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Los Angeles City Hall Seismic Rehabilitation Project - Base Isolation Technology
Reducing the forces transmitted to the building from the ground
City Hall has been placed atop a mechanical system of isolators, sliders and dampers called "base isolation technology" that will dampen the violent movements of the earth during a seismic event. By using Isolators and dampers the building is "decoupled" from the ground motion of any earthquake and the transmission of seismic energy to the building is dampened. This is done by lowering the vibrational frequency, allowing the building to move or displace, and lowering the shock acceleration of the seismic event thus reducing the tendency for the upper floors to move faster than the lower floors. In general, buildings that have been isolated in this way are subjected to 1/3 to 1/5 of the horizontal acceleration of conventional structures during a seismic event.
The isolator is a sandwich of alternating layers of 1/4" steel plate and 1/4 inch rubber which are vulcanized to form a single integrated unit. It is able to displace horizontally in any direction by 24 inches from the center. At left is a cross section showing the inside of an isolator.
An isolator fully extended as it would be during a seismic event. Every isolator will extend in any direction 21 inches.
These columns will carry between 500,000 pounds and 2.5 million pounds of load, depending upon the column size and location. 526 isolators will eventually be installed.
An inside view of an isolator
This isolator has been installed.