Research On Axial Pressure Behavior Of Prestressed Thin-Walled Steel Pipe Restrained Concrete Short Columns

The concept of combining prestressing technology with steel pipe concrete structures has now become a new research and development direction for steel pipe concrete,with the aim of improving the deficiencies of traditional steel pipe concrete structures and enhancing their mechanical properties.Therefore,with reference to the existing steel pipe concrete structure,it is proposed to form an active compressive restraint on the internal concrete by tensioning thin-walled round steel pipes with buttress bolts,so that the core concrete of the combined column can enter the three-way compressive stress state earlier under the axial compression load and thus enhance the concrete strength.At the same time,the composite column structure can effectively inhibit the inward local buckling of the steel tube and delay the local crushing of the concrete,thus improving the mechanical behavior of the axially compressed composite column,enhancing its mechanical properties and improving the utilization rate of steel and concrete.In order to study the axial compression mechanical properties of this prestressed thin-walled circular steel tube restrained concrete short column,the following work elements were carried out in this paper:(1)The basic mechanical properties of the material were tested,and the compressive strength,modulus of elasticity and Poisson’s ratio of concrete were obtained by measuring the standard concrete specimens made from the same batch as the combined short column specimens,and the tensile yield strength,tensile ultimate strength,modulus of elasticity and Poisson’s ratio of the thin-walled round steel pipe were obtained by tensile tests,which clarified the mechanical index of the buttress bolts and also laid the foundation for finite element modeling.(2)Sixteen specimens were designed and fabricated with the steel pipe diameter-thickness ratio(D/ t)and pre-stress degree(k)as the research variables,and axial compression tests were conducted on them to obtain the results of the loading process,failure mode,load-displacement curve and load-strain curve of each specimen,and the variation law of axial compression ultimate bearing capacity,yield stiffness and ductility of all specimens were analyzed.The results show that the initial prestressing action can effectively improve the failure mode of the specimens,which can change from compressive shear failure with poor ductility to compressive bending failure with good ductility,and finally develop into compressive drum failure with the best ductility.In addition,compared with the non-prestressed specimens,the axial compressive ultimate bearing capacity,yield stiffness and ductility of the specimens with prestressing increased significantly with the increase of the prestressing degree of the specimens,and the highest increase rate was 17.91%(when D/ t =56 and k =0.45),51.21% and 119.36%(when D/ t =74 and k =0.45)respectively;meanwhile,the above mechanical indexes also increased with the decrease of the diameter-thickness ratio of the specimens.(3)Based on the material test parameters,the finite element model of each specimen was established by ABAQUS finite element software,and the same loading conditions were set and simulations were completed,and then the simulation results were compared with the test results and found to be in good agreement,thus proving the reliability of the finite element simulation.Then a more detailed parameter expansion analysis of the ultimate bearing capacity of the combined short column was carried out based on the reliable finite element model.The results show that,within the reasonable parameters,the ultimate bearing capacity of the combined short column decreases with the increase of steel pipe diameter to thickness ratio,increases with the increase of concrete strength,and increases with the increase of prestressing degree;however,the utilization rate of the combined short column for thin-walled steel pipe increases with the increase of steel pipe diameter to thickness ratio,the utilization rate of concrete decreases with the increase of concrete strength,and the combination efficiency of thin-walled steel pipe and concrete increases with the increase of prestressing degree.Increase and increase.However,the larger pre-stress degree(when k =0.5)will make the member steel pipe there is a potential risk of local cracking,and eventually lead to sudden damage along the steel pipe cracking location.(4)According to the test and simulation results,the force mechanism of the combined short column is clarified,and then the equation is derived by combining the existing basic mechanical theory to establish the theoretical calculation formula for the axial compression load capacity of thin-walled round steel pipe concrete column,and the correction coefficients of the influencing factors(including the correction coefficients of the diameter-thickness ratio of steel pipe W,the correction coefficients of the flexural strength ratio of steel and concrete R,and the correction coefficients of the prestressing degree K)are introduced to obtain a perfect application for the The theoretical formula for the axial compression load capacity of prestressed thin-walled circular steel pipe restrained concrete column is obtained and compared with the test results and simulation results respectively.The results show that the error between the theoretical formula and the test results is within±8%,and the error between the theoretical formula and the simulation results is within ±4%.