| This thesis describes the investigations on the structural behaviors of cold-formed thin-walled steel lipped channel columns,which consisting of the experimental investigations and numerical analyses of cold-formed steel lipped channel columns experiencing distortional-local and distortional-global buckling interactions,and the research of design strength methods of distortional buckling,distortional-local and distortional-global buckling interactions.Concerning the experimental investigation on the cold-formed steel lipped channel columns experiencing the distortional-local buckling interaction,a set of 27 specimens with a nominal thickness of 1.5mm,several lengths,and the widths of web and lip were tested.Some detailed works involving column geometry selection,measuring member geometries and initial geometrical imperfections,and the tensile coupon tests were described.And the experimental results were also presented and analyzed,which consisting of failure modes,the applied load vs.strain curves,the applied load vs.displacement curves and the ultimate load-carrying capacities.The specimens eroded by distortional-local buckling interaction firstly experienced the distortional buckling.And when the applied load approached to the ultimate load-carrying capacity,local buckling located around the both end plates occurred and developed from both ends to mid-height cross section.Finally,the distortional-local buckling interaction governed the failure modes and ultimate strengths of specimens.As for the lengths of half-wave of local buckling,they were approximately to the widths of web for the specimens with short and middle lengths while they were a little larger than the widths of web for the specimens with long lengths.Due to the distortional buckling making the web have outward or inward deformations,concavo-convex conjointly deformations caused by local buckling could not be observed.Moreover,the ultimate load-carrying capacities of specimens decreased with the increase of length,however,it was not significantly.And increasing the widths of lip could increase the ultimate load-carrying capacities of specimens due to the fact that lips can restrain the distortional buckling effectively.Concerning the experimental investigation on the cold-formed steel lipped channel columns experiencing the distortional-global buckling interaction,a set of 33 specimens with a nominal thickness of 1.5mm were tested.Some detailed works involving column geometry selection,measuring member geometries and initial geometrical imperfections,and the tensile coupon tests were described.And the experimental results were also presented and analyzed,which consisting of failure modes,the applied load vs.strain curves,the applied load vs.displacement curves and the ultimate load-carrying capacities.All the specimens firstly experienced global(torsional)buckling and then the distortional buckling occurred with the increase of the applied loads.Finally,the distortional-global buckling interaction governed the failure modes and ultimate strengths of specimens.And the local deformation named “distortional plastic hinge” could be observed for each specimen.Due to the initial geometrical imperfections or the even distortional half-wave numbers,the “distortional plastic hinge” did not always locate at mid-height cross section of specimen.Moreover,the lengths of global half-wave were approximately to the lengths of specimens while the lengths of distortional half-wave were approximately to the 1/3 lengths of specimens.Note,although the elastic local,distortional and global(flexural-torsional)critical buckling stresses are nearly coincident,all the specimens only experienced the distortional-global buckling interactions rather than local-distortional-global buckling interactions.Concerning the numerical analyses on the cold-formed steel lipped channel columns experiencing the distortional-local or distortional-global buckling interaction,the rationality and effectiveness of finite element models were analyzed by means of the code ABAQUS initially,which were based on the experimental data from literatures.And then the parametric analyses using the finite element models for cold-formed steel lipped channel columns eroded by distortional-local and distortional-global buckling interaction were performed to evaluate the failure mode and ultimate load-carrying capacity of specimen.The parametric analyses consists of the mode of initial geometrical imperfection,the value of initial geometrical imperfection,yield stress of steel,the thickness of plate,the length of specimen,the web stiffener and the boundary condition.The results of parametric analyses showed that the mode of initial geometrical imperfection could govern the failure mode of specimen while the greater values of initial geometrical imperfection were,the lower ultimate load-carrying capacities of specimens were;values of initial geometrical imperfection had no influence on the failure mode of specimens;the higher yield stresses of plate were,the lower ultimate load-carrying capacities of specimens were,and due to the elastic critical buckling stresses,the low-strength structural steel may change the failure mode of specimens;the values of ultimate load-carrying capacities of specimens became greater with increase of the thickness of plate;the lengths of specimens had a decreasing trend in the ultimate load-carrying capacities of specimens,whereas they had a little influence for distortional-local buckling interaction and had a certain influence for distortional-local buckling interaction;the web stiffener could effectively prevent the local buckling and improve the stiffness of plate,however,it had a little contribution for precluding the distortional and torsional buckling.Moreover,comparing with fixed-ended boundary condition,the simple boundary condition could change the failure mode of specimen and reduce significantly the ultimate load-carrying capacities of specimens for distortional-global buckling interaction while a little for distortional-local buckling interaction.The accuracy for calculating the ultimate load-carrying capacity of cold-formed steel columns based on the direct strength method related to distortional buckling,distortional-local and distortional-global buckling interaction was analyzed and verified by using the experimental and numerical data collected from literatures.Based on the above results,the author proposed two methods for calculating the ultimate load-carrying capacity of cold-formed steel column eroded by distortional buckling,which were simpler than the current direct strength method.The proposed method 1 and method 2,which were followed the procedure adopted to handle columns failing in distortional and global buckling respectively,could effectively predict the ultimate load-carrying capacity of cold-formed steel column under axial compression.Moreover,due to the predictions of proposed method 2 being consistent with experimental and numerical data,the new methods based on the proposed method 2 for calculating the ultimate load-carrying capacity of cold-formed steel columns eroded by distortional-local and distortional-global buckling interaction were also proposed and evaluated.This thesis has the following points of innovation:(1)The cold-formed steel lipped channel columns from Q235 and Q345 structural steel experiencing distortional-local and distortional-global buckling interactions has been studied respectively and systematically by means of the method combining experiment and finite element analysis.And the failure modes and ultimate load-carrying capacities obtained from results can be used to supplement the relative researches and specifications.(2)Bsed on direct strength method,two methods calculating the ultimate load-carrying capacity of specimen experiencing distortional buckling have been proposed,which have more concise form and agree well with experimental and finite analyses results.Moreover,according to the proposed method 2,the methods calculating the ultimate load-carrying capacity of specimen experiencing distortional-local and distortional-global buckling interactions have also been proposed. |
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