Theoretical And Experimental Study On Distortional Buckling Of Web-Stiffened Cold-Formed Thin-Walled Lipped Channel Columns

Because the cold-formed thin-walled members are usually thinner in thickness,thus,with a large width to thickness ratio and an open section profile,stability problem needs to be addressed and considered carefully in the design of the structure members due to the weakness of their cross section.Distortional buckling mode,as discriminated from local or overall buckling mode may control the members'design in certain conditions.In this paper,a kind of web-stiffened cold-formed lipped channel columns are studied by combining experimental investigations and theoretical analysis with the emphasis focused on the mechanism of the distortional buckling mode,the countermeasures which can prevent the columns'distortional buckling from happening,and the simplified analysis method to determine the critical distortional buckling stress of the axially and eccentrically-loaded columns.The work presented in this paper can be summarized as following.1.A total of 18 specimens were tested statically.Different column length,axial force eccentricities and boundary conditions were chosen to observe the buckling modes,the possibility of interaction between different buckling modes,the failure modes and the load carrying capacities of the web-stiffened lipped channel section columns.The mechanisms of the buckling and failure modes under different conditions were analyzed.It is shown that the load position and the boundary condition both have significant effects on both the buckling modes and the ultimate bearing capacities of the specimens,while the column length just has observably influence on the buckling mode.Through comparison between test results and closed-form solution results,it was found that the closed-form solution should not be appropriate for the negative load eccentricity conditions.2.Finite element program of ANSYS were adopted to carry out the columns'parameter analysis with different load eccentricity,column length,stiffness of the web stiffener,flange width,lip depth and initial geometric imperfection considered.Numerical simulations were carried out for 186 members,by which the factors critical to the distortional buckling of the axially and eccentrically-loaded columns were found.The influences of the above factors on the columns'bearing capacities and the buckling modes were analyzed quantitatively.3.The experimental investigations on 18 web-stiffened cold-formed thin-walled lipped channel columns with horizontal batten sheets connected between the columns'two lips at reasonable space were presented.The strengthening effects of the batten sheets to the columns with different axial force eccentricities and column length were analysed.It is shown that the batten sheets can exert significant influence on both the buckling modes and the ultimate strength of the specimens under positive eccentric loading or axial loading conditions,while they just have few impacts on that of the specimens under negative eccentric compression.Theoretical explanation of the batten sheets'different influence on the bearing capacity of the eccentric compressive and axially–loaded columns was given.4.Parameter analysis of 80 column models strengthened by horizontal and/or inclined batten sheets were conducted by program ANSYS.The influence of the horizontal batten sheets'space to the members'buckling modes,failure modes,and ultimate load-carrying capacities were further discussed carefully.Measures of adding inclined batten sheets to control the distortion of positive eccentric loading columns were proposed under the conditions of horizontal batten sheets'insufficient restraint to the columns'distortional deformation.The strengthening effects of different kind of batten sheets arrangement were compared.5.The finite strip buckling analysis method of thin-walled members-CUFSM was also adopted to carry out parameter analysis,by which the influence of lip-stiffened flange dimensions,steel sheet's thickness and the stiffness of the web stiffener to the members'distortional buckling half-wavelength?,as well as the critical buckling stress?_cr were analyzed thoroughly.A correction coefficient k to the critical buckling stress of web-stiffened axially-loaded columns was given and the simplified closed-form formulas for critical load P_cr of the eccentrically loaded columns were also presented finally,which can simplify the column's design process and can be helpful for the application of such members in practical engineering.The comparisons of the analytical solution with the existing test data at home and abroad show that the correction coefficient k and the formula have good accuracy and wide applicability.