Notes:
Notes:
"In [October] 1989, City Hall, along with many other buildings, was damaged in the Loma Prieta earthquake. The engineers recommended a base isolation system to strengthen the building against future damage. During an earthquake the massive dome acts as a pendulum. A base isolation system separates the building from the ground, thus interrupting the energy before it enters the structure and accelerates the dome. This is done with the installation of rubber and steel discs (base isolators) under each column. A four-foot moat around the building allows it to move side to side without being attached to the earth. City Hall is now the largest base isolated building in the world and the only isolated national landmark. It has now been designed to remain operational even after a great earthquake. Construction work on the seismic system began in March of 1995 and is now complete."