Mechanical Behavior Of L-shaped Columns Composed Of Concrete-filled Steel Tube Mono-columns Connected By Double Vertical Steel Plates

The special-shaped columns composed of concrete-filled steel tube mono-columns have the advantage of special-shaped columns.The main benefit of this type of column is its ability to be embedded into walls,allowing columns to be placed without intruding on the usable space of a building,saving the material.It also has the characteristics of concrete filled steel tubular columns,such as excellent performances of seismic resistance,rapid construction speed and significant economic benefits and so on.Therefore,as a new type of green environmental protection structure,it has a wide application in the field of construction in the future.In this paper,the special-shaped column composed of concrete-filled steel tube mono-columns connected by double vertical steel plates（LCFST-D columns）was proposed.The experimental research,finite element simulation and theoretical analysis and calculation were carried out:（1）The axial compression test of four different patterns of L-shaped composed of concrete-filled steel tubes was carried out.The mechanical properties were studied,and the whole process was analyzed by finite element model.The results shown that the columns with concrete in the vertical steel plates and without the transverse stiffeners（LCFST-D columns）achieve the best mechanical properties.The bearing capacity of LCFST-D column was 25%higher than that of LCFST-S column.（2）The axial compression test of LCFST-D column with different slenderness ratio was carried out.The failure modes,bearing capacity,strain analysis and strength factor are analyzed.The effect of slenderness ratio on the bearing capacity and failure mode were obvious.ANSYS model was used to analyze the influence of material strength,steel tube size and thickness,width and thickness of vertical steel plate and column height on the axial compressive behavior of LCFST-D column.（3）Based on the stiffness conversion section method,considering the material characteristics and section form,the method for calculating the centroid of the LCFST-D column was established.The method for calculating the slenderness ratio of flexural buckling and flexural torsional buckling of LCFST-D columns are proposed.Through the prediction of the bearing capacity of LCFST-D column by various codes,it was found that the CECS159:2004 specification was the closest,but the average N_CEC/N_m value of the LCFST-D column was 0.925,the average error was larger than5%.Accordingly,based on the superposition theory and the confinement concrete by Mander,the equation for calculating the bearing capacity of the LCFST-D columns was deduced.The calculated values showed good agreement with the experiment results.The average error was less than 5%.Therefore,the formula was recommended.（4）The LCFST-D columns were tested under eccentric compression.The effects of different eccentricities on the mechanical properties of the LCFST-D columns were discussed.LCFST-D columns have high bearing capacity and ductility under eccentric load.The bearing capacity decreases with the increase in eccentric distance.The three mono-columns can share loads well.The M-N curves of LCFST-D columns were obtained through parametric analysis on the thickness of the vertical steel plate and the steel tube,based on the verified finite element model.The thickness of the steel tube was the major influencing factor.（5）Five specimens were tested under a constant axial force and cyclic lateral loading.The effects of the axial compression ratio,sectional dimensions of the mono-column,and the width and thickness of the vertical steel plates on the mechanical behavior and performances of seismic resistance of LCFST-D columns were investigated.The bearing capacity of LCFST-D column was 25%higher than that of LCFST-S column.Based on the experiments conducted on these LCFST-D columns under cyclic loading,finite element models were used to analyze the mechanical properties and seismic behaviour.The effects of material strength,axial compression ratio,size and thickness of steel tube,width and thickness of vertical steel plate,column height and other factors on the seismic performance of LCFST-D column were studied by finite element software.The seismic performance of the specimens improved significantly with a larger steel tube cross section size.（6）On the basis of the assumption of plastic yield of steel full section and superposition theory,the calculation formula of bending bearing capacity and shear formula of LCFST-D column were deduced.Based on the experimental and numerical analysis,recovery force model was obtained by linear regression method.It provided a reference for the elasto-plastic seismic response analysis of the LCFST-D column.The recommendations on the seismic design of the LCFST-D columns were proposed.It provided reference for structural design.