Analytical and Experimental Studies on Seismic Behavior of Mid-Story Isolated Buildings
Date Issued
2010
Date
2010
Author(s)
Wang, Shiang-Jung
Abstract
The mid-story isolation design method is recently gaining popularity for the seismic protective design of buildings located in the areas of high population. In a mid-story isolated building, the isolation system is incorporated into the mid-story rather than the base of the building. In this dissertation, the dynamic characteristics and seismic responses of mid-story isolated buildings are investigated using a simplified three-lumped-mass structural model for which equivalent linear properties are formulated. From the parametric study, it is found that the nominal frequencies of the superstructure and the substructure respectively above and below the isolation system have significant influences on the isolation frequency and equivalent damping ratio of a mid-story isolated building. Moreover, the mass and stiffness of the substructure are of greater significance than the superstructure in affecting the dynamic characteristics of the isolated building. Besides, based on the response spectrum analysis, it is noted that the higher mode responses may contribute significantly to the story shear force of the substructure.
Due to the existence of the substructure in a mid-story isolated building, the higher mode contribution to the isolated structure may not be negligible especially when the coupling of higher modes occurs. Through the equivalent linear analysis and shaking table tests, the adverse effect arising from the coupling of higher modes on the seismic responses of a mid-story isolated building is clarified. It is found that the coupling of higher modes may lead to the enlarged acceleration responses at the super-floor of the simplified structural model. In order to achieve the better seismic performance and functionality for the isolated structure and equipment inside, a simple method to guarantee the mid-story isolation design against the coupling of higher modes attributed to the improper design of the substructure and superstructure is presented.
The structural models with their isolation system located at the base and other stories are fabricated and tested to investigate the discrepancies between the seismic responses of base-isolated and mid-story isolated buildings. The test results indicate that the mid-story isolation design reveals the excellent seismic performance. However, there exist evident higher mode responses at the substructure and superstructure due to the significant higher modal participation mass ratios. Furthermore, the maximum deformation response of the isolation system is increased when the isolation system is installed at a higher story. Besides, there exists a phase lag of larger than 90 degrees between the seismic responses at the superstructure and substructure. Based on the test results, it is concluded that the peak inertia force and shear force responses acting at the superstructure are mainly attributed to the fundamental mode response. The contribution of the higher mode responses to the peak inertia force and shear force responses acting at the substructure is significant such that the design of the substructure should carefully consider the higher mode contribution.
The irrationalities of adopting the conventional equivalent lateral force procedure for the mid-story isolation design are discussed in this dissertation. The equivalent damping ratio contributed by isolation bearings should be conservatively predicted by the proposed method rather than the component damping ratio of the isolation system. The most rigorous situation for the displacement demand of the isolation system should be carefully considered and will be further studied. In addition, through the numerical studies, it can be seen that the modal response spectrum analysis including a sufficient number of modes is applicable for the preliminary design of mid-story isolated buildings if the effective period (or effective stiffness) and equivalent damping ratio contributed by isolation bearings can be appropriately and conservatively determined form an equivalent linear structural model.
Subjects
mid-story seismic isolation
higher mode
modal coupling
equivalent linear system
equivalent lateral force procedure
response spectrum analysis
Type
thesis
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