Study On Axial Compression Performance Of Wide-Limb L-shaped Columns Composed Of Concrete-Filled Square Steel Tubes

Concrete filled steel tubular composite special-shaped column（S-CFST column）solves the problems of indoor convex angle and exposed column,and has great advantages in space utilization and aesthetic effect.With the increase of the number of building layers,in order to achieve better axial compression bearing capacity and lateral stiffness,it is necessary to increase the limb length of S-CFST column.According to the definition of the specification,when the section width-to-thickness ratio of S-CFST column is greater than 4 and less than 5,it is a wide-limb concrete-filled steel tube composite special-shaped column（W-SCFST column）.W-SCFST columns have high axial bearing capacity and lateral stiffness,and have broad application prospects in high-rise buildings.In this thesis,the axial compression performance is studied by finite element simulation and theoretical comparison analysis.The main contents are as follows:（1）Based on the axial compression test of L-shaped double-plate connection concrete-filled steel tubular composite column（LCFST-D column）,a finite element model was established.The finite element results were compared with the experimental results to verify the correctness of the material constitutive model,boundary conditions and interaction relationship.The finite element results were compared with the experimental results to verify the correctness of the material constitutive model,boundary conditions and interaction relationship.The finite element model of W-LCFST column is established,considering the influence of section width-thickness ratio,steel thickness,steel strength,concrete strength and built-in perforated partition（PBL）on its axial compression performance.The results show that the axial bearing capacity of the model increases by 21.7%and the ductility coefficient decreases by15.8%when the width-thickness ratio of the section increases from 4 to 5.When the thickness of steel increases from 6 mm to 8 mm,the axial bearing capacity of the model increases by 19.8%,and the ductility coefficient increases by 27.5%.When the steel strength increases from Q235 to Q345,the peak bearing capacity of the model increases by 9.3%,and the ductility coefficient increases by 14.4%.When the concrete strength increases from C30 to C35,the peak bearing capacity of the model increases by 2.1%,and the ductility coefficient increases by 2.2%.Adding a group of PBL stiffeners,the peak bearing capacity of the model increased by 3.8%～4.6%,and the ductility coefficient increased by 14%～26%.Adding two groups of PBL stiffeners,the peak bearing capacity of the model increased by 7.9%,and the ductility coefficient increased by 29%.The addition of PBL stiffeners can significantly enhance the interaction between concrete and steel,increase the axial compression bearing capacity,improve the ductility of the specimen,and effectively improve the local buckling failure,which is conducive to engineering promotion and application.（2）The axial compression performance of W-LCFST long columns with column height of 900 mm-4200 mm was analyzed.The results show that with the increase of column height,the stress cloud diagram of the column changes from uniform stress distribution to stress aggregation in the column,and the failure mode changes from strength failure to instability failure.When the column height of the W-LCFST column is 1200 mm～1500 mm,the column height increases by 900 mm～1100 mm,the peak bearing capacity is basically unchanged,and the peak displacement increases by36%～56%.When the height of the W-LCFST column increases by 2700 mm～3000mm,the peak bearing capacity decreases by 4.7%～9.1%,and the peak displacement increases by 66%～150%.When the column height is 3000 mm,3600 mm and 4200mm respectively,the width-thickness ratio of the section increases from 4 to 5,the peak bearing capacity increases by 25.5%～28.4%,and the peak displacement increases by13%～18.5%.When the concrete material increases from C30 to C35,the peak bearing capacity of the model increases by 2.12%～2.19%,and the peak displacement is basically unchanged.When the steel material is enhanced from Q235 to Q345,the peak bearing capacity of the model increases by 3.9%～4%,and the peak displacement decreases by 2%～12%.When the thickness of steel increases from 6 mm to 8 mm,the peak bearing capacity increases by 20%～20.8%,and the peak displacement increases by 3.7%～9.6%.Increasing the width-thickness ratio of the section and the thickness of the steel can significantly increase the peak bearing capacity and increase the peak displacement.（3）Based on the Mander theory and the superposition theory,considering the existence of strong and weak constraint zones in the core concrete of the steel tube,a formula for predicting the bearing capacity of the W-LCFST short column section is proposed.The finite element results of the W-LCFST column are compared with the calculation results of the prediction formula,the Japanese specification（AIJ 1997）,the American specification（AISC-LRFD 1999）,and the technical specification for concrete filled rectangular steel tubular structures（CECS 159:2004）.The results show that the above calculation results are conservative,and the prediction formula proposed in this thesis is more accurate.The average error is 7%and the standard deviation is0.01,which can accurately calculate the bearing capacity of W-LCFST column section.The average error of peak bearing capacity between the calculation results of American code（AISC-LRFD 1999）and the finite element results is 29%,and the standard deviation is 0.01.The average error of the peak bearing capacity calculated by the Japanese specification（AIJ 1997）and the finite element results is 12%,and the standard deviation is 0.01.The average error of the peak bearing capacity between the calculation results of Technical specification for concrete-filled rectangular steel tube structures’（CECS 159:2004）and the finite element results is 13%,and the standard deviation is 0.03.（4）Based on the Steel Structure Design Standard’（GB50017-2017）and the peak load-displacement results of W-LCFST columns with PBL stiffeners,the normalized slenderness ratio（?）is calculated and divided into two types of W-LCFST medium and long column models.Nonlinear fitting is performed to obtain the calculation parameters of the stability coefficient.The regular slenderness ratio of the long column model in the first kind of W-LCFST is（?）≤0.215,the calculation parameter of the stability coefficientα₁=0.329,and the fitting degree R square is 0.956.The regular slenderness ratio of the long column in the second type of W-LCFST is（?）>0.215,the calculation parameters of the stability coefficient areα₂=0.949 andα₃=0.29,and the fitting degree R square is 0.912.