Design of cold-formed high strength steel CHS-to-RHS T- and X-joints at elevated temperatures

The static resistances of cold-formed S900 steel grade tubular T- and X-joints at elevated temperatures have been numerically investigated in this study. Circular hollow sections (CHS) were used as the braces, while square and rectangular hollow sections (SHS and RHS) were used as the chords for both T- and X-joints. In this study, both T- and X-joints were subjected to compression loads. The mechanical properties of cold-formed S900 steel grade hollow section members at elevated temperatures were used to perform the numerical investigation. The static resistances of CHS-to-RHS T- and X-joints were investigated at 400°C, 500°C, 600°C and 1000°C. The finite element (FE) models developed and validated by the authors for ambient temperature and post-fire investigations of cold-formed S900 steel grade CHS-to-RHS T- and X-joints were used in this study to perform numerical investigation at elevated temperatures. A comprehensive FE parametric study, including a total of 768 CHS-to-RHS T- and X-joints, was performed in this study using the validated FE models. Both CHS-to-RHS T- and X-joints were failed by chord face failure and a combination of chord face and chord side wall failure mode. The nominal resistances predicted from design rules given in European code and CIDECT, using mechanical properties at elevated temperatures, were compared with the resistances of CHS-to-RHS T- and X-joints at elevated temperatures. It is shown that the predictions from design rules given in European code and CIDECT are quite conservative but unreliable. As a result, economical and reliable design equations are proposed in this study for predicting the resistances of the investigated joints.