Cold-formed steel member bending stiffness prediction

• Cold-formed steel beams failing in local and distortional buckling are investigated.
• Bending stiffness of cold-formed steel beams is examined with existing methods.
• Predictions of the Effective Width and Direct Strength methods are inaccurate.
• New design expressions are proposed to predict member bending stiffness.
• The new method provides reliable predictions depending on cross-section slenderness.

The objective of this paper is to provide a simplified method for predicting the bending stiffness of thin-walled cold-formed steel members subject to elastic (or inelastic) local or distortional buckling. In thin-walled cold-formed steel members for serviceability and consideration of deformations (second-order effects) it is important to account for the loss of cross-section bending stiffness due to the use of slender elements and/or yielding. Although existing design specifications provide some guidance on how to predict the stiffness, limited information is available for cross-sections subject to distortional buckling or undergoing inelastic local and/or distortional buckling. Existing tests and shell finite element analysis on cold-formed steel beams failing in local or distortional buckling are used as the basis for the study conducted herein. Stiffness of the conducted tests and finite element models is compared with predictions based on the Effective Width Method and Direct Strength Method. Relationships between local and distortional cross-section slenderness and the observed and predicted secant stiffness up to the peak load are examined. New design expressions to predict cold-formed steel member bending stiffness depending on cross-section slenderness are presented. The new methods are proposed for adoption in cold-formed steel specifications.

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