Research And Design Recommendation On Web Bearing Capacity Of Cold-Formed Thin-Walled Lipped C-Section Members Under Local Compression

Cold-formed thin-walled lipped C-section members are usually used as floor beams and purlins of light steel houses.Due to the limitation of the lipped section shape,the installation of transverse stiffeners is difficult and uneconomical.Therefore,there are usually no stiffeners at the support of cold-formed steel beams or at the action of concentrated loads,it is necessary to determine the web local compression bearing capacity when designing.In this paper,the lipped C-section member whose flanges are rotation-restrainted is taken as the research subject.The web local bearing capacity under four load cases is studied,i.e.one side and two sides of end flanges(EOF & ETF),one side and two sides of middle flanges(IOF & ITF),and finally calculation method for local compression bearing capacity under above four load cases is obtained.The ABAQUS finite element analysis software is used to establish numerical models to study web bearing performance for the cold-formed thin-walled C-section members under local compression.Based on the latest international test results in the world,the correctness and applicability of the finite element model is verified from the perspective of ultimate bearing capacity,load-displacement curve and failure mode.Through the dimensionless variable-parameter analysis of the parameters of C-shaped section like the radius of the inner bend,web height,support length and yield strength,a series of valuable conclusions are obtained.The existence of the bending angle radius makes the member enter the state of elastic-plastic stress in andance,and lowers the bearing capacity of the member;The ratio of web height to thickness shows a non-monotonic relationship with bearing capacity,which is not consistent with the expression of this parameter in the North American Specification calculation formula;The difference of support length obviously changes the failure modes of members.By comparing the numerical analysis results with the calculation values from current North America Specification for the Design of ColdFormed Steel Structural Members AISI S100-16,it is found that when the formula is applied to the section and material properties of common components in China,it is mostly conservative under EOF and IOF load cases,but not safe under ITF load case,while the calculation results in ETF load case have a large deviation from the actual situation,which shows that it is not reasonable to directly introduce the relevant calculation formula from the North American Specification into Chinese National Specification due to the differences in section geometry and material properties;Also the error analysis results show that the yield strength of materials not considered in the specification is also an important parameter affecting the bearing capacity.The relevant calculation methods in the current mainstream design specifications are all obtained by limited regression analysis of experimental data and lack of theoretical background related to stablity calculation.Based on this,the Direct Strength Method(DSM)is introduced into the local compression problem of lipped C-section web in order to obtain a reliable formula calculating bearing capacity.Based on the analysis results of classical elastic plate stability theory,the calculation methods of elastic buckling critical load under four load cases are obtained by using eigenvalue buckling analysis and numerical correction.Based on the assumption of the yield line mode under the limit state,the plastic load calculation formulas under four load cases are derived by using the virtual work principle.The analysis results show that the Direct Strength Method has good applicability for EOF,ETF and ITF load cases,and the normalized slenderness ratio can reasonably reflect the contribution of web elastic buckling to Web Crippling.However,for the IOF load case acting on the middle side,the Web Crippling is more controlled by local plastic development,and the contribution of web buckling to Web Crippling is not significant,so the Direct Strength Method which can play a major role in both stability and strength problems is not applicable.For this reason,this paper modified the standard formula accordingly,and gave the calculation method of bearing capacity under IOF load case.Finally,based on the available experimental data,the proposed formula and the calculated value of the North American Specification are compared,and the results and background of the proposed formulas are proved to be superior.