Seismic base isolation


Seismic base isolation, also known as 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 that is in turn resting on the shaking ground, thus protecting a building or non-building structure's integrity.
Base isolation is one of the most powerful tools of earthquake engineering pertaining to the passive structural vibration control technologies.
The isolation can be obtained by the use of various techniques like rubber bearings, friction bearings, ball bearings, spring systems and other means. 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 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.
Isolation units could consist of shear or sliding units.
This technology can be used for both 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 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.
in Chicago
Base isolation units consist of Linear-motion bearings, that allow the building to move, oil dampers that absorb the forces generated by the movement of the building, and laminated rubber bearings that allow the building to return to its original position when the earthquake has ended.

History

Base isolator bearings were pioneered in New Zealand by Dr Bill Robinson during the 1970s. The bearing, which consists of layers of rubber and steel with a lead core, was invented by Dr Robinson in 1974.

Research

Through the George E. Brown, Jr. Network for Earthquake Engineering Simulation, researchers are studying the performance of base isolation systems. 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 earthquake 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 to the system level response. These tests will include a full-scale, three-dimensional test of an isolated 5-story steel building on the E-Defense shake table in Miki, Hyogo, Japan.
. Seismic
isolation research in the middle and late 1970s was largely
predicated on the observation that most strong-motion
records recorded up to that time had very low spectral
acceleration values in the long-period range. Records
obtained from lakebed sites in the 1985 Mexico City raised
concerns of the possibility of resonance, but such examples
were considered exceptional and predictable. One of the early
examples of the earthquake design strategy is the one given
by Dr. J.A. Calantariens in 1909. It was proposed that the
building can be built on a layer of fine sand, mica or talc that
would allow the building to slide in an earthquake, thereby
reducing the forces transmitted to building.
A detailed literature review of semi-active control
systems Michael D. Symans et al. provides
references to both theoretical and experimental research but
concentrates on describing the results of experimental work.
Specifically, the review focuses on descriptions of the
dynamic behavior and distinguishing features of various
systems which have been experimentally tested both at the
component level and within small scale structural models.

Adaptive base isolation

An adaptive base isolation system includes a tunable isolator that can adjust its properties based on the input to minimize the transferred vibration. Magnetorheological fluid dampers and isolators with Magnetorheological elastomer have been suggested as adaptive base isolators.

Notable buildings and structures on base isolation systems