Numerical analysis and design of cold-formed high strength steel RHS X-joints at elevated temperatures

A numerical program is carried out in this study with an aim to investigate the static performance of cold-formed high strength steel (CFHSS) X-joints with square and rectangular hollow section (SHS and RHS) braces and chords at elevated temperatures. The numerical investigation was performed through the finite element (FE) method using mechanical properties at elevated temperatures. The FE models developed and validated by the authors for identical CFHSS X-joints at room temperature and post-fire conditions were used in this study to perform numerical investigation at elevated temperatures. The SHS and RHS X-joints were subjected to axial compression loads through brace members. The validated FE models were used to perform a comprehensive FE parametric study at 400°C, 500°C, 600°C and 1000°C that comprised in total 756 FE specimens. Using mechanical properties at elevated temperatures, nominal resistances were predicted from design rules given in European code and Comité International pour le Développement et l'Etude de la Construction Tubulaire (CIDECT) and compared with the residual strengths of the investigated X-joints. 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. Generally, predictions from design rules given in European code and CIDECT are quite conservative but scattered and unreliable. Therefore, economical and reliable design equations are proposed in this study for predicting the resistances of cold-formed steel RHS X-joints made of S900 steel grade at elevated temperatures.