| Having experienced several great earthquake trails in the past 20 years, the technology of base isolation becomes the most popular seismic technology at home and board. And it is stepping into a stage of large extension in many countries including ours. Response spectrum has an important effect on cognizing the law for structural response and checking analysis result. The average characteristics of response spectrum related to many factors, such as focal mechanism, site conditions,earthquake magnitude, epicentral distance and so on. Among them, as the influence factors of ground motion, focal mechanism can affect not only the spectrum ’s values but also the shape of spectrum curve. Further more, it plays an important role in the seismic design of structure. Therefore, this article do the following research about the influence of the focal mechanism on the base isolated structure ’s response spectrum.(1)It studies the effect of spectrum ’s style and the upper structure damping ratio on the base-isolated structure, which simply the base-isolated structure as 2-DOF model. According the ATC-63 report, 22 records are selected as statistical samples.On the basis of the state space method, the real spectrum and pseudo spectrum of isolation layer are established. And by changing the damping ratio of the upper structure, isolated structure response spectrum with different damping ratio were set up. After contrastive analysis of the characteristics for the spectrum, some conclusions were obtained: It is unreasonable to use pseudo spectrum replace the true spectrum when studying on the base isolated structure ’s spectrum. And it is necessary to determine the material of the upper structure, so as to determine the upper structure ratio.(2)It studies the effect of focal mechanism on the linear elastic response spectra of isolated structures which considering non-proportional damping. 605 records in the Ⅱ site condition are selected as statistical samples.The earthquake records are divided into different groups according to the type of earthquake and focal mechanism. The acceleration, displacement and input energy response spectra under near-field and far-field ground motions are solved by using state-space method. The features of the response spectra are studied by means of normalizing and averaging,and the expressions of design response spectra are established by using piecewise linear fitting method. It can reflect the influence of focal mechanism for spectra.Finally, the expression is verified by the seismic response of a real base-isolatedstructure. And the value is compared with that from the design code. The result shows that the formula proposed in this paper is more safe and reasonable, and can be used in the seismic design for base-isolated structures.(3)It studies the effect of focal mechanism on the elastic-plastic response spectra of isolated structures which using the Bouc-Wen model to describe the nonlinear behavior of isolation layer. The elastic-plastic response spectra under near-field and far-field ground motions are solved by using state-space method and fourth-order Runge-Kutta method. After analysis the features of the response spectra, the expressions of the ratio between two response spectra and period are established by using curve fitting method. It can reflect the influence of focal mechanism for spectra.And in the foundation of the elastic response spectra,elastic-plastic response can achieved quickly by using it. Finally, the expression is verified by the seismic response of a real base-isolated structure. It shows that the expression has good computational accuracy when analyzing the isolated structures under earthquake motions. |
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