| The research and application of cold-formed thin-walled steel originated from European and American countries in the 1930 s.Because of non-campact cross-section and large width-to-thickness ratioes,cold-formed steel columns under axial compression are prone to local buckling.The stability of thin-walled section steel under compression should not be ignored in order to prevent the members from buckling and causing serious engineering problems.The critical stress or load of thin-walled steel members local buckling under axial compression is an important parameter to calculate the local or locally coupled buckling ultimate load.The effective width method usually takes the buckling critical stress of rectangular axial platen simply supported by two loading sides,that is,the critical stress without considering the panel effect of the member.Since Schafer put forward the direct strength method in 2002,the critical buckling stress or load of axial compression members considering the plate group effect has become a necessary condition for the calculation.However,the existing calculation formulae are most limited to the critical stress of simply supported members at both ends without considering the plate group effect.In order to establish practical calculation formulae for the local buckling critical load of commonly used axial compression members cross-section,this paper takes the C-shaped section columns in 4 standards and 1 design manual as the research object,that is,the Chinese national standard “Technical code of cold-formed thin-wall steel structures”(GB50018-2002),the Chinese construction industry standard “Cold-formed thin-walled steel sections for building structures”(JG/T 380-2012),the American national standard “North American Specification for the Design of cold-formed Steel Structural Members”(AISI S100-16),the American design manual “AISI Manual cold-formed Steel Design 2002 Edition”,and the British national standard “BS 5950-5:1998 Structure use of steelwork in building: part 5: code of practice for design of cold formed thin gauge sections”(BS 5950-5:1998).By combining theoretical analysis with finite strip method,the following research was carried out.(1)The range of 6 scale parameters(width-to-width ratios and the width-to-thickness ratios)for C-shaped section in the 4 standards and the 1 design manual was calculated and summarized to determine the geometric scale range of the research object.(2)Based on the formula of local buckling critical stress for axial compression rectangular plate,the buckling coefficient of local buckling critical stress simply supported or fixed supported at both ends of loading sides and simply supported at non-loading sides is expressed uniformly by introducing the effective-length factor of flexural buckling of the axial compression member.(3)The equivalent concept of local buckling critical load for axial members is presented.The practical formulae of local buckling critical load presented in this paper and proposed by other scholars are respectively used to calculate the critical load of 184 C-shaped columns.Meanwhile,the result was compared with the critical stress calculated by finite strip software CUFSM to verify the accuracy of the proposed practical formulae.(4)The local buckling coefficient is proposed considering the effect of plate group effect based on the result simulated by finite strip analysis.The formulae for calculating the local buckling critical load of C-,Z-and Hat-shaped columns are expressed uniformly by extending the section size range of C-shaped columns in the Chinese national standard GB50018-2002,the Chinese construction industry standard JG/T 380-2012 and the American design manual “AISI Manual cold-formed Steel Design 2002 Edition” to Z-and Hat-shaped columns.(5)The practical formulae of local buckling critical load in this paper is further verified by adopting the test data in the different papers and Schafer's direct strength method for local buckling of columns,which further demonstrates the accuracy and reliability of the proposed practical formulae.Study shows that the proposed practical formulae for estimating the local buckling critical load of the commonly used section columns is convenient,simple in form,both with the classic formula of the critical stress for the rectangular plate in harmony,fully reflecting the simply-supported and fixed-ended of boundary constraints on the effect of local buckling critical load,having high accuracy to estimate the commonly used C-,Z-,and hat-shaped columns,which can be used as reference for calculating the local buckling critical stress of cold-formed thin-wall steel axial compression columns in China,the USA and the UK. |
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