The objective of this thesis is the study of the stability of cold-formed thin-walled steel members. The interaction between global instability and local buckling of the profile walls has a reducing effect on the collapse load. The aim is to develop a numerical tool able to predict not only the collapse load, but also the equilibrium path. With this aim in view, the non-linear spatial beam tinite element recently developed by Vincent de Ville in Liege University was chosen for this purpose. The steel member is considered as an association of flat plates and corners. Local buckling is modeled by means of the concept of effective width of plates. The effective widths of the plates of the cross-section are updated at each iteration of the non-linear step according to the updated stress level of the finite element. These effective widths make up the new effective section of the finite element. In this way, the cross-section of the finite element varies continuously at each iteration, so that additional features had to be developed to take into account for the changes of the mechanical properties of the section, position of the center of gravity, orientation of the principal axes, area, flexural stiffness, etc. The arc-length method was used to solve the non-linear problem. Regarding the calculation of the effective widths, they can be calculated according to two different concepts: the former considers the plates of the section as isolated (simply supported), as it is proposed by Eurocode 3 and AISI-90; the second consists in taking into account local buckling interaction between the plates of the section by means of appropriate local buckling coefficients. Angle, channel and lipped-channel sections have been studied. Experimental results of beam-columns with the mentioned sections are compared to the numerical results found by finite elements. For channel sections only the second concept of calculation of effective widths gives a good estimation of the collapse load. In general, a good correlation between experimental and numerical results has been found for the sections studied. A graphical output has also been developed for plotting curves of load versus displacement and for the visualization in 3-D of framed structures discretised with the spatial beam finite element. This graphical post-processor has been developed with PHIGS. The members are drawn as they are with their deformations and their effective sections. Stress level can be seen by the colors surrounding the integration points.


Civil, Architectural and Environmental Engineering

Research Center/Laboratory(s)

Wei-Wen Yu Center for Cold-Formed Steel Structures



Publication Date


Document Version

Final Version

Document Type

Report - Technical

File Type