Seismic isolation retrofit of large historic building

Seismic isolation in the United States is gaining wide acceptance as an attractive and alternate means to upgrade historic structures seismically with minimal disturbance to architecturally significant features. One important historic-landmark structure that underwent such an upgrade is the Ninth Circuit U.S. Court of Appeals building located in San Francisco, which suffered damage during the 1989 Loma Prieta earthquake and was declared an unsafe building for further occupancy. The present paper summarizes the comprehensive approach adopted for implementation of the seismic-isolation technique for the seismic retrofit of this monumental structure. This historically significant structure is the first federal building to be retrofitted using seismic isolation. The paper discusses in detail the procedure adopted for evaluating the existing building strength, establishing the design approach, and evaluating and selecting the optimum isolator placement location and isolation system best suited for the building's structural performance and architectural usage.