S.D. Research May Buttress State’s Bridges

UC San Diego engineers have developed a revolutionary bridge-strengthening technique that can be used to retrofit more than a thousand California bridges vulnerable to earthquake damage, state officials announced Thursday.

The technology grew out of research funded after the 1989 Loma Prieta earthquake in the San Francisco Bay Area and is designed to avert disasters like the collapse of a mile-long stretch of the Cypress viaduct section of the Nimitz freeway, which killed 42 during the 1989 quake.

Pending final testing next month, the technique is expected to be used on the earthquake-damaged and still-closed California 101 and Interstate 280 viaducts in San Francisco, said Jim Roberts, chief of the California Department of Transportation’s division of structures.

Because the preliminary test results have been overwhelmingly positive, Caltrans officials believe this will also serve as a retrofitting option for double-column and double-deck bridges across the nation.

“Essentially, as soon as we validate the design, we are ready. We’ve gone a long way to solving the problems,” said Roberts at a press conference at UCSD.

The new “edge beam” concept features a beam supported by circular columns running along the outside of the structure. Scientists tout the new design as being flexible enough to bend with a major earthquake and durable enough to withstand the force.

The concept is considered revolutionary because it puts together all that is known about circular columns, modified hinges and retrofitted joints, and beams. And, for the first time, engineers will test the technique in a simulated earthquake before installing it.

“By simulating earthquake stresses in the lab, we can then apply these technologies to the real-world freeways and bridges we all drive everyday,” said Kyle Nelson, a Caltrans spokesman. “It’s revolutionary, and it has applications worldwide.”

M.J. Nigel Priestley, the UCSD professor of structural engineering who developed the new retrofit concept, said, “We’ve now made enormous improvements in our understanding of how structures perform and in our ability to design for earthquakes.”

The 1989 earthquake that devastated the San Francisco Bay area dramatically raised the priority of an 18-year-old state program to retrofit roadways for earthquake safety. After the quake, the state Legislature mandated that every bridge, freeway connector, overpass or other type of elevated road structure be examined and--if necessary--fixed by 1994.

After inspecting 24,000 highway bridges across the state, Caltrans engineers identified about 4,000 as potentially “at risk” in the event of a large earthquake.

“This is a life-and-death issue, there’s no question about it,” Roberts said. “Bridge retrofiting is the No. 1 priority.”

To finance the costly endeavor, a temporary quarter-cent sales tax was instituted shortly after the 1989 earthquake. That action brought in $80 million. More recently, the retrofitting program has been underwritten by the new gas tax and state funding.

Caltrans has set aside $750 million to retrofit about 750 bridges across the state, a project scheduled to be completed by the end of 1993, Roberts said. About two-thirds of the state’s bridges scheduled for such work are in the Los Angeles and San Bernardino areas, he said. Many of these are likely to be retrofited with the new edge-beam technique, said Nelson, a Caltrans spokesman.

Since the 1989 Loma Prieta earthquake, Caltrans has invested more than $7 million in research contracts at University of California campuses at Irvine, San Diego, Berkeley and Davis, at USC, the University of Nevada at Reno and two private consulting firms. Scientists at UCSD, which has the largest earthquake-simulation laboratory in the United States, won the largest share of that money, about $2 million.

At UCSD, scientists discovered three vulnerable points in older bridges: rectangular columns, deficient beams and lack of flexibility along the length of the roadway.

“We were not looking to increase strength but the ability to deform. It was not so much a deficiency of strength but being too brittle--they snap,” Priestley said.

Priestley and his team developed a retrofit for double-deck and other bridges that uses an 8-by-4-foot steel reinforced concrete “edge beam.” This beam runs along the outside of the structure and is supported by circular columns. Under stress, circular columns retain more concrete than their rectangular counterparts, said Frieder Seible, a UCSD professor of structural engineering.

On Aug. 5, UCSD researchers will undertake an unprecedented test of the new retrofiting techniques. For the test, they will use a half-scale model that has concrete columns 3.3 feet in diameter. The lower column is 10 feet tall; the upper column is 6 feet tall.

During these tests, they will use 13 computer-controlled hydraulic jacks to produce 260,000-pounds of force, or the equivalent of an 8.0 60-second earthquake on the San Andreas fault or a 7.5 tremor on the Hayward fault.

At the UCSD press conference Thursday, at the Charles Lee Powell structural engineering laboratory, Seible and Priestley gave a modest demonstration of the test they intend to run next month, ushering reporters past lines of huge, cracked columns toward two hydraulic jacks.

With the hydraulic jacks jammed up against a narrower column, Priestley explained that the 10-foot concrete column would move 5 inches.

To the naked eye, however, little seemed to occur. One reporter finally blurted out: “Is it happening yet?”

The results of such a small-scale test, Priestley conceded, are not dramatic. “It’s rather like watching grass grow.”