Investigation on flexural strength of cold-formed thin-walled steel beams with built-up box section

• The four-point bending test was conducted on built-up box sections (CU) and individual sections (C, U).
• Stress distribution at critical cross-section of built-up beams was discussed.
• The reasonability of the capacity superposition method and ‘equivalent method’ is assessed.
• The equivalent box section is suggested for built-up box beam bending about weak axis.

Built-up sections are widely used in cold-formed thin-walled steel frames. This paper mainly investigates the in-plane behaviour of built-up box beams with nested C section and U section under pure bending about strong and weak axis. The four-point bending test was conducted on a total of 7 groups of specimens, including built-up box sections (CU) and individual sections (C, U). A finite element model is developed and verified by the test results for further parametric analysis. The stress distribution at mid-span cross-section shows that the components in built-up beams can resist the bending moment together. For built-up box beams bending about strong axis, the reasonability of the capacity superposition method and equivalent method is assessed by the discussion on test results and numerical analysis results. The equivalent box section is suggested to calculate the flexural strength of built-up box beams bending about weak axis. Finally, the comparison between predicted and actual flexural strength indicates that this simplification of the proposed method can slightly under-estimate the flexural strength.