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.

Steve Loud, publisher of Composites News International, a trade journal, responded, “I believe composites will be shown to exceed steel’s jacket strength once the testing is done. Tests under way at UC San Diego, USC and the University of Arizona indicate that composites can offer more ductility improvement than can steel.”

Dispute also marks the public discussion of the effectiveness of base isolation.

After the Kobe earthquake, one of the leading base isolating firms, Dynamic Isolation Systems of Lafayette, Calif., issued statements noting that both of the base isolated buildings in Kobe (there are about 70 in Japan) had fared well.

A six-story Postal Ministry building, for example, was shaken on ground level by an acceleration force equivalent to 30% of gravity, the firm said. But above the lead-rubber isolators, the shaking was only 10% of gravity on the top floor. By contrast, DIS reported the acceleration on the ground at a nearby conventional building was 27% of gravity, but rose to a dangerous 97% on the top, fifth floor, which sustained heavy damage.

However, just as during the Northridge quake, the base isolated buildings in Kobe were not in the epicentral area and so did not encounter the highest shaking intensities sustained in the quake.

Critics such as Thomas Heaton of the U.S. Geological Survey’s Pasadena office say they are waiting to see what happens when a powerful earthquake is centered precisely under base isolated buildings. Los Angeles could have a quake, Heaton suggests, that would overwhelm such a system.

Heaton advocates other choices that afford strong protection from quakes, including shear walls built to 50% over code. Citing the Olive View Hospital in the San Fernando Valley, Heaton said that “reinforced concrete shear walls can be made to go through almost any kind of shaking.”

All Heaton would acknowledge about base isolation is that it “is one means of keeping a building in range of preservability for a certain size earthquake.” However, to be surer of effectiveness, he said, City Hall designers would have to provide for more displacement during shaking than the four feet they have allotted in the moat that will surround the building’s foundation.

Heaton and such colleagues as David Wald and John F. Hall of Caltech, have also questioned whether earthquake pulses might not resonate up a tall base isolated building in such a way as to damage parts of it.

Youssef, in an interview, responded sharply that base isolated projects “are the only reliable systems that can protect historic, decorative buildings and keep them functional.”

The Olive View Hospital, Youssef maintained, “is built like a battleship,” while “at Los Angeles City Hall, to do a conventional reinforcement, you’d have to run walls through the rotunda and up to the tower. The costs to City Hall’s historic elements would be excessive.”

Youssef concedes that a huge earthquake--in the mid- or high magnitude 8 range--could compromise his system, but he said that providing larger displacement would be counterproductive, because during smaller quakes--in the magnitude 6 range that are more likely to occur--the building could move and shake as if it were a conventional structure.

It is better, he said, to build for the most likely range of quakes than for the one catastrophe occurring perhaps every 2,500 years.

As for resonance of earthquake pulses--the reason most base isolation systems have been limited to buildings of less than 12 stories--Youssef said he had taken care of it through designing for the use of dampers, as well as other kinds of extra reinforcement.

“We never say there’s a guarantee (against damage) in the biggest possible earthquakes,” he said. “But people need to recognize that when that scenario occurs, when that kind of earthquake hits Downtown L.A., the base isolated buildings will still fare better.”

Youssef said he is building in redundancies in the system, to better protect City Hall. And, rebutting another designer, Marc S. Caspe, who has been extolling the virtues of his sliding-friction base isolation system, Youssef said he is convinced his system is the best.

In another interview, George Housner, professor emeritus of earthquake engineering at Caltech, described himself as a believer in base isolation. For Los Angeles City Hall and some other major base isolation projects, “I don’t see any better alternative,” he said.

(BEGIN TEXT OF INFOBOX / INFOGRAPHIC)

Minimizing Quake Damage The use of seismic base isolation systems in medium high-rise buildings is growing in the United States and Japan, although it remains much less common than conventional flexible reinforcement. The most common base-isolated system is the rubber-spring shock absorber, sometimes accompanied by dampers. The aim of base isolation is to minimize shaking in a structure during a temblor.

Conventional Reinforcement Shown is an example of how a steel or reinforced concrete column can rupture during a strong quake, compromising the integrity of the building. Elements of ductibility--the capacity to bend without breaking--in the steel or concrete prevent rupturing in most quakes.

Basic Isolation System

Here a column is further insulated from shaking by a rubber-spring shock absorber called a seismic base isolator. It is surrounded by a moat--four feet wide in the case of Los Angeles City Hall--that provides room for movement in any direction.

Basic Isolater This is one of the several designs of base isolaters. Vulcaized rubber layers that can move in any direction are laminated between steel sheets to form a movable, flexible base. Source: Marc J. Caspe Researched by KENNETH REICH / Los Angeles Times