Research On The Distortional Buckling Capacity Of Stainless Steel Beams With Lipped C-Section

Stainless steel structures have the advantages of attractive appearances,stable mechanical properties,good corrosion resistance,easy maintenance and low life cycle costs,leading to the broad application prospect.The stress-strain relationship of stainless steel shows significant nonlinearity,while the cold-formed stainless steel members with open sections are likely to develop local and distortional buckling.Therefore,members of this kind have complex nonlinear buckling behaviors.So far,the relevant specifications and researches for cold-formed stainless steel members with open sections still need to be supplemented.Based on the above background,this dissertation focuses on the distortional buckling capacity of stainless steel beams with lipped C-section.The experimental researches,numerical simulation and parametric analysis were carried out,and the calculation formula for the distortional buckling capacity of stainless steel beams with lipped C-section was finally proposed on the basis of the direct strength method.For the domestic austenitic S30408 stainless steel,the mechanical property tests were conducted on 6 flat coupons and 6 corner coupons.The stress-strain curves and various material mechanical properties were obtained,including the initial elastic modulus,nominal yield strength,ultimate tensile strength and strain hardening coefficient.Conclusions were drawn from the mechanical property tests results:(a)the nominal yield strength of corner coupons was about 40%~60% higher than that of flat coupons,and the elongation after fracture of corner coupons was significantly lower than that of flat coupons;(b)the two-stage RambergOsgood model fitted well with the stress-strain curves of stainless steel,while the strain hardening coefficients in the model needed to be corrected according to the test results.Based on the mechanical property test results,bending tests were performed on 8 stainless steel specimens with lipped C-section.Through the bending tests,the development process of distortional buckling waves was revealed.As the load changes,the curves of the vertical displacements at mid span,vertical displacements at loading points,support rotations,lateral displacements at mid span,lateral strains and longitudinal strains were obtained,and the distortional buckling load and bearing capacity of specimens were also achieved.Results of the bending tests showed that failure modes of bending test specimens are all distortional buckling.The development process of distortional buckling waves experiences four stages,which are respectively the initial stage,loading to 90% of the bearing capacity stage,loading to the bearing capacity stage and the bearing capacity declining stage.The load-vertical displacement at mid span curves,load-vertical displacement at loading points curves and load-support rotation curves present the basically same development trend,which can be di-vided into three stages,including the linear stage,nonlinear stage and failure stage.From the comparison of the bearing capacity of different specimens,it can be known that when other section parameters are fixed,the increase of section thickness or curl width will lead to the enhancement of bearing capacity.The comparison between the buckling load and the bearing capacity shows that the post-buckling strength exists in specimens under distortional buckling.The finite element software ABAQUS was used to establish the refined finite element models and carry out numerical simulation analysis on the distortional buckling behaviors of the bending test specimens.The finite element analysis results and experimental results were compared to verify the accuracy of the finite element models.For the convenience of the subsequent extensive parametric analysis,geometric models of the refined finite element models were simplified reasonably.The simplified analysis models can get the relatively fast operating speed and still have the required simulation accuracy.Based on the simplified analysis models,the parametric analysis was conducted on the distortional buckling capacity of stainless steel beams with lipped C-section.Influences of four parameters on the distortional buckling capacity were investigated,including the cold working effects(residual stress,corner region strength,corner radius),the strain hardening coefficient,the initial geometric imperfection,and the distortional buckling slenderness.Conclusions can be drawn from the parametric analysis:(a)the residual stress has little effect on the distortional buckling capacity,while the corner region strength can enhance the distortional buckling capacity,which should not be neglected.The capacity enhancement turns more apparent with the increase corner radius;(b)when the strain hardening coefficient of stainless steel varies,the distortional buckling capacity shows little difference,indicating that the strain hardening coefficient has little effect on the distortional buckling capacity;(c)for specimens with different lipped C-sections,the distortional buckling capacity decreases when the initial geometric imperfection amplitudes increase;(d)there is a strong correlation between the distortional buckling slenderness and the distortional buckling capacity.As the slenderness goes up,the ratios of the distortional buckling capacity to both the edge yield moment and the full-section plastic moment continuously go down.The finite strip program CUFSM was employed to study the elastic distortional buckling stress of lipped C-section beams.By analyzing effects of the section parameters(section height,flange width,curl width and section thickness)and the coupling between different parameters,the simplified calculation formula was fitted.The comparison between the formula results and the finite strip program results verified the calculation accuracy of the fitting formula for the elastic distortional buckling stress.Based on the direct strength method,the calculation formula for the distortional buckling capacity of stainless steel beams with lipped C-section was studied.Analysis results show:(a)the direct strength formula in North American Cold-Formed Steel Specification for the Design of Cold-Formed Steel Structural Members can not accurately predict the distortional buckling capacity of stainless steel beams with lipped C-section;(b)the fitting formula in this dissertation,which was obtained by taking the full-section plastic moment as the reference capacity,shows better fitting accuracy for calculating the distortional buckling capacity;(c)comparison of the formula results and test results verifies the accuracy of the fitting formula;(d)the reliability indexes under different load combinations are all larger than the allowable reliability index in the specification,proving the reliability of the fitting formula.Finally,from the perspective of engineering designs,the calculation process of distortional buckling capacity of test specimens was given,by using the two fitting formulas of elastic distortional buckling stress and distortional buckling capacity.The accuracy of this calculation method was verified by comparing the calculation results and test results.Analysis shows that the distortional buckling capacity can be directly calculated by the fitting formulas according to the section parameters and mechanical property parameters of stainless steel,without the help of finite strip program and finite element software.Researches in this dissertation can make up for the lack of experimental researches and calculation methods of distortional buckling capacity of stainless steel beams with lipped C-section,and also provide an important theoretical basis for the follow-up revision of Chinese Technical Specification for Stainless Steel Structures,as well as a meaningful reference for related designs and researches of this field.