Existing comment:
Seismic Isolation
Much new knowledge was gained over the decades about earthquake engineering after the Bridge was completed in 1937. Two retrofit strategies have been used recently on the Bridge: (1) making some key elements stronger, and (2) using seismic isolators on approaches to the Bridge to reduce the earthquake motions that they experience.
The isolators reduce the severity of shaking transmitted up into the structure and make the vibrations slower and less jerky. You can see seismic isolators, short black cylinders about 3 feet (1 m) in diameter, under the roadway decks of approach spans at the north and south ends of the Bridge.
When a bridge is rigidly connected to the ground, it directly experiences the rapid, jerky earthquake shaking. If the bridge is mounted on seismic isolators, the isolators deform back and forth during the earthquake allowing a bridge to rock back and forth, softening the vibrations that the structure experiences. When a bridge is isolated, the forces can be reduced by as much as two-thirds as compared to a non-isolated bridge.
Invented in New Zealand in the 1970s, a common type of isolator like the one shown above is made of layers of steel and rubber bonded together. Sometimes there is a hole in the middle in which a lead plug is inserted to increase energy absorption. These are also called lead-rubber bearings. |