Study On Axial Compression And Bias Performance Of Stiffened Thin-walled CFST Short Columns

Using thin-walled steel tube in concrete filled steel tube(CFST)reduces the steel consumption and welding difficulty,which significantly improves the economic benefit of CFST columns.However,the local buckling of thin-walled steel tube limits the application and promotion of thin-walled CFST structures.To improve the structural behavior of thin-walled CFST columns,this paper proposed some innovate stiffeners,including binding steel plate stiffener and inscribed square steel tube stiffener for circular thin-walled steel tube,and inscribed octagonal steel tube stiffener and inscribed circular steel tube stiffener for square thin-walled steel tube.Experimental studies and finite element analyses were carried out on the behavior of the thin-walled CFSTs with the proposed stiffeners under axial and eccentric compression,and the models for predicting the load-carrying capacities were established.The main research contents are as follows:(1)Fourteen stiffened circular thin-walled CFST and twelve stiffened square thin-walled CFST stub columns were tested to failure under axial compression.The test parameters were diameter-thickness ratio(steel ratio),compressive strength of concrete,yield strength of steel tube,stiffening form,etc.The failure modes,load versus axial strain curves,and load versus tube stresses curves of the tested specimens were reported.The failure and deformation modes,the strain and stress development in steel tube,and confinement and load-carrying mechanism were analyzed.The influence of different stiffeners on the mechanical properties of the short columns was investigated.The test results showed that the ultimate strength and ductility of the circular thin-walled CFST specimens were significantly improved by the inscribed square steel tube stiffeners.The local buckling modes of thin-walled square tube were improve by both the inscribed octagonal steel tube stiffeners and inscribed circular steel tube stiffeners,leading to an enhancement of tube confinement and a better usage of residual strength of steel tube after buckling.Therefore,the load-carrying capacity and ductility of the stiffened square specimens were increased.(2)Ten stiffened circular thin-walled CFST and ten stiffened square thin-walled CFST short columns were tested under eccentric compression.The test parameters were diameter-thickness ratio(steel ratio),compressive strength of concrete,yield strength of steel tube,stiffening form,etc.The failure mode,load versus deflection curves,and load versus tube stresses curves of the tested specimens were reported.The failure and deformation modes,and the strain and stress development in steel tube were analyzed.The influence of eccentricities and different stiffeners on the mechanical properties of the short columns was investigated.The test results showed that the failure mode of all the thin-walled circular CFST specimens was characterized as bending failure.The ultimate strength and deformability of the circular specimens were significantly improved by using binding steel plate stiffeners and inscribed square steel tube stiffeners.While for the unstiffened square specimens,local buckling was observed on the compression face,but their bend deformation was insignificant.The square specimens with inscribed octagonal steel tube stiffeners and inscribed circular steel tube stiffeners were failed by bending,and their strength and deformability were significantly improved compared to the unstiffened ones.(3)Based on ABAQUS software,the finite element(FE)models of circular and square stiffened thin-walled CFST stub columns under axial and eccentric compression were established,and the predicted results are in good agreement with the experimental results.On this basis,the complete loading process analysis and parameter analysis were carried out.The results showed all the stiffeners can effectively delay the local buckling of the thin-walled steel tube,thus improving the confinement effect as well as the utilization efficiency of materials.In addition,the increase of steel ratio,compressive strength of concrete,and yield strength of steel tube can improve the sectional bearing capacity.As the eccentricity increases,the axial load-carrying capacity of the specimen decreases and the ductility increases.(4)Based on the experimental results and FE analyses,the load-carrying mechanism of thin-walled steel tube with different stiffeners was presented.The calculation models to predict the biaxial stresses and equivalent confinement of steel tube were established.For the stiffened thin-walled CFST stub columns under axial compression,the load-carrying capacity was calculated by considering the enhancement of concrete compressive strength caused by confinement,and the reduction of longitudinal stress in steel tube due to the lateral tension.For the stiffened thin-walled CFST stub columns under eccentric compression,the characteristic points on the axial load(N)versus moment(M)interactive curves were predicted on the basis of full-section plasticity assumption in EC4,in which the contribution of the stiffeners were taken into account.The predicted results of the proposed methods are in good agreement with the experimental and FE results,providing a theoretical basis for the design and application of stiffened thin-walled CFSTs.