Research On The Buckling Mechanism And Ultimate Strength Design Methed Of Cold-formed Double-limbs Built-up Open-section Columns

In recent years,cold-formed steel?CFS?double-limbs built-up open-section columns,which are formed by two indentical C-section members using self-drilling screws,are widely used in CFS structures due to their superior mechanical behavior.However,the existing researches are still in the primary stage,which mainly aims at investigating the qualitative influence of screw spacing,screw layout and cross-sectional shape on the bearing capacity through experimental investigations and numerical simulations.Such methods ignore the stability problem of this kind of member and,at the meantime,can not reveal the quantitative relationship between the above parameters and bearing capacity.For this reason,the engineers can only make some conservative estimates in the design of this kind of member and lack of theoretical guidance.In this scenario,the instability mechanism of the CFS double-limbs built-up open-section columns was studied.The influences of discontinuous interface,cross-sectional plate interaction and single profile interaction were revealed.Subsequently,a new design method,which is closer to the mechanical behavior of this kind of member,was proposed to quantify the bearing capacity.The main research contents are as follows:?1?Experimental investigations were conducted on the buckling behavior of CFS double-limbs built-up open-section columns under compressionA totoal of 18 single C-section columns and 18 CFS double-limbs built-up open-section columns.The built-up effects on bearing capacity,cross-sectional deformation and post-buckling strength reserve of CFS double-limbs built-up open-section columns were analyzed by comparing the difference of the test results between single C-section columns and CFS double-limbs built-up open-section columns.?2?The buckling formula of built-up plate was established for the first time.The formulae of buckling critical load calculation for the screw assembly plate considering the shear slip deformation and self tapping screw constraint were established,which extended the application scope of the classical thin plate small deflection theory in the screw built-up plate and solves the lack of relevant theories both at home and abroad.?3?The local buckling P_crLrL and distortional buckling P_crDrD formulae of CFS double-limbs built-up open-section columns were established for the first time and their application were extended to the caculation of local buckling P_uL and distortional buckling P_uD capacity.Based on the research of built-up plate,the models for P_crL and P_crD were proposed,respectively.Then,the expressions for P_crL and P_crD were derived,respectively,and solves the lack of relevant theories both at home and abroad for CFS double-limbs built-up open-section columns.It worth to mention that the proposed models took both the column length L and the number of half-waves m as the characterisitc parameters for longtitudinal deformation to consider the effects of L and m on P_crL,which sloved the problem for the traditional way that only takes half-wavelength as the characteristic parameter and cannot accuately and effeciently obatined the relationship of P_crL,L and m.Finally,in order to take the effects of post-buckling reserve,material properties and imperfections into consideration,the derived formual P_crL and P_crD were intorduced into Schafer's local buckling design curves P_uLL and distotional buckling design curves P_uD,respectively,and thus the local buckling and distortional buckling capacity calculation method were obtained.?4?The flexure buckling formula P_crGrG of CFS double-limbs built-up open-section columns bending in virtual axis was established for the first time and its application was extended to the caculation of flexure buckling capacity P_uG.the flexure buckling model for P_crG was proposed and the expression for P_crG was derived based on energy method,and thus the lack of analytical flexure buckling formula for CFS double-limbs built-up open-section columns bending in virtual axis was solved.Finally,in order to calculate the flexure buckling capacity of CFS double-limbs built-up open-section columns bending in virtual axis,the derived formual P_crG was intorduced into available flexure buckling design curves P_uG.6.A new bearing capacity design method,which is closer to the buckling behaviour of CFS double-limbs built-up open-section columns,was established.Based on the aforementioned P_crL,P_crD and P_crG and evaluating the appropriateness of current avaible design method for CFS built-up member,the critertion of local-distortional buckling was presented and a new direct strength methodwas proposed based on the presented criterion.The comparison results shown that the proposed method can not only avoid the unsafe problem of classic direct strength method for ignoring the effect of local-distortional interaction,but also improved the excceding conservation problem of the current available local-distortional design curves.The main innovation points were summarized as following:?1?The critical buckling formulae for screw connected built-up plates were derived for the first time,and thus the applicable scope of the classical small deflection theory was extended.?2?The local,distortional and flexure buckling mechanism were revealed for the first time and,at the meantime,the ctitical local,distortional and flexure buckling formulae were derived,which lays a theoretical foundation for a more rational bearing capacity design method of such members.?3?The effects of end conditions were considered for the derived local buckling P_crLrL and distortional P_crD formulae,which solved the problem for the traditional way that simply ignores these effects on P_crL and P_crD.In this condition,the effects of end conditions on P_crLrL and P_crD were clarified.?4?A more comprehensive bearing capacity design method was established for the CFS double-limbs built-up open-section columns,which can be used successfully in determining the bearing capacity of local buckling,distortioan buckling,local-distortional interactive buckling and other kinds of buckling modes.