## 3 Design Method for Seismically Isolated RC Frame-Core Tube Tall Building  
### Traditional Design Method  
For the traditional design method of seismically isolated tall buildings, the seismic performance targets, including the limits of the \(\beta\) and the MBD under the MCE, are determined first. According to the GB50011-2010 [17], \(\beta\) is required to be no more than 0.40 and 0.38 for the structures without and with the NFVDs in the isolation system, respectively. This indicates that the seismic forces of the superstructure are similar to that of a fixed-base structure with one-degree lower seismic intensity. Due to this fact, the seismically isolated structures are designed following a step-by-step iterative design method as shown in Figure 8.  
Figure 8: Traditional design method for seismically isolated tall building. (Q1\(\sim\)Q3 denote the critical problems). The design method can be divided into 5 steps:  
(1) Design of the superstructure. The superstructure is first designed as a fixed-base structure with a one-degree lower seismic intensity. An appropriate superstructure is the basis for the seismic design of the isolated building. The \(T_{\text{f}}\) is the most critical index to be determined in this step, because it is highly associated with the seismic force of the structure and the seismic absorbing coefficient. (2) Selection of the isolator numbers and sizes. The numbers and sizes of isolators are determined according to the appropriate compressive stress of each isolator under gravity load (referred to as \(\sigma^{8}\) hereafter). It is notable that the requirements of \(\sigma^{8}\) for different isolators, including the isolators under the columns, the corner of core tube, the outer wall and the inner wall, are different. Due to the significant overturning effect of the RC frame-core tube tall buildings, the isolators under the columns and the corner of the core tube may possibly be under tension, a higher \(\sigma^{8}\) is usually required for these isolators. However, an appropriate \(\sigma^{8}\) for different isolators of RC frame-core tube buildings has rarely been investigated and reported, which is critical for the selection of numbers and sizes of isolators. (3) Selection of the isolator types. Determining the type of the isolators (LRB or NRB) is a key step in the design of such buildings. This can significantly affect the seismic performance of the isolation system and the entire structure. Specifically, an appropriate yield ratio is required first, as mentioned above. In addition, a rational distribution of the LRB isolators should be carefully designed to avoid torsion effect. However, a comprehensive investigation on the appropriate yield ratio and the distribution of the LRB isolators has rarely been proposed for the RC frame-core tube seismically isolated tall buildings. (4) Calculation of the design indexes. After the preliminary isolation system is established, critical design indexes, including \(\beta\), MBD under the MCE, \(\sigma^{\text{p}}_{\text{max}}\) and \(\sigma^{\text{t}}_{\text{max}}\), are assessed to validate the feasibility of the isolation system. The \(T_{\text{is}}\) is initially obtained through modal analysis. Then, a set of appropriate ground motions are selected according to \(T_{\text{f}}\) and \(T_{\text{is}}\). The THAs are conducted for these two structures and critical design indexes are calculated. (5) Validation and/ or modification of isolation system. Three design indexes, including \(\beta\), \(\sigma^{\text{p}}_{\text{max}}\) and \(\sigma^{\text{t}}_{\text{max}}\), are evaluated. If one of these indexes cannot meet the requirements, the sizes, numbers or types of the isolators will be constantly modified (i.e., iterate from step 2) until these indexes can satisfy the requirements. Subsequently, the MBD under the MCE is examined. The introduction of the NFVDs in the isolation system is recommended, if the MBD under the MCE exceeds the limit value specified by the allowable width of the isolation gap.