Study On Stability Of Cold-formed Thin-walled Steel Members With Stiffened Sections Under Compression

As the main load-bearing part of cold-formed thin-walled steel structure,cold-formed thin-walled steel column has been widely used in many light steel structures.Because of large width and thickness of thin-walled members,local buckling easily occurs when the member is under compression.Stiffening ribs can significantly improve the local buckling capacity of members and increase the stability when installing stiffening ribs on the plates.For the calculation method of the bearing capacity of thin-walled steel members,the influence of distortion buckling has not been taken into account in the existing codes and the shortcomings of the complicated calculation process become more and more prominent.In recent years,the Direct Strength Method(DSM)has effectively solved the problem without considering distortion buckling.And there are much studies on the stability performance and bearing capacity calculation methods of common stiffeners such as V-shaped and E-shaped members,but less studies on other types of stiffeners.A new type of cold-formed thin-walled steel compression member with cap stiffener in the middle of web and middle cap stiffener in the web and flange is proposed.The mechanical properties are studied by experiments,theoretical analysis and computer simulation.The stability performance of 30 simply supported cold-formed thin-walled steel members with new cross-section is studied through tests.The results show that the critical local buckling stress and ultimate bearing capacity can be effectively improved by setting the middle cap stiffeners.Comparing with the non-stiffened common complex flanged channel steel section specimens,the ultimate bearing capacity of the web-cap type stiffened complex flanged channel steel section specimens increases by 50%-60%,the web-flange type stiffened complex flanged channel steel section specimens increases by 80%-140%.The main failure mode of short columns is local and distortion buckling,and for the middle and long columns is locally and distorted.In eccentric compression members,the distance and direction of effective centroid deviation of eccentric compression members can be changed by setting cap stiffeners in the middle.Using finite element software,the finite element analysis model for complex crimped channel steel members with middle cap stiffening is established.The results show that the load-bearing efficiency of plate-type middle-cap stiffeners can be significantly improved.Under the conditions of axial compression,positive eccentric compression and negative eccentric compression,the load-bearing efficiency of web-type middle-cap stiffeners and web-flange-type middle-cap stiffeners is 41%and 43%,17%and 15%,37%and 71%higher than that of common complex flanged channel steel members,respectively.The bearing capacity decreases significantly with increment of the length of members;the increase ratio of ultimate bearing capacity of members with different cross-section types varies significantly with the thickness of plates;the change of flange width has little influence on the failure mode and ultimate bearing capacity of members under two boundary conditions;and the plate stiffening has some influence on the increase range of bearing efficiency of members under two boundary conditions.Setting stiffeners can effectively reduce the width-thickness ratio of the cross-section and improve the ultimate bearing capacity of the members;the bearing capacity of the members with different cross-section forms is not the largest at the position of axial compression.Establishing the elastic buckling stress analysis model by finite strip software CUFSM for simply supported axially compressed members of complex crimped channel steel with intermediate cap stiffening.The results show that the elastic local buckling stress of the component can be significantly increased by installing cap stiffeners in the middle of web,web and flange,and the effect on the elastic distortion buckling stress and the overall elastic buckling stress of the component is small.Based on the experimental and computational results,the direct strength method formulas considering local,distortion and global correlative buckling are established for the stiffened intermediate cap section members with simply supported and clamped ends.The formulas for calculating the effective centroid offset of the section under simply supported boundary conditions at both ends and the stability bearing capacity of eccentrically loaded members in the bending moment action plane are established.