Study On The Novel High-rise Large Span Cassette Structure And Its Seismic Performance

In recent years,many high-rise buildings have been constructed in China,but these buildings have similar structures.Although the frame structure has many advantages,the large element cross-section and occupied space division restrain further development.To optimize these cases,the cassette structure was introduced in high-rise large span buildings.Composed by open-web sandwich slab and grid frame,cassette structure has a larger span,better antiseismic performance and larger energy-dissipating capacity than the original frame structure.Current design process of cassette structure is coarse,and the structural performance in highrise buildings lacks research.Therefore,the main studies of this paper are focus on the modified design processes of the structure,the structural performance of the structure under earthquake,and structural assessment considering the uncertainty of earthquake.The main research work and conclusions are listed as follow:(1)The modified design process of the open-web sandwich slab considering the line stiffness ratio and shear deformation is proposedThe current design procedure assumes that an open-web sandwich slab can be transformed to an equal-bending-stiffness solid beam.However,many parameters,such as line stiffness ratio and length-to-height ratio of the chord lying between the two shear keys,can largely influence the stiffness and strength of the open-web sandwich slab,which is not considered in the current design procedure.Therefore,based on experiment and numerical studies,several design threshold values and an integrity value were proposed,and the recommended values in the design process of open-web sandwich slab were given to modified the current design process.(2)The modified design process of the grid frame is proposedThe cassette structure is a large-span high-stiffness building,in which the grid frame is the vertical system and guarantees its remarkable performance under lateral loads.Although the grid frame has a large lateral stiffness,the original design method is inaccurate,since it ignores the relative rotation in the beam-column joint and the shear effect in the columns,which can cause an undesired fail mode and brittle tendency under large earthquakes.Based on the theoretical calculations,tests and simulations,a modified design method is presented and the relative rotation in the beam-column joint and the shear effect are considered.A practical building was used as a case study and the overall performance of the frame,original grid frame and new grid frame under earthquakes were conducted.Furthermore,the IDA analysis was used to consider the uncertainty of the earthquake,and the fragility analysis based on the probability method was also used.Among these results,the new grid frame shows a better structural performance and ductility than the original grid frame and frame structure.(3)The seismic performance of high-rise cassette structure is studiedBased on the substructure tests,the overall performance of cassette structure under earthquake was conducted.The study showed when the building height was low(approximately50 m),the structure deformation shape was shear,and bending deformation was the majority in the structure elements.In this case,cassette structure had obvious advantages over frame structure.Through pushover analysis and IDA,the advantage of cassette structure is obvious.Compared with traditional structures,cassette structure features large stiffness,good seismic behavior and good span performance.It can be used to reduce the amount of material used and performs well in tall buildings.Based on the shake table test,the seismic performance of the cassette structure satisfies the design purpose.Under the design DBE and MCE levels,the drift of the structure is smaller than the threshold value,and no obvious damage can be found.When the level of ground motions increases,an obvious plastic behavior can be seen,and the drift increases.However,even under a large ground motion,the structural integrality can be maintained,and only two story-drift-ratios exceed the code limit,which shows a good antiseismic performance of the structure.Furthermore,finite element models are established and the probabilistic seismic analysis is conducted based on the shaking table test,and the seismic performance of cassette structure is studied roundly.(4)The case study of cassette structure and frame structure is proposedAn existing frame office building is selected and redesigned as a cassette structure to compare the performance of the two kinds of structures.Three-dimensional(3D)finite element models are built for the two prototype structures,where fiber elements are used to simulate plastic behavior.To assess the advantages fully,seismic analysis and economic analysis are performed.Eighteen Federal Emergency Management Agency(FEMA)recommended seismic records are used,and the roof displacement,story drift ratio,plastic hinges development and Park–Ang damage index are compared.From the results,cassette structure shows a much better performance then the frame structure.Therefore,replacing a frame structure with a cassette structure can be a good option.