Strength Evaluation of Cold-Formed Steel Columns Using The Results of Finite Strip And Finite Element Linear Stability Analysis

Abstract

Most cold-formed steel columns display open and rather thin-walled cross-sections which mean that their structural behaviour is strongly affected by local and global buckling. Th e local mode, that occurs for shorter profi les, is characterized by (i) the local plate mode (LPM) characterized by the simultaneous flexural buckling of the web and fl anges and (ii) by the distortional mode (DM) characterized by the displacements of flange-stiff ener edges (that remain plane). The global mode occurring for long profi les is characterized by (i) the fl exural mode (FM) characterized by the translation of the whole section in the direction of the major principal axis and (ii) by the fl exural-torsional mode (FTM) characterized by the simultaneous translation and rotation of the whole section. Th e possibility of using the results of linear stability analysis in the national codes of thin-walled cold-formed steel structural elements (for instance, European and Brazilian Codes) arises, i.e. local and global buckling instability modes and corresponding bifurcation stresses determining the ultimate strength of members. Two powerful numerical methods are chosen to perform a linear stability analysis of a cold-formed steel structural member: (i) the Finite Strip Method, (i1) the Semi-Analytical Finite Strip Method (trigonometric functions are used in the approximation of displacement) used for simply supported boundary conditions, (i2) the Spline Finite Strip Method (‘spline’ functions are used in the approximation of displacement) used other boundary conditions and (ii) the Finite Element Method. The linear local and global stability results of for Z, C and rack cold-formed columns are used to obtain ultimate strength through the procedures adopted in the Eurocode 3, Part 1.3 and in the Brazilian Code (NBR 14.762/2001). The obtained numerical estimates by specifi cations are compared with experimental results available in literature.

State-of-the-Art: Integrating Fastener Technology and Design Guidelines for Enhanced Performance of Cold-Formed Steel Sections

“…Furthermore, the presence of springs within CFS studs has negligible impact on distortional buckling and no influence on local buckling, yet they play a crucial role in preventing global buckling by providing constraint [19]. The study concludes that the smeared spring hypothesis can only be valid when the half-wavelength to connector spacing ratio for global buckling is less than 0.25 [19].…”

“…Although generally conservative, the modified slenderness technique is perhaps slightly unconservative, deviating by approximately ten percent for CFS stub columns that are put up back-to-back [18]. Furthermore, the presence of springs within CFS studs has negligible impact on distortional buckling and no influence on local buckling, yet they play a crucial role in preventing global buckling by providing constraint [19]. The study concludes that the smeared spring hypothesis can only be valid when the half-wavelength to connector spacing ratio for global buckling is less than 0.25 [19].…”

Distortional buckling resistance of cold-formed steel

“…(7), W is the elastic modulus of the gross section in relation to the extreme fiber that reaches yielding, f y is the yielding stress of steel and dist is the reduced slenderness associated with the distortional buckling given by Eq. (8) and M dist is the critical bending moment that causes elastic distortional buckling and is obtained by linear stability analyses. …”

“…The formulation was validated by Prola and Pierin [8] when the software results were compared with experimental results obtained by [9][10][11][12].…”