The Building Code of Japan (the Building Standard Law of Japan) has been changed from the former prescriptive into performance-based type in June, 2000. This paper presents the evaluation procedures of seismic performance of seismically isolated buildings against major earthquake motions newly introduced to the Building Code of Japan in October, 2000. The evaluation procedures apply the single-degree-of-freedom (SDOF) system, equivalent linearization and response spectrum analysis. The basic concept of seismic design spectra for major earthquake motions is: 1) basic design spectra defined at the engineering bedrock, and 2) evaluation of site response from geotechnical data of surface soil layers. The principle of evaluation procedures is that the predicted response values should not exceed the estimated limit values. The evaluation criteria of each portion of a building are: 1) the isolation system shall be designed so that the maximum response displacement does not exceed the design limit displacement, and 2) the structure above the isolation system and the foundation and structural elements below the isolation system shall be designed so that the working stress does not exceed the allowable stress. The scope of application of the evaluation procedures is: 1) buildings not exceeding 60 meters in height, and 2) base isolation buildings on the soil layers excluding very soft soils.
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