Bolstering bridges against quakes

Sun Journal

California: Officials hope large bearings will protect the huge toll structures, allowing them to stay open after shocks.

September 07, 1999|By Douglas Birch | Douglas Birch,SUN STAFF

LA JOLLA, Calif. -- Any day now, California could be rocked by an earthquake as big as the one that struck northwest Turkey, killing more than 14,000 people. A severe quake on this crowded stretch of Pacific Coast would cause tremendous property damage. Yet even if it were centered in a major city, earthquake experts agree, far fewer would die.

One reason is that after major quakes in the 1970s and 1980s, Californians didn't just adopt and enforce stricter building codes. In many areas, they took the far more expensive step of strengthening older structures.

It's a step, many structural engineers think, that the rest of the country will eventually take.

The state accelerated its quake preparations in 1989, when an earthquake measuring 7.1 on the Richter scale struck south of San Francisco. The Loma Prieta quake knocked down parts of the 1950s-era Cypress freeway viaduct in Oakland, some older buildings in the bay area and a 50-foot section of the roadbed of the San Francisco Bay Bridge. Property damage totaled $6 billion. Sixty-three people died.

In the aftermath, San Francisco buttressed its City Hall, Opera House and Civic Auditorium, and spent $350 million shoring up some privately owned brick and concrete-block buildings. Utilities spent hundreds of millions of dollars strengthening pipes carrying water and gas. The state Department of Transportation, called CalTrans, began buttressing its 2,200 bridges.

Using technology developed in New Zealand and tested at the University of California, San Diego (UCSD), the state wrapped many bridge-support columns in steel jackets. By 1994, when a 6.7 earthquake struck Northridge, near Los Angeles, most older bridges survived. Today, all but a few dozen bridges have been strengthened.

But agency officials now face their biggest challenge.

Over the next three years, at a cost of about $900 million, nine of the state's huge toll bridges are to be equipped with the latest in earthquake protection, devices called isolation bearings. These quake-prone landmarks include the Coronado Bridge over San Diego harbor and the western span of the San Francisco Bay Bridge. (The state's best-known bridge, the Golden Gate, which an independent authority operates, has been strengthened using a variety of techniques.)

Until recently, engineers focused on preventing bridges from collapsing. "As we say, our first job is to keep the bridge out of the bay," says Dorie E. Mellon, a senior CalTrans engineer. But state officials wanted more. They wanted to keep bridges, critical links in the state's transportation system, open to traffic even after a severe quake. They wanted to strengthen the bridges quickly by installing a few devices, rather than rebuild them piece by piece. And they wanted to buttress the structures in a way that wouldn't alter their appearance.

"The toll structures are big, they're old and they're historic," Mellon says. "They are symbols of their cities. So we have to preserve their historic qualities."

The force that topples buildings in an earthquake is inertia. When the ground moves, a building resists that motion. It wants to stand still while the ground dances underneath it. If a building is too rigid and the motion too violent, its columns will snap off and the structure will collapse.

One isolation bearing device is a barrel-shaped stack of rubber discs, filled with a lead core. The barrel keeps vibrations in the support columns from reaching the structure, just as rubber mounts prevent the vibration of car engines from jolting people riding in the car.

Another device is essentially a giant shock absorber, a piston and cylinder filled with either thick fluid or soft metal. Shock absorbers increase a building's ability to dampen energy from an earthquake -- just as a car's shock absorbers help the car dampen energy from the pounding of its wheels as it rides over bumps and potholes.

Finally, there are friction pendulum devices, the oddest and most innovative of the designs. These are shallow steel bowls that perch on top of the piers beneath the bridge's roadbed. The roadbed sits on a series of semicircular bearings, about the size of basketballs, which normally rest in the middle of the bowl.

Nothing connects the roadbed to the top of the piers: The road is designed to slide freely on its bowls if its supporting columns start to shimmy. It's a little like putting the bridge roadway on roller skates.

"These big bearings are the key element for the survival of these bridge structures," says Giamario Benzoni, an earthquake engineer at UCSD. "But to date, large-scale versions of the isolation devices have never been tested. Engineers think they will work, based on computer analysis and tests of smaller devices. But no one is certain."

No test facilities existed in the United States big enough or powerful enough to test these big new devices. So CalTrans was faced with taking the results of tests of smaller bearings and assuming the bigger ones would work in a similar way.

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