Research On Local And Distortional Buckling And Its Interaction Of Cold-formed Steel Triple-limbed Open-section Built-up Columns

The triple-limbed open-section built-up column is constructed by connecting cold-formed steel C-section limbs using self-drilling screw.It is widely used in cold-formed steel structures due to its ability to simultaneously fulfill structural,connection,and load-bearing requirements.This type of complex built-up section component exhibits significant torsional stiffness,and its buckling deformation mode are susceptible to initial defects.Existing research on built-up columns with built-up sections has primarily focused on the load-carrying capacity of members under a single buckling mode,with limited investigation into the instability mechanism and coupling effect of different buckling modes.Currently,there is no systematic research on the instability mechanism of triple-limbed open-section built-up members under different buckling modes,which hinders the provision of systematic theoretical guidance for engineering design.In this study,through experimental research,numerical simulation,and theoretical derivation,the local buckling and distortional buckling instability mechanisms of triple-limbed open-section built-up columns were revealed,and calculation methods for local buckling,distortional buckling,and local-distortional interaction buckling load-carrying capacities for this type of member were proposed.The main research content and conclusions are presented below:（1）A total of 60 specimens,including triple-limbed open-section built-up columns and single C-section columns,were designed and fabricated for axial compression tests,covering three buckling modes:local buckling,distortional buckling,and local-distortional interaction buckling.The effects of cross-sectional dimensions and self-drilling screw spacing on the load-carrying capacity and stiffness of the specimens were investigated.The cross-sectional and longitudinal deformation characteristics of the triple-limbed open-section built-up columns were analyzed.The results showed that the triple-limbed open-section built-up columns exhibited significant assembled effects.The section form,buckling mode,and screw spacing had certain influences on the buckling half-wavelength of the specimens.The width-thickness ratio of the section significantly affected the ultimate load-carrying capacity of the columns.（2）A finite element model was established by ABAQUS software and validated based on experimental results.Parametric analyses were conducted on cold-formed steel triple-limbed open-section built-up columns,investigating the effects of factors such as cross-sectional dimensions,self-drilling screw spacing,boundary conditions,and slenderness ratio on the local buckling critical stress,distortional buckling critical stress and local-distortional interaction buckling bearing capacity of the members.The results showed that the geometric dimensions of the built-up column cross-section significantly influenced the critical stress for local buckling and distortional buckling.When the longitudinal spacing of the screws exceed the height of the web,it had negligible influence on the critical stress for local buckling and distortional buckling.When the ratio of member length to web height exceeded 3,the boundary conditions only had limited constraints on the buckling deformations at the near-end location of the member.（3）Considering the effects of shear slip deformation and constraints from self-drilling screws,an analytical formula for the critical buckling stress of screw built-up plates was derived,expanding the applicability of thin plate stability theory and further developing classical thin plate stability theory.Additionally,the influence of plate geometry dimensions,self-drilling screw layout pattern,and screw diameter on the critical buckling stress of screw built-up plates.When the ratio of longitudinal spacing of the screws to the width of the built-up plate was less than 0.7,the critical buckling stress of the screw built-up plates decreased with increasing screw spacing.When the length-width ratio of the plate was less than 3,the effects of boundary conditions on the critical buckling stress of the screw built-up plates should be considered.（4）Based on the buckling deformation characteristics of screw built-up plates and built-up columns,a calculation model for local buckling and distortional buckling was established.By deriving analytical formulas for the critical buckling stress of built-up columns based on the energy method,and incorporating them into the corresponding design curves of the direct strength method,formulas for calculating the local buckling and distortional buckling capacity of built-up columns with triple-limbed open-section were proposed.The calculated results were in good agreement with experimental and finite element results,meeting the accuracy requirements.This approach solved the long-standing dilemma of lacking calculation methods for local buckling and distortional buckling capacity of built-up columns with this type of section,and expands the applicability of the direct strength method.（5）Based on the experimental and numerical simulation results,an analysis of the coupled buckling deformation modes of local and distortional buckling was conducted.A simplified calculation process and a unified calculation form were proposed for the calculation of the ultimate bearing capacity related to local and distortional buckling,and were compared with existing calculation methods.The results showed that the prediction of the local-distortional buckling capacity of built-up columns with triple-limbed open-section based on existing direct strength method design curves is conservative.The proposed calculation formula in this study exhibits good calculation accuracy and can serve as a basis for engineering design.（6）After clarifying the influence of coupled buckling interaction on the post-buckling strength of built-up columns,a novel criterion for the identification of local-distortional and distortional-local interaction buckling modes for triple-limbed open-section built-up columns is proposed for the first time.When the ratio of the critical local buckling stressσ_{cr L} to the critical distortional buckling stressσ_{cr D},falls within the range of 0.71 to 1.76,the column exhibits local and distortional interaction buckling.Local-distortional interaction buckling occurs when both theλ_L andλ_D are less than 2.On the other hand,distortional-local interaction buckling occurs when bothλ_L andλ_D are greater than 2.