Research On The Instability Mechanism And Bearing Capacity Calculation Method Of Cold-Formed Double-Limb Closed-Section Built-Up Columns Under Axial Compression

Cold-formed thin-walled steel(CFS)double-limb closed built-up columns,which are composed of a single-limb C-section member and a single-limb U-section member connected by self-drilling screws,has gradually become the focal point for scholars at home and abroad as a new type of special members.However,the theory and application researches at home and abroad are still in the primary stage.Most scholars mainly carried out experimental researches or numerical simulation analyses on the bearing capacity of the built-up members,and reveal the influence of the basic parameters such as section forms,screw spacing or screw arrangement,but lack of the systematic theoretical research,without considering the stability theory of "screw non-rigid connection",the complex buckling correlation theory between single-limb and between plates,and the calculation method of bearing capacity of built-up columns.It cannot guide the practical application of engineering scientifically and effectively.In this scenario,in this paper,the instability mechanism and bearing capacity design method of CFS double-limb closed built-up columns were studied in-depth in the theory and experiment.From the basic theory of screw built-up plate(plate level),the complex buckling theory between single-limb and between plates(plate group level),and the calculation method of bearing capacity for built-up columns(member level),comprehensively revealed the instability mechanism and design method of bearing capacity for CFS double-limb closed built-up columns in this paper.The main research contents are as follows:(1)Experimental research.The local buckling(30 specimens),distortional buckling(30 specimens)and global buckling(18 specimens)of CFS double-limb closed-section built-up columns were tested.The influence of section geometrical dimensions,specimen length and screw spacing on the buckling modes,the number and length of the buckling half-wave and the ultimate capacity of specimens were studied.The influence of "splicing effect" under different buckling modes on the critical buckling load,buckling modes,post-buckling strength and bearing capacity of CFS double-limb closed-section built-up columns is revealed.(2)Numerical simulationThe finite element model is established and the numerical simulation analysis is carried out based on the experiment.The critical buckling load,elastic buckling performances,failure characteristics,the relationship curve between ultimate bearing capacity and axial displacement and the curve between strain and deflection of test specimens are obtained.The rationality of the finite element model is verified.The effects of the boundary conditions at the loading end,the geometric dimensions of the cross-section and the screw spacing on the critical buckling loads of the built-up plates and built-up columns under local buckling,distortional buckling and global buckling modes are revealed.(3)The calculation theory of critical buckling load of screw built-up platesBased on the theoretical analysis,the coordination deformation of micro elements in the small deflection buckling of screw built-up plate is analyzed.The stability calculation theory of screw built-up plate considering the influence of shear slip deformation is proposed and verified to solve the defects of the classical thin plate theory in solving the buckling problem of the screw built-up plate according to the energy method.The application scope of the classical small deflection theory in screw built-up plates through the study in the theoretical research of "the level of built-up plates" was expanded.(4)The critical load formulas and bearing capacity calculation methods of local buckling,distortional buckling and global buckling of CFS double-limb closed-section built-up columnsThe instability mechanisms of the built-up columns under the three buckling modes were deduced based on the research of screw built-up plate,and the critical load calculation models considering the interaction of plates in the built-up columns under the three buckling modes are established respectively.The analytical expressions of critical buckling loads for three buckling modes are derived based on the energy method.Finally,the buckling critical loads under the three buckling modes are applied to the corresponding bearing capacity design curves of direct strength method in the American AISI code,and the bearing capacity calculation methods of the built-up columns under the three buckling modes were obtained for the first time.