Behavior of concrete-filled cold-formed steel built-up section stub columns

This paper presents a test campaign concerning the behavior of concrete-filled cold-formed steel built-up section stub columns. The test program consisted of five types of cold-formed steel built-up sections infilled with concrete of three different grades (C40, C80 and C120). A pair of identical open sections were built-up to form a closed section by discrete self-tapping screws and connected using steel strips where necessary. A total of 42 stub columns, including 32 concrete-filled specimens and 10 hollow counterparts, was axially compressed between fixed-ends. Details of material properties tests and stub column tests are described in this paper. The stub column compressive strengths, full range axial load-shortening histories as well as load–strain responses and failure modes are presented and discussed. The test strengths were compared with the design predictions by the current American Specification ACI318, despite that this design rule was originally developed for concrete-filled steel columns with conventional tubular sections rather than built-up sections. The comparison demonstrated that the American Specification ACI318 provides generally unconservative and unreliable compressive strength predictions for the concrete-filled cold-formed steel built-up section stub columns. Necessary improvements shall be made in order to yield accurate compressive strength predictions.