Analytical and Experimental Studies on Seismic Behavior of Mid-Story Isolated Buildings

近年來隔震技術發展逐漸成熟，因應經濟與施工條件、建物特性、都市人口集中以及土地取得不易等因素，除了基礎隔震建築物的蓬勃發展外，中間樓層隔震建築物之實務案例亦與日俱增。所謂中間樓層隔震建築物即是將隔震層設置於基礎以上之樓層，國內最常見之中間層隔震設計是將隔震層設置於一樓頂。然而，中間樓層隔震建築物可能因下部結構的存在而造成對於隔震設計非預期之影響，因此，本文利用一簡化三自由度結構模型模擬一中間樓層隔震建築物，三個自由度之堆疊質量分別代表中間樓層隔震建築物之上部結構、隔震層以及下部結構，隔震系統之遲滯行為以一等效線性系統模擬，針對此一簡化三自由度結構模型之動力特性以及耐震行為進行深入探討。由參數分析結果可發現，中間樓層隔震建築物下部與上部結構之勁度與質量對於等效隔震週期以及隔震系統等效阻尼比的發揮有相當大之影響，尤以下部結構之結構特性對於整體隔震效益之發揮影響最為嚴重。此外，由反應譜分析結果可發現，高模態反應對於下部結構層間剪力之影響不容忽視。 由於中間樓層隔震建築物存在一下部結構，因此高模態反應對於上部結構與隔震層之影響亦不可忽略，尤其在高模態發生耦合的情況下。經由等效線性分析以及振動台試驗研究結果可發現，高模態耦合會造成隔震層之加速度反應急遽放大。因此，本文提出一簡易分析方法可防止中間樓層隔震設計發生高模態耦合的問題，如此將可避免下部與上部結構因不當設計而造成高模態耦合的情況發生。 同時，本文針對基礎隔震與中間層隔震結構模型進行振動台試驗研究，試驗結果顯示中間層隔震設計可發揮良好之隔震效益。然而，由於其第一模態參與質量明顯小於基礎隔震結構，因此，在中間層隔震結構之試驗結果中可明顯看到高模態的反應。此外，當隔震層設置於較高樓層時，隔震系統會有較大之變形反應，且上部與下部結構之位移與受力反應會存在一超過90度之相位差。由試驗結果亦可得知，上部結構之慣性力與層間剪力主要仍為第一模態反應，但是下部結構之慣性力與層間剪力則主要由高模態反應控制，因此，未來在中間樓層隔震設計中必須合理地考慮高模態效應。 藉由數值分析與試驗研究結果，本文探討若將目前耐震設計規範中適用於基礎隔震設計之等效線性靜力分析程序，應用於中間樓層隔震設計之諸多不合理且不合宜之處。在考慮上部與下部結構特性對於整體阻尼比之影響，隔震系統之等效阻尼比應被更為準確且保守地估計。此外，對於中間樓層隔震系統之位移需求，應考慮上部與下部結構產生相位差之最保守設計，對此仍須進行後續更深入的研究。經由數值分析可發現，在適當且保守地考慮隔震系統之等效週期（或等效勁度）與等效阻尼比下，考慮足夠模態數之反應譜動力分析可作為中間樓層隔震建築物之初步設計方法。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