Cold-formed high strength steel CHS-to-RHS T- and X-joints: Performance and design after fire exposures

The post-fire structural performance of cold-formed high strength steel (CFHSS) T- and X-joints with circular hollow section (CHS) braces and square and rectangular hollow section (SHS and RHS) chords is numerically investigated in this study. The tubular members had the nominal 0.2% proof stress of 900 MPa. The CHS-to-RHS T- and X-joints were subjected to compression loads through brace members. The nominal values of peak fire temperatures (ψ) were 300°C, 550°C, 750°C and 900°C. Tests carried out by the authors investigating the post-fire static response of cold-formed S900 steel grade CHS-to-RHS T- and X-joints were formed the basis of the numerical investigation carried out in this study. The test results reported by the authors were used to develop accurate finite element (FE) models. The validated FE models were used to perform extensive numerical parametric studies comprising 768 FE specimens. The validity ranges of critical geometric parameters were extended beyond current limits mentioned in international codes and guides. The residual strengths of test and FE specimens were compared with the nominal resistances predicted from design equations given in Eurocode 3 and Comité International pour le Développement et l’ Etude de la Construction Tubulaire (CIDECT) Design Guide 3 using the measured post-fire residual material properties. Generally, design rules given in Eurocode 3 and CIDECT are shown to be quite conservative with very dispersed and unreliable predictions. As a result, accurate and reliable design rules are proposed in this study.