspension bridge those forces were four times as great as in the 1987 Whittier quake.

With forces such as those, experts do not take great comfort in the lack of damage to the Vincent Thomas. It is the largest bridge in Southern California, and state officials consider its approaches vulnerable in even a moderate earthquake.

“We have to hurry up and do our strengthening before the known Big One” occurs on the nearby Newport-Inglewood Fault, said Ahmed M. Abdel-Ghaffar, a professor of structural engineering at USC who has studied the Vincent Thomas for years. “It could be tomorrow, it could be a long time. Nobody knows.”

Paying for that work, which Caltrans figures will cost $32 million, is shaping up as a north-south legislative battle.

After the 1989 Loma Prieta quake, the Legislature ordered a statewide survey of all bridges, including freeway overpasses, local bridges and toll bridges, to determine their vulnerability in an earthquake.

With retrofitting funds in the state’s recession-racked budget running short, Gov. Pete Wilson last year proposed using toll revenues to cover the $615-million cost of upgrading seven of the state’s nine toll spans. But legislators objected, particularly those in the San Francisco Bay Area, where all but two of the state-maintained toll bridges are located.

State Sen. Quentin L. Kopp (I-San Francisco), who chairs the Senate Transportation Committee, said gas taxes should be used to pay for earthquake retrofitting for all roadways, including toll bridges. He is proposing raising gas taxes by 2 cents per gallon for 42 months to pay for the strengthening.

Using tolls alone to pay for the work on toll bridges “would be terribly unfair to people who are already paying for these structures’ maintenance with their gas taxes,” he said. “The toll bridges are part of the state highway system. They are not some independent entities.”

Assemblyman Richard Katz (D-Sylmar), who chairs the Assembly Transportation Committee, disagrees with Kopp. “Toll bridges were created and paid for by the tolls and the tolls are the appropriate way to pay for any seismic work,” he said.

Tolls pay for maintenance of the bridge structures, but the gas tax-supported highway fund pays for work on the roadway itself.

Katz acknowledged that a regional bias accounts for at least part of his thinking on the issue. Bay Area bridges need the bulk of the retrofitting work Caltrans is recommending and, if the money for it comes out of general highway funds, Southern Californians would end up paying for bridges they do not use regularly.

However, he said he might support raising money for retrofitting of all types of structures by issuing bonds or raising the statewide sales tax by a quarter-cent.

Abdel-Ghaffar, who wrote his doctoral dissertation at Caltech on the vibrational characteristics of the Vincent Thomas, said the bridge approaches are the most vulnerable part of the structure. The main span of the bridge is “very strong” and is supported by 990 piles sunk to bedrock, but the approaches could fail or become detached.

A 1992 report issued by the federal General Accounting Office warned that a 7.0-magnitude quake in the Newport-Inglewood Fault zone could cause soil liquefaction on Terminal Island that would destabilize and damage three of the island’s bridge approaches. Soft soils intensify quake damage because shaking essentially turns them to liquid, undermining columns or foundations.

Caltrans has recognized seismic hazards at the approaches to the Vincent Thomas Bridge but considers them less severe than at some other locations in the county.

If the bridge’s approaches failed, it would throw the main span “out of function” and could endanger or seriously damage the bridge, Abdel-Ghaffar said.

He said Caltrans should install cables to tie the approaches to the bridge structure. The superstructure could be strengthened substantially, he said, if shock-absorbing lead-rubber composite bearings were installed to connect the roadbed to the main columns.

Using the data from 26 accelerographs installed on the bridge to measure the forces it is subjected to in an earthquake, Abdel-Ghaffar said the two towers were “really taking a great deal of force from all the way at the top to the deck and down to the base.”

The east tower at the deck, for example, was jolted in the direction the roadbed runs with a force equal to 55% of the force of gravity.

“Accelerations are far, far higher than we had anticipated,” Abdel-Ghaffar said. “This is maybe a good kind of push” to make the bridge stronger.

“If you lose just one part of it,” the vital link to the Los Angeles Harbor that carries more than 27,000 cars a day would be useless, he said.