| The cold-formed steel triple-limbs built-up columns with half open section,which are generally used in shear wall chords,corner column of exterior walls,side columns of door and window openings,and other parts that need to be strengthened,have been widely used in field of building structure due to their excellent properties.However,there are no systematic studies on this type of component.The main reason is that such built-up sections cannot behave whole section members.And the discontinuity of connection interface is caused by discrete screws,which leads to the significant shear slip deformation when components appear buckling.However,the shear slip deformation can be restrained by screws.This case makes stability analysis of cold-formed steel triple-limbs built-up columns with half open section more complicated.For this purpose,taking cold-formed steel triple-limbs built-up columns with half open section as research object,the test study,numerical simulation and theoretical analysis were combined to reveal the real instability mechanism of local buckling,distortional buckling and global buckling of such components from three levels: screw composite plates,built-up section plates group and global components.On basis of this,the design methods of bearing capacity of cold-formed steel triple-limbs built-up columns with half open section were proposed.The detailed research contents and main conclusions are as follows:(1)Firstly,54 triple-limbs built-up columns were designed,which differed in buckling modes,dimensions of cross-section and longitudinal spacings of screws.And then the axial compression tests were conducted on triple-limbs built-up columns and 24 single columns(Cshaped steels and U-shaped steel)forming built-up columns.The buckling deformations,failure modes and ultimate bearing capacity of triple-limbs built-up columns affected by pure local buckling,distortional buckling and global buckling modes were studied systematically.The results indicate that all single columns and triple-limbs built-up columns appear expected typical failure modes including local buckling,distortional buckling and global buckling during the tests.The axial compression performance of triple-limbs built-up columns has the built-up effect of 1+1+1>3.The built-up effect decreases gradually with increase of longitudinal spacings of screws.Additionally,the longitudinal spacings of screws have no significant effect on failure mode and buckling half wavelength of specimens.But the longitudinal spacings of screws have effect on ultimate bearing capacity of specimens.(2)On basis of test studies,a nonlinear finite element model was developed to predict axial compression performance of cold-formed steel single columns and cold-formed steel triplelimbs built-up columns.And the comparisons were performed between simulation results and test results.The results indicate that the complexity of this model can be used to predict the failure modes,ultimate bearing capacity and global stiffness of single columns and triple-limbs built-up columns.The verified model can be further applied to the studies of cold-formed steel built-up members with different dimensions of section,properties of materials,layout of screws and diameters of screws,which provides subsequent theoretical analysis with calculation basis.(3)Taking the screw composite flange plate of cold-formed steel triple-limbs built-up columns as the research object,the possible buckling deformation modes were analyzed.And the corresponding calculation models were established,and the theoretical calculation equations of the buckling critical stress of the screw composite plates with three-sided consolidation with one upper plate and free lower plate were derived,which extended the application of the classical theory to screw composite plate with different boundary conditions.The results indicate that the main factor affecting buckling critical stress of screw composite plates is ratios of width to thickness.And the buckling critical stress of screw composite plates decreases rapidly with increase of ratios width to thickness.When the ratio of length to width is less than 1.5,the buckling critical stress of the screw composite plate will be affected.In addition,the buckling critical stress of the screw composite plates can be affected only after the screws have effective constraints.And the effect of diameters of screw on the buckling critical stress of the screw composite plates is weak,which can be ignored.(4)On basis of studies of the screw composite plates,the deformation modes of the local buckling and the distortional buckling of the cold-formed steel triple-limbs built-up columns were analyzed.And the calculation models of the local buckling and the distortional buckling were established,respectively.Besides,the theoretical calculation equations of the critical stresses of local buckling and distortional buckling of the triple-limbs built-up columns were derived.In order to consider the influence of initial defects,properties of materials and postbuckling strengths,the buckling critical stress was introduced into the bearing capacity design curve of the direct strength method.And a design method for predicting the local buckling and distortional buckling capacity of cold-formed steel triple-limbs built-up columns was established for the first time.The results indicate that the ultimate bearing capacity of coldformed steel triple-limbs built-up columns with failure of pure local buckling and distortional buckling can be predicted accurately by combination of calculation equations for critical stress of local buckling and distortional buckling and bearing capacity of local buckling and distortional buckling of the direct strength method.(5)The calculation models of flexural buckling were established,and the constraint effect of screws and shear-slip deformations were considered.And the theoretical equations for calculating the flexural buckling critical loads of cold-formed steel triple-limbs built-up columns were derived.In order to consider the influence of initial defects and properties of materials,the buckling critical loads were introduced into the bearing capacity design curve of the direct strength method.And the calculation methods for predicting the flexural buckling capacity of cold-formed steel triple-limbs built-up columns were established for the first time.The results indicate that the ultimate bearing capacity of cold-formed steel triple-limbs built-up columns with pure flexural buckling failure can be predicted accurately by combination of calculation equations for critical stress of flexural buckling and bearing capacity of flexural buckling of the direct strength method. |
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