Mechanical Behavior Of Double-plate Connected Special-shaped Concrete-filled Steel Tube Composite Columns

Concrete-filled steel tube structures have the advantages of both steel structures and concrete structures.This type of structures can effectively reduce the cost of construction,improve the fire resistance performance,improve building comfort and construction speed.Special-shaped concrete-filled steel tubular composite columns can be hidden in walls,avoid the interior convex angle,improve the utilization rate of interior space,and reduce the size of column section,so it has a good application prospect in residential buildings.In this paper,the flexural mechanical properties and calculation methods of double-plate connected special-shaped concrete-filled steel tube composite columns are discussed through experimental research,finite element numerical analysis and the bearing capacity calculation formula derivation.The main research contents of this paper are as follows:(1)The compressive bending test of L-shaped double-plate connected concretefilled steel tube composite columns(L-CFST column for short)with different loading axes was completed.The effects on failure modes and mechanical properties of doubleplate connected L-CFST columns with different loading axes,loading directions and eccentricity were analyzed.The failure modes,load-vertical displacement curves,transverse deflection curves,load-strain curves and cross-sectional strain changes were analyzed.Test results show that the double-plate connected L-CFST column has good overall performance,main failure mode was overall buckling failure.The bearing capacity of double-plate connected L-CFST columns were increased by more than 19%compared with that of single-plate connected ones.(2)ABAQUS finite element model was established for double-plate connected LCFST columns,and the correctness of the finite element simulation was verified by comparison with the compressive bending test results.The effects of steel strength and concrete strength combination,steel tube and steel plate thickness and eccentricity on the bearing capacity of double-plate connected L-CFST columns were analyzed by finite element method.It was found that it was not effective to increase the bearing capacity of double-plate connected L-CFST columns by increasing the strength grade of concrete.The results of finite element calculation provide data for the verification of theoretical formulas.(3)The compressive bending test of L-CFST columns was completed,and the load was applied to both ends of two single-columns.The failure modes,bearing capacity,ductility and other mechanical properties of double-plate connected L-CFST columns and single-plate connected L-CFST columns were analyzed.The results show that the failure modes of both specimens were overall buckling failure toward the positive direction of u-axis.Bearing capacity of double-plate connected L-CFST specimen was7.8% higher than that of single-plate connected one,both of which have good overall performance and ductility.The mechanical properties of double-plate connected LCFST column under single,double and three-limb loading were parameterized by finite element models.Comparing with the loading modes of the knife-hinge loaded specimens,it was found that both loading modes can give full play to the mechanical performance of specimens.Connecting plates were not only the coordinate deformation function of connecting each limb column,but also plays an important role in the overall bearing performance of specimens.(4)The compressive bending test of double-plate connected L-CFST columns with unequal limbs was completed.The effects of different limb elongation modes on the failure modes and mechanical properties of double-plate connected L-CFST columns with unequal limbs were studied.Test results showed that when the limb aspect ratio was 4.0,the failure form of the specimen was the joint failure of the whole bending and the partial failure of the column end,and the bearing capacity of the specimen could not be fully exerted.Therefore,when the length-width ratio of the long limb increases to 4.0,structural measures should be added at the end to avoid the damage caused by stress concentration at the end to affect the bearing capacity of the special-shaped columns with unequal limbs.(5)ABAQUS finite element model was established for double-plate connected LCFST columns with unequal limbs,and the correctness of the finite element simulation was verified by comparison with the compressive bending test results.The effects of different limb elongation modes on bearing capacity of double-plate connected L-CFST columns with unequal limbs were studied.It is found that the bearing capacity of special with two-side elongation is not much different from that of specimens with single-side elongation.It can be adjusted flexibly according to the requirements of structure type or arrangement in structural design.(6)Based on the superposition theory and the full section plastic assumption,the axial compressive stability bearing capacity calculation formula,the pure flexural bearing capacity calculation formula and the N-M correlation curves of double-plate connected L-CFST columns were derived.The results of axial compression test and finite element model show that the formula of axial compression stability bearing capacity has a good accuracy.The N-M correlation curve of flexural capacity has little error with the overall calculation of test and finite element Therefore,the formulas proposed in this paper can be used to calculate the compressive bearing capacity of double-plate connected L-CFST columns.In order to be used in engineering design,simplified formulas of standard specimens were derived.The moment of inertia and pure bending bearing capacity of commonly used specimens were listed,which was convenient for searching and using in structural design.