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City Hall Getting Latest Quake Upgrade : Safety: Downtown structure will be tallest yet to have 'base isolation' work. Experts debate the effectiveness of hazard mitigation techniques.


Since Northridge and Kobe, a surge of interest has developed in California in new earthquake hazard mitigation techniques, with particular emphasis on seismic base isolation to protect structures from intense shaking and on lightweight composite materials to use in bridge retrofits.

There are skeptics of both of these attempts to reduce quake losses. Practical tests remain to be completed on the composites, and some suggest that it will not be known how well base isolation really works until a major quake occurs right under a building with such a system.

But after years of slow progress, base isolation is being adopted in a growing number of major projects. The final approval by the Los Angeles Board of Public Works of engineer Nabib Youssef's plan for Los Angeles City Hall marks a significant advance for the exponents of base isolation systems.

The 26-story City Hall--seriously damaged, particularly on its upper floors, by the Northridge earthquake--will be the tallest building yet to have base isolation installed.

Approval of the project came in spite of scientific and engineering critics who questioned whether it is too tall for such a system, which has usually been limited to buildings between three and 11 stories.

The term base isolation covers a variety of systems that seek to reduce shaking in a building by installing shock absorbers in the form of rubber springs or, less commonly, friction sliding bearings at the base of the building.

At City Hall, 430 rubber spring shock absorbers will be placed underneath the building's vertical columns just above the foundation.

Viscous dampers, another kind of shock absorber, will also be installed both at the base and at upper levels to further impede shaking and reduce movement of the building within a four-foot moat to be built surrounding the foundation. In addition, more conventional shear walls will be built up into the tower.

The cost of the retrofit project has been estimated at $146 million, plus $22.9 million to move all 1,000 employees who work on the fifth through 26th floors to another site for three years while the work goes on. Those with offices below the fifth floor, including the mayor and City Council members, will remain.

Youssef said in a recent interview that the base isolators will only cost $3.4 million of the total retrofit amount and will take 18 to 24 months to install, beginning next month.

Among other California buildings that have been or are being base isolated are the city halls in San Francisco and Oakland, the Rockwell International headquarters in Seal Beach, USC University Hospital, Kerckhoff Hall at UCLA and the San Bernardino Medical Center.

Meanwhile, Caltrans officials are accelerating tests of new composite materials to use in place of steel in retrofit jackets protecting bridge columns from rupture.

Bridge retrofitting in California started after the 1971 Sylmar-San Fernando earthquake was accelerated after the 1989 Loma Prieta quake, and then gained impetus after the 1994 Northridge quake. Current Caltrans plans call for $1.75 billion to be spent on retrofitting 2,350 interstate and state highway bridges by the end of 1997. Another $650 million is projected for retrofitting seven major toll bridges.

Since Caltrans reports that 1,300 of these retrofits have yet to begin, and additional retrofits are to be performed on about 1,000 city and county bridges, it is clear that new, reliable and less costly retrofitting would be of major importance.

The composites are fiber-reinforced plastics that are wrapped around the columns in thin layers to impede or prevent their rupture and to increase ductility, or flexibility during shaking.

Use of stronger materials in the column jackets could prevent the type of freeway collapses that marked the 1971, 1989 and 1994 California earthquakes and January's Kobe quake.

Some bridge retrofits using carbon fiber composites are already in use on a test basis on the Santa Monica Freeway. Tests of an earlier system, using glass and aramid fibers, yielded mixed results. Other testing will take place shortly on the Santa Monica Freeway on a new system trade-marked Snap Tite, a glass-fiber composite.

Jim Roberts, Caltrans director of engineering, said, "I definitely think the composites hold a lot of promise. . . . You can do such retrofits with lighter equipment, and it is much faster to install, but we need competitive bidding to see which one is cheaper."

Composite producers assert that their products will not only be stronger than steel but cheaper as well. But Roberts has reservations on the strength question, saying that composites might prove better suited to areas with fewer earthquakes and less intense shaking.

He also said he believed steel was better suited to the Bay Area toll bridges, the state's largest.

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