Research On The Influence Of Setback To Seismic Performance Of High-rise Building Structures And Study Of Limit Method

Setback is a common type of structural irregularity, mainly including four forms: step back at the roof of podium, step back at middle height of the structure, small tower at the top and reducement of core-tube. Previous earthquake showed that the structural members adjacent to the location of setback were easily broken in strong earthquake. Due to the serious damage of local structural members, the building collapsed before the overall seismic capacity was fully utilized. The vertical structural irregularity due to setback is discussed in this dissertation, focusing on the influence of setback on the seismic performance of reinforced-concrete frame and wall-frame structures. The mainly research work and conclusion are summarized as follows.(1) Based on the continuum theory, a simplified model is established to simulate the behavior of structures with vertically varying stiffness. Axial deformation of the columns is considered in the model, which improves the computation accuracy. The comparison result shows that the simplified model agrees with the Finite Element Model(FEM). Using the simplified model, influence of varying stiffness or mass on the elastic response of frame/wall-frame structures is studied. Result shows that setback influences the lateral deformation continuity and the whipping effect. The first storey above the location of setback is the part of greater force and larger deformation. The lateral stiffness ratio γ is relevant to the lateral deformation continuity and whipping effect, which is defined as the ratio of lateral stiffness between the storey above and below the setback. γ is proposed to be the limitation index to control the influence of setback.(2) According to the Chinese present standards, a group of frame/wall-frame structures with different forms of setback are designed. Relation between the lateral stiffness ration γ and the nonlinear response in different intensity earthquake is studied by nonlinear time-history analysis. The validity of γ is certified in respect of structures’ deformation, yielding mode and stress distribution in structural members. Finally, γ is suggested as 0.7 for frame and 0.9 for wall-frame structures. For frame structures, the lateral stiffness is defined as the ratio of storey shear to inter-storey drift. For wall-frame structures, the lateral stiffness is defined as the ratio of storey shear and inter-storey drift ratio.(3) The failure mechanism of vertical structural members adjacent to the location of setback is analyzed in detailed. Result shows that formation of weak part is relevant to normal section capacity redundancy ratio ρ. The seismic weak part can be located by ρ. For structures with multi-lines of seismic defense, ρ is effected by re-distribution of internal force. A modified calculation method of ρ based on precautionary intensity earthquake is proposed. The validity of the method is certified by two examples: an 8-storey frame structure and 24-storey wall-frame structure. By comparing the damage situation under rarely occurred earthquake, the method of capacity redundancy ratio ρ is in consistency with nonlinear time-history analysis.(4) To solve the problem that vertical structural members adjacent to the location of setback are easily to break in earthquake, a seismic design method based on the pre-designed yielding mode is proposed. In this method, structural members are classfied into two types: crucial members and energy dissipation members. Firstly, an elastoplastic model is established assuming that the crucial members will keep elastic in moderate or major intensity earthquake. Secondly, the seismic capacity demand of crucial members is calculated by equivalent linearization spectra analysis, followed by cross section design. Frame/wall-frame structure examples are designed by the proposed method and the method in present standards respectively. Nonlinear time-history analysis shows that the structures designed by the proposed method have a better yielding mode, and have no weak part in earthquake. However, weak part appears in the structures designed by the method in present standard.