Research On Mechanical Properties Of Concrete Filled Double Skin Steel Tube Under Eccentric Compression And Compression-torsion Combined Forces

In order to study the mechanical properties of concrete filled double skin steel tube(CFDST)under the combined action of compression and torsion,the section size and axial compression ratio are used as parameters to perform the compression and torsional static test on 12 square CFDST,and the finite element model was established to analyze the effect of material strength and slenderness ratio on the bearing capacity.And the numerical model was established with the parameters of corrosion rate of local corrosion and eccentricity of load.The influence of corrosion rate and eccentricity of load on the bearing capacity and failure mechanism of such members are analyzed.The results show that when the axial compression ratio is relatively small(n<0.5),the axial compression ratio has little effect on the torsional bearing capacity.After entering the plastic,the torque-rotation curve has a significant yield platform with slight increases in bearing capacity,and the component has good ductility.When the axial compression ratio is relatively large(n> 0.5),the torsional bearing capacity decreases significantly with the increase of the axial compression ratio.When n=0.6,the torsional bearing capacity has been reduced to 70% of pure torsion.After entering plasticity,the bearing capacity decreases sharply and the ductility is poor,showing a completely different failure mode.In addition,the strength of the outer steel tube has a greater impact on the torsional bearing capacity,and the concrete strength has little effect on the bearing capacity.The torsional stiffness decreases significantly with the increase of the slenderness ratio,but the bearing capacity is not obviously affected by the slenderness ratio.The circular CFDST has a higher degree of coincidence of the T-θ curves at different positions of the middle section.Compared with the square outer tube,it has a more uniform restraint effect on the internal structure.At the beginning of torque loading,the hoop strain and shear strain at the corner of the outer steel tube with square cross-section are similar to the middle.At the later stage of loading,the shear strain in the middle is greater than that in the corner,but the hoop strain in the corner is greater than that in the middle.It shows that there is stress concentration in the corner,which has a more obvious restraint effect on the internal structure,at the same time,the interaction force of the corner is greater than that of the middle during the whole process.The local corrosion of the outer steel tube results in a decrease of stiffness in the elastic range.The greater the corrosion rate is,the earlier the local buckling of the corrosion area will occur,which will cause plastic failure and reduce the bearing capacity.Load eccentricity has a greater effect on bearing capacity,the bearing capacity decreases sharply with the increase of eccentricity when eccentricity is less than 1.5,and the decreasing tendency of the bearing capacity gradually becomes flat when eccentricity is greater than 1.5.The restraint effect of the outer steel tube on the internal structure is greatly affected by eccentricity.The constraint effect is obvious when eccentricity is small,and the outer steel tube only generates a circumferential tensile strain.With the increase of eccentricity,local buckling first appears in the steel tube on the side close to the load point in the corrosion area,at the same time,the circumferential compress strain occurs on the steel tube on the side far from the load point in the corrosion area.The concept of critical corrosion rate is proposed.And the relationship between the critical corrosion rate and the thickness of the outer steel tube and the relationship between the critical corrosion rate and steel strength are analyzed.The longitudinal stress of the sandwich concrete in the erosion pit area during the whole loading process is obviously increased compared with other positions,and there is obvious horizontal expansion after entering the plastic,but other positions are still in the elastic stage.Finally,the formula for calculating the bearing capacity of square CFDST columns under local corrosion is proposed.