Seismic Performance Of Square Concrete-Filled Steel Tubular Columns With Stiffened Plastic Hinge Region

Concrete-filled square steel tube columns（CFST）has been widely applied in the structures of high-rise buildings and underground spaces.The local buckling of square steel tube in CFST columns has been well observed when hit by strong earthquakes,which reduced the stability and load capacity of the CFST columns as well as the structures.Aiming at improving the seismic behavior of CFST columns by enhancing the interaction between the steel tube and core concrete,a variety of stiffening methods have been proposed to strengthen the local stability of the steel tube to delay the local buckling.Numerous researches have been conducted to investigate the static mechanical behavior of the stiffened concrete-filled square steel tube columns（SCFST）.However,there are few,if any,experimental data and theoretical research focuses on the dynamic performance of SCFST columns,and the stiffening method still needs to be further optimized.Based on analysis of advantages and disadvantages of the previous stiffening methods,this paper proposed a new type of stiffening method and verified the effectiveness of the new method through experimental and theoretical investigation into the seismic performance of SCFST columns.The main research work and conclusions are listed below.（1）Four CFST columns and eight SCFST columns were fabricated and tested under reversed cyclic lateral force and constant compression to verify the effectiveness of the proposed new stiffeners in improving the seismic behavior of the CFST columns along with investigation of effects of the width-to-thickness ratio,the axial compression ratio,and the shear-span ratio of columns on the hysteretic performance of CFST and SCFST columns.The experimental results have indicated that the proposed stiffeners could improve the energy consumption capacity and enhance the lateral load capacity of SCFST columns by14%to 38%comparing with CFST columns.The stiffeners also effectively delayed the local buckling of steel tube with improved local stability of steel plate and modified the local buckling mode.With increasing shear-span ratio,the lateral load capacity of SCFST decreased with more obvious local buckling of square steel tube.For CFST and SCFST columns with shear-span ratio of 3,with decreasing width-to-thickness ratio,the energy consumption capacity,the lateral load capacity and the corresponding drift ratio to the maximum lateral load increased,and the local buckling was effectively delayed.On one hand,when the axial compression ratio varied between 0.2 and 0.35,the axial load had little effect on the occurrence and location of local buckling.On the other hand,the local buckling occurred earlier and more obviously when loaded with the axial compression ratio of 0.5.（2）The resistance mechanism of steel tube and infilled concrete in CFST and SCFST columns under cyclic lateral loading while subjected to constant axial compression was studied.The nonlinear deformation of steel tube,the mechanism of stiffeners improving the local stability of steel tube,and the bonding force transfer mechanism at the interface between steel tube and core concrete were analyzed.The analytical results indicated that the Poisson’s ratio of steel tube was between 0.2 and 0.45 when only the axial load was applied,and the steel tube provided little confining effect to the concrete core.After the reversed cyclic lateral load was applied,along with the drift ratio,the steel tube commenced providing“confining effect”to core concrete once the Poisson’s ratio of the compressed core concrete became larger than that of the steel tube.The compressed core concrete was in a three-dimensional compressive stress state,while the tensioned core concrete was in a three-dimensional stress state of tension-compression-compression.The compressed portion and the tensioned portion of steel tubes were in a tension-compression bi-axial stress state and tension-tension bi-axial stress state,respectively.The nonlinear deformation of steel tube in CFST and SCFST columns was concentrated within the heights of 200 mm and 100 mm from the column root,respectively.The steel tube out of this height provided weak confinement on the core concrete.According to the plate stability theory,due to the reduced width of the steel tube plate by the stiffener,the buckling-resistant capability of the steel plate was improved to enhance the local stability of the steel tube and the overall mechanical properties of the member.（3）The applicability and accuracy of the design methods recommended in several representative design codes and/or standards for assessing the ultimate lateral load capacity of CFST（4 columns in this paper and 31 columns collected in previous references）and SCFST（8 columns in this paper and 15 columns collected in previous references）were investigated through comparing the test results with the calculated ones.The comparison results indicated that the AIJ code provided the best prediction of the test results with discrepancy varying between 1%and 53%(M_exp/M_u,AIJ=1.07),while the AISC code gave the most conservative evaluation of the lateral load capacity of the CFST columns with discrepancy of 1%～404%(M_exp/M_u,AISC=2.38).The Chinese GB50936 code gave 1%to 81%larger prediction of the test results(M_exp/M_u,GB=1.21),and the empirical equation by Han et.al underestimated the test results by 2%to 198%(M_exp/M_u,Han=1.35).For SCFST columns,due to ignorance of the confinement effect by the stiffeners,the current codes all gave much more conservative predictions.The discrepancies between the experimental and the calculated results by the AIJ code(M_exp/M_u,AIJ=1.42),Chinese code(M_exp/M_u,GB=1.71),and the AISC code(M_exp/M_u,AISC=9.32)were 14%and 139%,15%and 191%,and larger than73%,respectively.（4）By modifying design equations recommended in current codes and taking the confinement effect by stiffener into account,this paper developed a new calculation method that can unified the evaluation of the lateral load capacity of CFST and SCFST columns in this paper and previous references.The prediction results by the proposed method showed much higher accuracy than current design methods.For CFST columns,the discrepancy between the experimental results and the calculated ones ranged from 1%to 53%with an averaged M_exp/M_u,Pro of 1.04,while the discrepancy for SCFST columns ranged from 0%to67%with an averaged M_exp/M_u,Pro of 1.23.（5）To make full use of the proposed stiffeners to improve the seismic performance of SCFST columns,a calculation method for determining the minimum height of stiffeners was proposed.The calculated minimum height is to avoid either being too short to cause premature failure of the non-stiffened portion of the steel tube or being too long to cause the construction difficulty and increase the cost of the stiffeners.（6）To establish an accurate numerical analytical method for the seismic performance of SCFST columns,this paper proposed a stress-strain model suitable for the confined concrete in SCFST columns based on well-known Mander model.To verify the effectiveness of the proposed model,Fiber Element（FE）model was established and the hysteretic loops of SCFST columns were calculated using the Open SEES platform.The calculated results showed that the hysteretic behavior of SCFST columns could be fairly well evaluated by using the proposed confined concrete model and the Steel02 steel model recommended in the Open SEES.The average ratio of the calculated lateral load capacity to the experimental results of all the tested SCFST columns V_cal,ave/V_exp was 0.95 with a standard deviation of 0.08.