Axial Behavior Of G550 Steel Cold-formed Thin-walled Built-up Section Columns

With the development of the cold-formed thin-walled steel residential system in China,the cold-formed thin-walled steel residential structure have started from low-rise building to multi-story building.The high-strength cold-formed thin-walled built-up section steel is gradually applied,but there is no detailed method for calculating the bearing resistance of builtup section columns.Chinese current code is applicable to steel with thickness of 2-6mm,but whether it is applicable to steel components with thickness of 1mm remains to be studied.Since there are many requirements for seismic fortification in China,cold-formed thin-walled steel columns will produce uplift force under horizontal load,so it is necessary to study the axial behavior of cold-formed thin-walled steel members under monotonic and cyclic load.Firstly,the experimental data of cold-formed thin-walled single-limb and built-up section columns under axial load in the past ten years are summarized.Some single-limb and built-up sections are selected to verify the calculated values of the existing design methods by Chinese code(GB50018-2002)and North American Specification(AISI S100).The results show that: the specification cannot accurately predict the bearing resistance of the columns with the track as boundary condition.Therefore,there is no suitable calculation method for bearing resistance of cold-formed thin-walled built-up section steel columns.Since the calculated value cannot accurately predict the compression bearing resistance of cold-formed thin-walled steel sections with actual boundary condition,a total of 48 highstrength cold-formed steel columns are tested under monotonic and cyclic load.Short tracks were fixed by self-drilling screws to the member ends to consider the real support handled in construction.The test,in which the boundary conditions are connected by the track,simulates the practical application of the project.The cross-section of the test consists of two types of single-limb sections and four types of built-up sections,in which the built-up sections are connected by single-limb sections through self-drilling screws.Each section adopts four lengths of 500 mm,1000mm,1500 mm and 2000 mm.Based on the experimental phenomena and results of 48 cold-formed thin-walled steel columns,the effects of loading system,slenderness ratio and section form on bearing resistance and failure mode are mainly investigated.It is found that the initial loading stiffness of specimen decreases with the increase of the length of the specimen.When the slenderness ratio is less than 30,the global buckling is not considered.When the slenderness ratio larger than 40,the global buckling is the main failure mode.Besides,the distortional buckling will not occur in closed-section.In addition,according to the experimental strain gauge data analysis of the local buckling critical bearing resistance,it is found that the existence of lips and the effective connection of the screws can increase the ultimate bearing resistance of the section.According to the results of cyclic loading test,the loading law of the hysteresis curve of single limb sections and built-up sections is discussed.It is found that cyclic loading will not change the bearing resistance and failure mode of the specimen,and monotonic loading can be used as the envelope of cyclic loading.Before buckling,the specimen is in the elastic section.After buckling failure,when the unloading force is 0,the specimen has residual deformation.When the unloading displacement continues to 0,the specimen enters the tensile section,and the unloading stiffness decreases which is equal to the shear stiffness of the screw group.It is found that after the specimen buckles,the strength of one circle is reduced by 5% compared with the strength of the previous circle.Analyze the amplification factor of different specimen,it is recommended to design the built-up section as a closed section.In addition,it is found that the normalized stress ratio is approximately 0.4 when the split section is a short column.Based on the test results,the built-up coefficient method and the finite strip method are proposed to calculate the bearing resistance of the built-up section.Two methods are used to calculate the bearing resistance of the specimens in the literature,and the calculated value is compared with the test value and the standard calculated value.It can be seen that the doublelimb section can be calculated by the American standard,while the other section forms are safer to be calculated by the finite strip method.