Base isolation

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Tomb of Cyrus is said to be the oldest base-isolated structure in the world
Snapshot of shake-table testing of a base-isolated (right) and a regular (left) building model
LA City Hall, the tallest base isolated building in the world[1][not in citation given]

Base isolation, also known as seismic base isolation[2] or base isolation system,[3] is one of the most popular means of protecting a structure against earthquake forces.[4] It is a collection of structural elements which should substantially decouple[disambiguation needed] a superstructure from its substructure resting on a shaking ground thus protecting a building or non-building structure's integrity.[5]

Base isolation is one of the most powerful tools 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 a base isolation system which are intended to provide the aforementioned decoupling[disambiguation needed] 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[6] and sliding units.[7] The first evidence of architects using the principle of base isolation for earthquake protection was discovered in Pasargadae,[8] 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 for both new structural design[9] 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 is also used on a smaller scale - sometimes down to a single room in a building. Isolated raised-floor systems are used to safeguard essential equipment against earthquakes. The technique has been incorporated to protect statues and other works of art - see, for instance, Rodin's Gates of Hell at the National Museum of Western Art in Tokyo's Ueno Park.[10]

Base isolation demonstration at The Field Museum in Chicago

Research on base isolation[edit]

Through the George E. Brown, Jr. Network for Earthquake Engineering Simulation (NEES), researchers are studying the performance of base isolation systems.[11] The project, a collaboration among researchers at University of Nevada, Reno; University of California, Berkeley; University of Wisconsin, Green Bay; and the University at Buffalo is conducting a strategic assessment of the economic, technical, and procedural barriers to the widespread adoption of seismic isolation in the United States. NEES resources have been used for experimental and numerical simulation, data mining, networking and collaboration to understand the complex interrelationship among the factors controlling the overall performance of an isolated structural system. This project involves shaking table and hybrid tests at the NEES experimental facilities at the University of California, Berkeley, and the University at Buffalo, aimed at understanding ultimate performance limits to examine the propagation of local isolation failures (e.g., bumping against stops, bearing failures, uplift) to the system level response. These tests, including a full-scale, three-dimensional test of an isolated 5-story steel building on the E-Defense shake table in Miki, Hyogo, Japan, will help fill critical knowledge gaps, validate assumptions regarding behavior and modeling, and provide essential proof-of-concept evidence regarding the importance of isolation technology.[12]

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