Theoretical Analysis And Experimental Study On Mechanical Property Of L-shaped Concrete Filled Square Steel Tubular Composite Special-shaped Columns Bearing Capacity Under Axis Compression

Steel-concrete composite structure can realize the complementary material properties of steel and concrete.It has been widely used in civil engineering structures,especially in high-rise buildings.When steel-structure residential buildings adopt this kind of column,it not only saves space,but also facilitates the placement of furniture and articles,and meets the aesthetic requirements.At present,the types of special-shaped columns mainly include reinforced concrete special-shaped columns,steel reinforced concrete special-shaped columns and concrete filled steel tubular special-shaped columns.On the basis of summarizing and analyzing the above types of special-shaped columns,this paper puts forward a new type of special-shaped columns,L-shaped concrete filled square steel tubular composite special-shaped columns,that is,L-shaped,T-shaped,cross-shaped and other special-shaped sections formed by welding of multiple square concrete filled steel tubular columns.The mechanical properties and working mechanism of L-shaped concrete-filled square steel tubular composite special-shaped columns under axial compression are studied in depth through experimental study,theoretical analysis and numerical simulation,.Five specimens of L-shaped concrete filled square steel tubular composite special-shaped columns were tested under static axial pressure.The effects of steel tube thickness,steel strength,slenderness ratio and other factors on the failure mode,ultimate bearing capacity,ductility and other mechanical properties of the specimens were analyzed.A finite element analysis model consistent with the test was established,and the effects of steel strength,steel tube thickness and concrete strength on the members were quantitatively analyzed.Thecross-sectional stress distribution of two different types of specimens of short column and medium length column in each loading stage were analyzed,and puts forward the constraint mechanism of L-shaped concrete filled square steel tubular composite special-shaped columns.The formulas for calculating the strength and bearing capacity of composite columns in some national codes are compared and analyzed,and the coagulation of L-shaped square steel tubular is obtained according to the results of experiment and numerical simulation.The formulas of strength and bearing capacity of soil composite special-shaped columns are given.Comparing the results obtained by two commonly used methods for calculating the stability coefficient of composite columns(i.e.load ratio method and column curve method)with those obtained by experiments and numerical simulation,a formula for calculating the stability bearing capacity of L-shaped concrete filled square steel tubular composite special-shaped columns is put forward according to the actual needs of the project.The results show that the failure process of L-shaped concrete filled square steel tubular composite special-shaped columns can be divided into three stages: elastic stage,elastic-plastic stage and failure stage.The ultimate bearing capacity and ductility of members increase with the increase of steel tube thickness and steel strength,and decrease with the increase of slenderness ratio.The failure mode of short specimens is that local buckling instability coexists with the overall bending instability.The local buckling degree decreases with the increase of steel strength and steel tube thickness.The failure mode of medium-long members is that the overall buckling instability coexists with the local buckling instability caused by it.The local buckling degree of members is obviously smaller than that of short members.The steel strength has the greatest improvement on the ultimate bearing capacity of members,and the steel strength,steel tube thickness and concrete strength have the same improvement on the ultimate bearing capacity of members.All of them are 27%,24.2% and 6.4%.It is found that the stress distribution in the middle section of L-shaped concrete filled square steel tubular composite special-shaped columns core concrete is uniform,and the stress value at the edge and corner is larger and uneven,which indicates that the steel tube has strong restraint on the middle section concrete and weak restraint on the edge and corner concrete.There is a gap between the calculationresults of strength and bearing capacity in various national codes,most of which are conservative.Among them,the calculation results of AIJ code are close to the actual bearing capacity of specimens and less discrete.The calculation results of the formula of strength and bearing capacity of L-shaped concrete filled square steel tubular composite special-shaped columns based on this code are in agreement with the experimental and simulated values.The calculation results of stability coefficient obtained by load ratio method and column curve method are close,and the safety redundancy is better.The calculation method of load ratio method is simple and meets the actual needs of engineering,which can be used for reference in engineering design.