Base isolation

Main article: Earthquake engineering

Snapshot of shake-table testing of a base-isolated (right) and a regular (left) building model

Base-isolated LA City Hall

Base isolation, also known as seismic base isolation or base isolation system, is one of the most popular means of protecting a structure against earthquake forces. It is a collection of structural elements which should substantially decouple a superstructure from its substructure resting on a shaking ground thus protecting a building or non-building structure's integrity.

Base isolation is the most powerful tool of earthquake engineering pertaining to the passive structural vibration control technologies. It is meant to enable a building or non-building structure to survive a potentially devastating seismic impact through a proper initial design or subsequent modifications. In some cases, application of base isolation can raise both a structure's seismic performance and its seismic sustainability considerably. Contrary to popular belief base isolation does not make a building earthquake proof.

Base isolation system consists of isolation units with or without isolation components, where:

1. Isolation units are the basic elements of base isolation system which are intended to provide the mentioned decoupling effect to a building or non-building structure.
2. Isolation components are the connections between isolation units and their parts having no decoupling effect of their own.

By their response to an earthquake impact, all isolation units may be divided into two basic categories: shear units and sliding units. The first evidence of architects using the principle of base isolation for earthquake protection was discovered in Pasargadae , a city in ancient Persia, now Iran: it goes back to 6th century BC. It works by having a wide and deep stone and mortar foundation, smoothed at the top, upon which a second foundation is built of wide, smoothed stones which are linked together, forming a plate that slides back and forth over the lower foundation in case of an earthquake leaving the structure intact^{[citation needed]}.

This technology can be used both for new structural design and seismic retrofit. In process of seismic retrofit, some of the most prominent U.S. monuments, e.g. Pasadena City Hall, San Francisco City Hall, Salt Lake City and County Building or LA City Hall were mounted on Base Isolation Systems. It required creating rigidity diaphragms and moats around the buildings, as well as making provisions against overturning and P-Delta Effect.

Base isolation demonstration at The Field Museum in Chicago   Mausoleum of Cyrus, the oldest base-isolated structure in the world[citation needed]

See also

• Earthquake engineering structures
• Geotechnical engineering
• Seismic retrofit
• Shock absorber
• Shock mount
• Vibration isolation