Background

The modern era of passive structural control had its beginnings in New Zealand in the early 1970s. Numerous research and development efforts were under way around the world by the end of that decade, and the first applications of seismic isolation in Japan and the United States began in the early 1980s. In Japan, the first isolated structure was a small house built as a demonstration project in 1982 by a real estate development company. The first isolated building constructed in the United States was a county administration building in southern California, completed in 1985.

As the Japanese economy boomed throughout the second half of the 1980s, many more demonstration projects were built by a large number of construction companies to showcase their advances in technology. Funding for these buildings typically came from the construction companies' government-mandated research investments, and the buildings used many different types of proprietary isolation systems. Of the 85 isolated buildings approved for construction before the Kobe earthquake, approximately 35 were either dormitories, research centers, or other facilities owned by construction companies or bearing manufacturers and built for demonstration purposes.

The development of structures incorporating passive damping moved more slowly than did buildings that incorporated seismic isolation. Although relatively few were contractor-owned demonstration projects, many of the early applications were intended as much to mitigate motions due to wind and typhoons as to prevent damage in strong earthquakes. These buildings also used an array of different proprietary devices. Comprehensive data for these buildings are more difficult to obtain than for isolated buildings, but designers and construction companies responding to the present survey indicated that their firms were responsible for 28 buildings with passive damping systems prior to the Kobe earthquake.

No building with passive structural control in Japan had experienced a significant earthquake until shaking from the Kobe earthquake was recorded at two isolated buildings located approximately 30 km from the epicenter. Both the Matsumura-Gumi Research Laboratory and the West Japan Postal Savings Computer Center (West-1) experienced ground accelerations of approximately 0.3g. The West-1 building performed very well, reducing accelerations in the superstructure to approximately 0.1g, and it became the focus of significant attention after the earthquake because at the time it was the largest seismic isolated building in the world (Naeim and Kelly, 1999). Even though these two isolated buildings were well outside of the region of strongest shaking, engineers, construction companies, the media, and other groups in Japan used these buildings to promote the potential benefits of seismic isolation.

USE OF ISOLATION AND DAMPING TECHNOLOGY ACCELERATES IN JAPAN

Beginning in mid-1995, the rate of construction approvals in seismic isolated buildings and buildings with passive damping systems increased substantially. The clear differences between the pre-earthquake and post-earthquake construction trends are illustrated in the Figure to the left, which plots the available data for all isolated buildings licensed prior to June 1998 .

Though the data are less complete, similar trends in construction approvals have been found for buildings incorporating passive damping systems. Before the earthquake, about 28 buildings with passive damping systems had received construction approval. More than 125 have been approved since. It is estimated that more than 50 percent of all high-rise buildings designed in Japan today employ some form of passive damping system 24 of 36 buildings reviewed by the Building Center of Japan (BCJ) high-rise committee in 1997 incorporated energy dissipation devices (Building Center of Japan, 1997; Comartin, 1998).

USE OF ISOLATION AND DAMPING TECHNOLOGIES IN THE UNITED STATES

There was a great deal of interaction among researchers in New Zealand, Japan, and the United States throughout the 1980s, ensuring that advances in the technologies of seismic isolation and passive energy dissipation were disseminated widely. The rate of applications in New Zealand and the U.S., however, could not keep pace with Japan and its booming economy. By 1990 there were only four isolated buildings and no buildings with passive dampers completed in the U.S. By the end of 1993, three weeks before the 1994 Northridge earthquake, eleven isolated buildings had been completed, and two buildings had been retrofit using passive dampers.

When the Northridge earthquake struck southern California on January 17, 1994, at least five isolated buildings experienced measurable ground motions. The USC University Hospital in east Los Angeles, approximately 35 km from the epicenter, sustained stronger shaking than any other isolated building ever had previously. Peak ground accelerations of 0.38g below the building were attenuated to less than 0.13g over most of the superstructure, making this building a focal point for international study, and many researchers and engineers from around the world came to Los Angeles in the weeks following the earthquake (Asher et al., 1997).

The favorable performance of the USC University Hospital demonstrated that seismic isolation was a viable earthquake-resistant design approach. However, another important lesson from the Northridge earthquake confirmation of the  potential for severe, impulsive ground motions in the near-source region came from numerous strong motion recordings in the northern San Fernando Valley and subsequent seismological studies. These records have led to a new generation of designs for  isolated buildings, and as the technology has matured, the number of applications has increased steadily. At the end of 1998, there were approximately 40 isolated buildings completed or beginning construction in the United States, with more than 20 buildings incorporating passive damping systems for seismic protection either completed or in the design phase.