Seismic Performance Of Assembled Joint To CECFST Column And Vulnerability Analysis Of Assembled CECFST Frame

In recent years,steel-concrete composite members has been widely applicated.Various types of steel-concrete composite members emerge one after another.Concreteencased concrete-filled steel tube(CECFST)column and partially-encased concrete(PEC)beam have favorable mechanic performance and fire resistance because the concrete encased in the steel surfaces.Innovative composite frames using CECFST columns and PEC beams have broad application prospects.However,there is a dearth of researches on the composite joints between CECFST column and PEC beam and frames using these two members.This paper makes an attempt to assemble CECFST columns to PEC beams using extended end-plate connections and all-bolted connections,which has wide application prospects in assembled architectures of practical engineering.For the purpose of getting the performance of the joints and frames,this paper studied the seismic performance of assembled joint to CECFST column and vulnerability analysis of assembled CECFST column frame,which has important theoretical and engineering significance.The main work of this paper could be concluded as follows:(1)Four specimens with different beam types and connection types were designed and fabricated,which have been tested under low cyclic loading.Failure modes of each specimen have been studied.Furthermore,evaluating indicators including horizontal load versus displacement hysteretic and skeleton curves,moment versus angle skeleton curves,displacement ductility coefficient,strength and stiffness degradation and energy dissipation capacity have been thoroughly studied.The mechanic mechanism has been studied by observing the stress distribution of key locations combined with the experimental phenomenon.The results shows that the joints have favorable seismic behavior.(2)FEA models of each joint are established by ABAQUS software,and the constitutive relation,contact pairs,element type,boundary condition and mesh size used in the model are defined.The analytical results are compared with the experimental results to verify the reliability of FEA models.In this paper,load transfer mechanism of CECFST column in extended end-plate joint has been studied,and the load transfer paths of outer concrete under bolt tension and compressive zone pressure are discussed.On this basis,the calculation methods of flexural capacity and initial stiffness of two types of joints are proposed.For the extended end-plate joints,the influence of concrete between the flanges of PEC beam is considered in the calculation method of flexural capacity.According to the load transfer path of the CECFST column wall and considering the influence of outer concrete,the initial stiffness calculation formulas of the extended endplate joint and all-bolted joint are deducted by using component method.Finally,two methods are used to evaluate the stiffness of the joints,which lays a theoretical foundation for the nonlinear time history analysis and seismic vulnerability analysis of the frames.(3)FEA models of each joint are established by using Open Sees software,including fiber section model,constitutive relation model and semi-rigid joint domain model.The load versus displacement hysteretic curves of each joint are obtained by simulation,and the rationality of the modeling method is verified by comparing with the experimental results.Based on the Open Sees modeling method and the joint calculation method derived in the previous sections,the composite frames formed by CECFST columns to PEC beam with two different types of joints are established.The nonlinear time history analysis of two frames under different ground motions is carried out,and the seismic capacity of the frame is evaluated with the maximum and residual inter story displacement angles as indexes.Based on the incremental dynamic analysis,the seismic vulnerability of the frames is studied,and the performance level of the frames under the collapse probability and non-collapse probability is revealed,which provides reference for practical engineering.