Research On Mechanical Behavior Of Concrete-Filled Square Steel Tubular Stub Columns With Inner I-Shaped CFRP Under Axial Load

Concrete filled steel tube(CFST)with the advantages of high bearing capacity,good plasticity,good ductility,convenient construction,corrosion resistance,fire resistance and good economic performance has been widely used in long span bridge,factories and high-rise building engineering.Carbon fiber reinforced polymer(CFRP)has been gradually popularized in civil engineering because of its high strength,light weight,corrosion resistance and easy molding.This paper combines the advantages of two kinds of materials,which put the I-shaped pultruded CFRP profiles in concrete square steel tube,making the CFRP profiles binded by core concrete.It can restrain the local buckling of CFRP profile flange,improve the ductility of component,reduce the size of structural column,and reduce the weight of structure,and increase the construction area.In this paper,a comprehensive analysis was presented on the mechanical behavior of Inner I-shaped CFRP concrete-filled square steel tubular stub columns subject to axial load by the test and finite element analysis methods.The test can be divided into two parts:the full section compression test of I-shaped CFRP profile and the axial compression test of inner I-shaped CFRP concrete-filled square steel tubular stub columns.Firstly,the full section axial compression test of 5 I-shaped CFRP pultruded short columns is carried out.The mechanical behavior and failure process are analyzed,and the compressive strength,elastic modulus,Poisson's ratio and other mechanical properties of the I-shaped CFRP pultruded profiles are determined.Secondly,5 groups of inner I-shaped CFRP concrete-filled square steel tubular stub columns and one group of CFST were designed with the variation parameters of steel ratio and steel strength,with two specimen in each group.Through the observation and analysis of the test,the failure mode,bearing capacity and working mechanism of the axial compression member are studied.On the basis of tests,the finite element model is established by choosing the constitutive relation,element type,contact form and boundary condition.Finally,combining the test and finite element simulation results,in-depth study of the I-shaped pultruded CFRP profiles,concrete core and steel stress strain distribution characteristics,load ratio,square steel tube to the core concrete of the binding.The influences of concrete compressive strength,steel ratio and steel yield strength on ultimate bearing capacity and initial axial compression stiffness of concrete members are analyzed.The design indexes of bearing capacity are deduced according to the test and finite element simulation results,and their applicability is checked by referring to the calculation formula in the reference code.The results show that the axial compressive short column of I-shaped CFRP pultruded section is the brittle failure form in the middle part of the specimen,and the failure zone of the section can be described more accurately by using the Tsai-Wu strength criterion;The whole process of concrete-filled steel tubular stub columns with I-shaped CFRP section can be divided into elastic stage,elastic-plastic stage,descending stage and gentle stage;Component CFRP profile,core concrete,steel tube can work well under the action of axial force,with the increase of steel ratio and concrete strength,the initial compressive stiffness and ultimate bearing capacity were improved.With the increase of steel yield strength,the ultimate bearing capacity of the component increases,and the initial compressive stiffness does not change significantly;The GB59036(2014)and the EC4(1994)code can predict the ultimate bearing capacity of the test specimen accurately.The design indexes of the bearing capacity deduced by the finite element simulation results are in good agreement with the experimental values.