Keywords: steel cold-formed member, local buckling, buckling interaction, thin-walled member.

The strength of steel cold-formed members is obtained on the basis of the traditional effective width method (EWM), a semi-empirical formulation to take into account the plate buckling of thin-walled members which is included in codes for structural design [1,2]. The application of the EWM for folded sections is based on approximate solutions of local buckling by isolating each single plate element of the cross-section and its final results compare approximately with experiments.

The "other way" to perform buckling interaction for design purposes is the direct strength method [3]. As opposed to the EWM, the DSM takes advantage of computational possibilities to perform accurate buckling analysis of the folded plate member and the results are included in the design process. The favourable consequences are: (i) an understandable process of design; (ii) less complex strength equations and strategies and (iii) a more accurate theoretical strength results.

It is in this context of coexistence of traditional and new methods of design that the Brazilian code for steel cold-formed structural members [2] is under revision, stimulating the development of a deep investigation concerning different methods of the design of cold-formed columns and beams. As a consequence, the effective area method (EAM), which was originally conceived as a direct method for steel cold-formed columns [4] was recalibrated and the results obtained [5] encouraged its extension to beams, resulting in a general direct method that was named the effective section method (ESM). The presentation of the ESM is the main goal of the present paper. The ESM was calibrated against experimental results for the cold-formed columns and beams and the results indicated accurate statistical results and a final partial resistance factor of around 1.1 to 1.2, for both columns and beams.

In addition, the ESM is combined with useful and original equations for direct access to the local buckling actual behaviour of the section, greatly simplifying its application and allowing designers to avoid repetitive buckling analysis using computational methods. Finally, a very important feature is that the ESM can be presented and applied side-by-side with the EWM, contributing to the integration of the methods and improving the attractiveness of the ESM.

1

AISI S100-2007, North American specification for the design of cold-formed steel members, American Iron and Steel Institute, 2007.

2

ABNT NBR 14762, Dimensionamento de estruturas de aço constituídas por perfis formados a frio, Associação Brasileira de Normas Técnicas, 2001.

3

B. Schafer, T. Pekoz, "Direct strength prediction of steel cold-formed members using numerical elastic buckling solutions", In Proc. Fourteenth Int. Specialty Conf. on Cold-Formed Steel Structures, Univ. of Missouri-Rolla, 1988.

4

E.M. Batista, "Étude de la stabilité des profils à parois minces et section ouverte de types U et C", Collection des publications de la Faculté des Sciences Appliquées, 119, University of Liège, 1989.

5

E.M. Batista, "Local-global buckling interaction procedures for the design of cold-formed columns: effective width and direct method integrated approach", Thin-Walled Structures, accepted for publication, April 2009. doi:10.1016/j.tws.2009.04.004

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