CHALLENGING

San Francisco's elegant City Hall was constructed soon after the 1906 earthquake that devastated the city. The 1989 Lama Prieta damaged the building, and repair and earthquake protection began soon after. Courtesy of Forell/Elsessor Engineers, Inc.

Base Isolation Bearings: A Case Study

Seismic base isolation is a system of protecting buildings from earthquake damage by using "bearings" or supports, typically made of layered rubber and steel pads, to separate buildings from the ground on which they sit. The bearings allow a building to move freely on shaking ground. This approach challenges the more customary view that we make a building stable by fixing it more firmly to the earth.

In describing the traditional method of limiting earthquake damage in The Seismic Design Handbook (1989), Ronald Mayes wrote, "The vocabulary of seismic design is limited.... The basic approach has not changed [over the years]: construct a very strong building and attach it securely to the ground. This approach of arm wrestling with nature is neither clever nor subtle...." The new technology breaks from this by allowing a building to move, but to move without destruction.

The use of isolation bearings was developed by many researchers in countries around the globe, especially in the Pacific region. One ambitious application has been the work of Eric Elsesser and his associates in retrofitting the San Francisco City Hall following the Loma Prieta earthquake of 1989.


The traditional supporting structure of the San Francisco City Hall included steel columns firmly anchored to the ground. In an earthquake, such a rigid system causes a building to shake and vibrate, especially in the upper floors. Using seismic base isolation, each column is separated from the ground and cushioned by a large rubber-and-metal support. During an earthquake, these bearings allow the entire building to move with the earth and avoid large-scale destructive vibrations. Courtesy Forell/Elsesser Engineers, Inc.