Design of cold-formed austenitic stainless steel CHS T-joints under fire condition

This paper presents the investigation on the fire resistance of cold-formed austenitic stainless steel circular hollow section (CHS) T-joints with braces subjected to axial compression. The austenitic stainless steels are types of EN 1.4301 (AISI 304) and EN 1.4571 (AISI 316Ti having small amount of titanium). A finite element model (FE) was first developed and validated against the existing test results of austenitic stainless steel (EN 1.4301) CHS T-joints at ambient temperature in terms of axial resistance, failure mode and load-deformation response. An extensive parametric study was then carried out on the austenitic stainless steel CHS T-joints. The key parameters include the ratio of brace diameter-to-chord diameter (β), the ratio of brace thickness-to-chord thickness (τ) and the ratio of chord diameter-to-chord thickness (2γ) as well as different high temperature (under fire) levels. Their effects on the load resistance of the T-joints were investigated. Similar to those at ambient temperature condition, at each high temperature level, the higher β values yield higher ultimate loads while the lower 2γ values yield higher ultimate loads, and the parameter τ has little effect on the ultimate loads. The reduction factors of Young's modulus in material properties unconservatively predicted the reduction factors of the T-joint strengths, while those of yield stress predicted conservatively. The applicability of the current design equations for predicting the strength of the T-joints was assessed. The CIDECT and the European design equations for ambient temperature condition are recommended for the strength design of the CHS T-joints fabricated by austenitic stainless steel under fire up to 950℃. These predictions are generally conservative and reliable.