Aseismic Base Isolation: Its History and Prospects


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Title: Aseismic Base Isolation: Its History and Prospects

Author(s): James M. Kelly

Publication: Special Publication

Volume: 70


Appears on pages(s): 549-586

Keywords: bearings; earthquake resistant structures; earthquakes; rubber; structural analysis; structural design; tests; vibration.

Date: 1/1/1981

The concept of base isolation is a natural one based on accepted physical principles. It has not, however, been readily accepted by the structural engineering profession, perhaps because the concept runs counter to accepted methods of aseismic design. In essence, a base-isolated structure is decoupled from the damaging horizontal components of earthquake ground motion by a mechanism which prevents or reduces the transmission of horizontal accelera- tion into the structure. While many base isolation schemes have been proposed over the last one hundred years, virtually all remained unimplemented until the concept became a practical possibility with the recent development of multilayer elastomeric bearings, a development which began with the design of bearings for bridges and those used to isolate structures from ground-borne acoustic vibration. This paper describes the development of base isolation and extensive experimentation on the concept carried out on the shaking table at the Earthquake Engineering Research Center of the University of California, Berkeley. Several isolation systems have been tested, including schemes incorporating handmade isolation bearings of natural rubber as well as commercially manufactured bearings. Two large structural models were used in the tests. The results from these tests have established the effectiveness of this approach to aseismic design and have demonstrated that the peak accelerations experienced by a building on an isolation system are substantially lower than those felt by a conventionally founded structure. Other mechanisms have been tested in combination with the elastomeric bearings, including a mechanical fuse in the form of a notched pin designed to fracture at a specified level of shear force and which acts as a wind restraint, and several forms of energy-absorbing device designed to provide increased levels of damping to the isolation system and which control relative displacements at the bearings. A fail-safe skid system has also been tested; this system produces a Coulomb frictional damping and in the event of earthquake ground motion far greater than that for which the bearings were designed acts to prevent structural collapse.