| High-rise buildings have become the most common building form in China,and framecore tube structure is the main structure form of high-rise buildings,especially super high-rise buildings.The seismic conceptual design requires that the frame should have reasonable stiffness and bearing capacity in the frame-core tube structure,and it also should be strengthened according to Chinese and foreign design codes.The stiffness and bearing capacity of the frame are enhanced by controlling the minimum frame-shear ratio and the frame shear amplification according to Chinese design code and technical points of expert peer review.The existing research results show that this plays an important role in improving the seismic safety of frame-core tube structures.However,the regulation of frame-shear ratio and its limit value is based on the qualitative seismic concept rather than the quantitative research results,which has become a hot issue in the structural design of high-rise buildings in recent years.Aiming at this problem,base on the cooperative working principle of dual system and the secondary aseismic system,the physical significance,distribution law,influencing factors,variation rules of frame-shear ratio and its influence on the overall seismic performance of frame-core tube structure are systematically studied by using the continuum analysis,static pushover analysis and incremental dynamic time history analysis.The main contents and conclusions are as follows:1)Based on the cooperative working principle of dual system,the basic differential equations of frame-core tube structure with uniform stiffness considering bending shear coupling effect and nonuniform stiffness are established by using continuum analysis method and solved by numerical method.The derivation and solution of the equations are verified by finite element analysis.The results show that: the frame should have reasonable stiffness to ensure adequate cooperative effect of dual system in frame-core tube structure;the frame-shear ratio index can quantitatively reflect the relative stiffness between the frame and the core tube,and it is reasonable to adopt the frame-shear ratio in Chinese design code.The essence of this limit is to ensure the relative stiffness ratio(stiffness characteristic value)between frame and core tube should not be too small.For high-rise especially super high-rise frame-core tube structure with nonuniform stiffness,the distribution of frame-shear ratio curve along the height presents the characteristics of "large in the middle and small at both ends".The "maximum frame-shear ratio" can be used to ensure the relative stiffness of frame and core tube,and the limit value of frame-shear ratio on top floors can be reduced.2)Based on the principle of equal stiffness,9 and 12 frame-core tube models with different stiffness ratio(frame-shear ratio)were designed.The static pushover analysis of these models were carried out by using perform3 d software,and the plastic development process and variation rule of frame-shear ratio were compared.The analysis shows that: the internal force redistributes between the core tube and the frame with the plastic development of the structure,and the change of frame-shear ratio curve exhibits two modes which determined by the stiffness ratio(frame shear ratio);when the initial maximum frame-shear ratio is greater than 5%,the change rule is "first increase and then decrease",and when the initial maximum frame-shear ratio is small than 5%,the change rule is "always increase";the former reflects the advantages of the dual system which give full play to the secondary aseismic system in the process of internal force redistribution,and the overall performance of the latter is close to the single system without the secondary aseismic system.From the point of secondary aseismic system,the frame should have reasonable stiffness in frame-core tube structure,and the initial maximum frame-shear ratio must be greater than 5% base on the results of models in this paper.3)Based on the principle of equal stiffness,five frame-core tube models with different frame-shear ratio(including a single system)are designed.The incremental dynamic time history analysis(IDA)is performed by ABAQUS software.The dynamic pushover curves,plastic development and damage,variation rules of frame shear ratio and frame overturning moment ratio,stiffness degradation,collapse probability and collapse margin ratio of different models are compared and the influence of frame-shear ratio on the overall seismic performance is studied.The results show that: under the same input conditions,the dual system exhibits better seismic performance than the single system;the model with larger frame-shear ratio exhibits smaller wall damage,slower stiffness degradation and better ductility.Under the precautionary rare earthquake,the single system and the dual system with different frame-shear ratio can reach the predetermined performance requirements;however,with the further increase of earthquake intensity,the dual system has higher seismic redundancy and the model with larger frame-shear ratio exhibits higher seismic redundancy.The collapse margin ratio of the structure increases with the increase of the frame-shear ratio,and the single system has significantly lower collapse margin ratio than the dual system.4)The IDA results show that: with the increase of peak acceleration,the proportion of overturning moment shared by the frame gradually grows and increases more for model with larger frame-shear ratio.Under the precautionary and over precautionary rare earthquake,the overturning moment of the single system model is basically born by the core tube,and that shared by the outer frame of the dual system model increases more,which reveals the outer frame plays an important role in anti-overturning,thus delaying the stiffness degradation of the whole structure.The change of the curve and the maximum value of average frame-shear ratio exhibits two modes with "always increase" for single system and "first increase and then decrease" for dual system,which reflects the difference between the single system and the dual system.According to the change mode of the proportion of overturning moment shared by the frame of the bottom floor and the frame-shear ratio,the frame stiffness with the maximum frame-shear ratio greater than 5% can meet the requirements of the secondary aseismic system of the frame-core tube structure base on the results of models in this paper.5)The overall seismic performance evaluation index of frame-core tube structure based on the stiffness degradation under dynamic action is proposed as "stiffness degradation coefficient",which is defined as the weighted average stiffness degradation rate of the horizontal dynamic stiffness of each order of the structure,and is verified by shaking table tests of two scale models.Five frame-core tube models are evaluated and compared by that index,and the results show that: the stiffness degradation coefficient increases with the increase of the peak acceleration.Under the precautionary and over precautionary rare earthquake,the stiffness degradation coefficient of the single system model is significantly higher than that of the dual system model,and the model with larger frame-shear ratio has lower stiffness degradation coefficient.It’s proved that the dual system exhibits better seismic performance than the single system,and the model with larger frame-shear ratio has higher seismic redundancy.6)The elastic-plastic dynamic time history analysis shows that the internal force of the frame-core tube structure redistributes under the strong earthquake,and the frame should have a certain strength(bearing capacity)to undertake the seismic force transferred from the core tube.Taking four dual system frame-core tube models as examples,this paper compares and analyzes the frame shear adjustment methods in current Chinese and American codes,and puts forward two practical frame shear adjustment methods based on elastic-plastic time history analysis and equivalent linearization analysis respectively. |
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