Experimental Study On The Axial Compression Behavior And Hysteretic Performance Of Multi-Cell Ultra High Performance Concrete-Filled Double Skin Steel Tubular Short Columns

Due to the advantages of high strength,good durability and low carbon emission,Ultra high performance concrete(UHPC)has been favored by scholars and engineers recently.Applying UHPC to concrete-filled steel tube(CFST)and concrete-filled double skin steel tube(CFDST)with high strength and good ductility can form UHPC-filled steel tube(UHPCFST)or UHPC-filled double skin steel tube(UHPCFDST),and the basic mechanical properties of these members have been widely studied worldwide.However,because of the ultra-high compressive strength of UHPC,higher strength steel or thicker steel tubes are required to achieve a confinement factor comparable to ordinary CFST.At present,according to the relevant literature,some of the texperiments tend to set a very small width-to-thickness ratio of the steel plate to obtain a large confinement factor,which is not economical enough although the mechanical properties are good.Another part of the literature investigates UHPCFST with thin-walled steel tubes with a large width-to-thickness ratio,which,despite their low steel ratio,the strength of steel can not be fully utilized due to local buckling.Therefore,the author proposed to apply UHPC and high strength steel to the newly developed multi-cell concrete-filled double skin steel tube(MCFDST)developed by the research group to form multi-cell ultra high performance concrete-filled double skin steel tube(MUHPCFDST)This new structure was an improved form based on UHPCFDST by welding the inner and outer steel pipes into a whole by connection plate and inherits the advantages of high strength,good ductility and large stiffness-to-weight ratio of UHPCFDST,which has research significance and engineering value.In this paper,the mechanical behavior of UHPCFST,UHPCFDST and MUHPCFDST columns under axial compressive loadings and low cycle reciprocating loading were studied experimentally,theoretically and numerically.The main research contents are as follows:(1)Pre-experiment results with quadrilateral cross-section and normal strength materials confirm that the axial compression performance of MCFDST is better than CFST and CFDST,which confirms the significance of conducting mechanical performance studies on MUHPCFDST,UHPCFDST and UHPCFST columns.(2)MUHPCFDST,UHPCFDST and UHPCFST short columns with quadrilateral,hexagonal and circular cross-sectional shapes were tested and the failure mode,bearing capacity and ductility of those members were investigated,systematically.Based on the existing UHPC test literature,a uniaxial constitutive model considering the content of steel fiber is established.A material subroutine considering the constraint of steel pipe on UHPC is developed and applied to the corresponding numerical analysis model.The correctness of the model is verified by comparing with the experimental results.(3)Based on the verified numerical model,a large number of parameter analyses are carried out to study the effects of section shape,steel strength,UHPC strength,section void ratio,outer steel pipe wall thickness and connecting plate thickness on the axial compression performance of members;The calculation methods of axial compression bearing capacity of these kinds of short column members are proposed.(4)Low cycle reciprocating loading tests of UHPCFST,UHPCFDST and MUHPCFDST members with hexagonal sections were carried out.Based on the test results of the lateral force-displacement curves,vertical load – displacement curves,moment – displacment curves,strain response,ductility and energy dissipation the seismic performance of three kinds of hexagonal members is systematically discussed.The validity of the numerical model is verified by comparing with the experimental results.(5)Based on the verified numerical model,parametric analysis were conducted to study the enfluence of steel strength,UHPC strength,hollow ratio,outer tube thickness and strip thickness on the hysteretic performance of hexagonal MUHPCFDST,UHPCFDST and UHPCFST members.Finally,the author proposed a modified M/M_u-N/N_u curve for these columns,based on the method provied in T/CCES 7-2020.