Plans to Retrofit Toll Bridges Described

State engineers got a chilling look Tuesday at how earthquakes could affect some of California’s intercity urban toll bridges and the challenges faced by the state in retrofitting them.

One of the most elaborate seismic retrofitting projects ever undertaken--a five-year, $175-million strengthening of the Golden Gate Bridge due to begin next summer--is essential to reliably prevent its destruction in a giant earthquake on the San Andreas fault, engineers said Tuesday.

UC San Diego engineering professor Frieder Seible called retrofitting of the state’s eight great toll bridges, for which design work has been largely completed, an “unprecedented challenge of the technical skills” of seismic engineers.

The eight bridges are the Golden Gate, the San Francisco Bay, the Richmond-San Rafael, the San Mateo, the Carquinez and the Benicia-Martinez in the Bay Area, the Vincent Thomas Bridge in San Pedro and the bridge to Coronado in San Diego.

In several presentations at the National Seismic Conference being held on bridge and highway retrofitting, the famed Golden Gate often held the spotlight. But a striking video presented by a San Francisco engineer, Mark A. Ketchum, depicted the expected shaking on the San Francisco Bay Bridge in a magnitude 8 quake.

It was a macabre sight. The bridge roadways rocked so wildly in the simulation that it seemed inevitable that some vehicles and their occupants would be tossed into the bay.

The Golden Gate was described by Darryl D. Matson, a Vancouver, Canada, engineer who has been among those working on its $12.2-million retrofitting design, which is in addition to the basic $175-million cost, as so “vital a link that there is probably not enough you could spend on maintaining it as a link.”

But as it is, said engineer Mervin Giacomini of the Golden Gate Bridge Highway and Transportation District, the retrofitting cost will amount to 13% of the $1.4 billion it would cost to replace the span if a quake destroyed it. (The bridge cost $27 million in pre-inflation dollars to build in the mid-1930s).

The retrofitting work will begin on what was described by engineer Gerald V. Davis of the Sacramento firm of Imbsen & Associates as the “very vulnerable” north viaduct approach to the bridge. Davis said that each of the towers, plus most other structural elements, will have to be rebuilt.

Davis explained that the San Andreas fault, generator of the great earthquake of 1906, is less than seven miles offshore from the Golden Gate and that eventually a magnitude 8-plus earthquake is expected. When it comes, it will exert forces 2.2 times that of gravity on the bridge towers, he estimated.

“The existing towers are incapable of handling the expected stresses,” he said. “Those towers may tip over during an earthquake.”

Giacomini said plans call for the work, expected to be financed 80% by the federal government and 20% by bridge tolls, to be completed in four phases ending in 2001.

A public hearing to be held in San Francisco tonight will feature reassurances by authorities that the appearance and historical elements of the bridge will not change materially, the district engineer said. Saying that there is no substantial opposition in any quarter to the retrofitting plan, he predicted that tonight’s hearing would be “a love fest.”

“We promise that the color of the spans will stay exactly the same,” he said. Although major work will be done to reinforce the entire structure, there will be little that is noticeable when the work is complete, he added.

The bridge will be kept open most of the time during the retrofitting, Davis said.

Giacomini said the schedule calls for permits to be issued for the first phase in March, with advertising for bids to proceed in April and construction to begin by summer.

Meanwhile, other engineers reporting Tuesday said the Bay Bridge retrofitting, to be done through Caltrans, will cost about 20% of what it would cost to replace that span, although no precise dollar amount is available.