Bridge Retrofit for Tremors Passes the Test : Safety: Caltrans is ready to employ UCSD’s revolutionary design to begin strengthening the state’s bridges to withstand major earthquakes.

A cutting-edge retrofit technology designed to strengthen California’s bridges to better withstand major earthquakes has been successfully tested at UC San Diego, paving the way for safer, more resilient bridges, researchers said Tuesday.

With this technology, engineers believe retrofitted bridges will be flexible enough to bend with a major earthquake and sturdy enough to withstand extreme force--a design that Caltrans hopes to use to avoid a disaster like the collapse of a mile-long stretch of the Cypress viaduct section of the Nimitz freeway, that killed 42 during the 1989 Bay Area quake.

“UCSD has produced a proven, safe and cost-effective design,” said Jim Roberts, chief of the California Department of Transportation’s division of structures. “The retrofit techniques we are developing in California will be the model for the rest of the world.”

The concept behind the design, touted as revolutionary, could be used to retrofit more than a thousand California bridges, including 71 in San Diego County, Caltrans spokesman Jim Drago said from Sacramento.

Caltrans officials had been waiting for UCSD’s final test results--including subjecting a bridge model to a simulated earthquake--before using the design on the state’s older double-deck bridges, including the damaged California 101 and the Interstate 280 viaducts in San Francisco, which have been closed since the 1989 quake.

The “edge beam” design passed recent tests with flying colors--clearing the way for construction to begin on the California 101 viaduct, for an estimated $60 million, and the Interstate 280 viaduct, for an estimated $90 million, Drago said.

The new design is the first to use engineers’ current knowledge of a variety of bridge features, such as circular columns, modified hinges and retrofitted joints, researchers said.

The design features a steel-reinforced concrete edge beam, no bigger than 8 by 4 feet, that is put on the outside of the structure. This edge beam is surrounded by circular columns; each column is reinforced by a steel cage around a concrete inner core. The retrofit also includes a cap beam, which is 9 by 8 feet and runs between the column.

A key advantage of the design, engineers say, is that the structure requiring strengthening does not need to be closed to install the new retrofit technology.

For the test, the first of its kind, researchers used 13 computer-controlled hydraulic jacks to generate a force greater than a magnitude 8.0 earthquake on the San Andreas Fault, which is considered the maximum possible for an earthquake in San Francisco, said Frieder Seible, professor of structural engineering at UCSD and co-investigator of the project.

Each computer is capable of triggering 260,000 pounds of force. Although engineers hoped to conclude the tests last month, they ran into some glitches in orchestrating all 13 computers to simultaneously generate the earthquake, Seible said. As a result, testing was completed last week.

The series of tests, conducted at the Charles Lee Powell Structural Systems Laboratory at UCSD, included a two-week around-the-clock earthquake simulation of a retrofitted, half-scale bridge model, which was 50 feet long with columns 10 feet and 6 feet high. The bridge model was outfitted with 350 instruments that monitored the stress, displacement, rotation and tilt during the simulated earthquake.

After this grueling two-week pounding, Seible and his co-investigator, M.J. Nigel Priestley, UCSD professor of structural engineering, discovered that the bridge model suffered only “very minor” cosmetic damage.

After subjecting the bridge model to the simulated 8.0 Richter-scale quake, the UCSD engineers stepped up the force to full capacity of the equipment--or 1 1/2 times more pressure.

“Even at that load and displacement level, the structure didn’t show any signs of strength degradation--it didn’t weaken at all,” Seible said. “This was extremely encouraging. So our bridge specimen really behaved extremely well.”

Although it will take about two years to fully analyze the data, Seible said he expected to send a preliminary report to Caltrans officials within the next few days.

“The model performed exceptionally well. Our test can give Caltrans a lot of confidence that the proposed measures are technically sound,” Seible said. “The performance was even better than we hoped for. The concept is very safe and sound, and can be implemented now without any reservations.”

Priestley, who developed the retrofit concept, was unavailable for comment.

Based on the test results, Caltrans will award a contract and begin work on the California 101 viaduct, which is expected to be completed in mid-1992, and Interstate 280 viaduct, which should be finished at the end of 1992, said Drago of Caltrans.

In the San Diego area, there are no double-beam bridges like those in San Francisco, said Jim Larson, a local Caltrans spokesman. But the technology could be adapted to the 71 bridges that are candidates for retrofitting, he said.

“We recognize the fact that damage cannot be prevented in a major earthquake,” said Roberts of Caltrans’ division of structures. “Our goal is to minimize damage to structures so they can be easily repaired.”