| Summary: | 碩士 === 臺灣大學 === 土木工程學研究所 === 98 === The mid-story seismic isolation design method, in which the isolation system is incorporated into the mid-story rather than the base of the building, is recently gaining popularity for the seismic protective design of buildings. In addition to be capable of satisfying some particular architectural concerns of aesthetics and functionality, the adoption of the mid-story isolation design, as an alternative to the base isolation design, can enhance the construction feasibility especially at highly populated areas. In the past, the dynamic behavior of mid-story isolated buildings has been numerically investigated using a simplified structural model in which three lumped masses are assigned at the substructure, super-floor above the isolation system and superstructure. It was disclosed that the dynamic characteristics and seismic responses of a mid-story isolated building are significantly affected by the mass and stiffness of the substructure.
In order to experimentally investigate the discrepancies of dynamic characteristics and seismic responses in base-isolated and mid-story isolated buildings, a series of shaking table tests on three seismically isolated structural models are performed in this dissertation. These three structural models are respectively isolated at the base of the superstructure, the top of an one-story substructure and the top of another two-story substructure. Therefore, the first specimen is a conventional based-isolated building while the other two are the so-called mid-story isolated buildings. In addition to the isolation system composed of four lead-rubber bearings (LRB), the implementation of two linear viscous dampers (VD) into the isolation system is adopted in the experimental study to demonstrate the seismic performance of the isolated superstructure. Furthermore, based on the comparison of test results of two mid-story isolated structural models with their isolation systems installed at different stories, the effect of substructure properties on the dynamic behavior of mid-story isolated buildings is thoroughly discussed.
It can be found from the analytical results and test results that the dynamic behavior of a mid-story isolated structure is not be identical to, or even becomes more complex than, that of a base-isolated structure. The contribution of higher modes to the seismic responses of mid-story isolated buildings should be paid more attention. Therefore, the assumption of a single degree of freedom system for the superstructure of a mid-story isolated building (i.e. the isolation system is the only lateral deformation system as the base isolation design) may not be conservative especially when the substructure is not stiff sufficiently. In this dissertation, the irrationalities of adopting the conventional equivalent lateral force procedure for the mid-story isolation design are discussed in detail. Furthermore, two modal response spectrum analysis procedures appropriately considering the contribution of higher modes are adopted to predict the peak seismic responses of the test structural models. Based on the comparison of test results and numerical predictions by these two analysis procedures, the feasibility of the modal response spectrum analysis procedures for the preliminary design of mid-story isolated buildings is also examined in this dissertation.
The Civil Engineering Research Building of the National Taiwan University (NTU) is a mid-story isolated structure with which several accelerometers and displacement transducers are equipped for the seismic monitoring. In this dissertation, the data recorded by these measurement instrumentations during the past major earthquake events are not only used for the structural system identification but also compared with the analytical results considering an appropriate numerical model for the seismic isolation bearings. It is disclosed that both the determination of the elastic stiffness of seismic isolation bearings and the earthquake characteristics around the site should be considered carefully for the seismic isolation design.
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