RHS-to-RHS cold-formed S960 steel fire exposed X-joints: Structural behaviour and design

This paper presents a detailed finite element (FE) analysis and design of cold-formed high strength steel (CFHSS) fire exposed X-joints with square and rectangular hollow section (SHS and RHS) brace and chord members. The nominal 0.2% proof stress of SHS and RHS members was 960 MPa. The static behaviour of SHS and RHS X-joints was numerically investigated corresponding to four post-fire temperatures, including 300°C, 550°C, 750°C and 900°C. The RHS X-joints were subjected to axial compression loads through brace members. The post-fire residual strengths of cold-formed S960 steel grade SHS and RHS X-joints were experimentally investigated by the authors. The test results were used to develop an accurate FE model. Through the validated FE model, a comprehensive FE parametric study comprising of 756 FE specimens was performed in this investigation. Overall, SHS and RHS X-joint specimens were failed by chord face failure, chord side wall failure and a combination of these two failure modes. The validity ranges of critical geometric parameters were extended beyond current limits mentioned in international codes and guides. Using the measured post-fire residual static material properties, nominal resistances were predicted from design rules given in Eurocode 3, CIDECT and literature. The residual joint strengths of test and FE specimens were compared with predicted nominal resistances. Generally, it has been shown that the existing design rules are quite conservative but unreliable. As a result, accurate and reliable design rules are proposed in this study.