Structural Behavior of Stainless-Clad Bimetallic Steel CHS T- and K-Joints at Elevated Temperatures

This study presents an experimental program on the structural behavior and strength characteristics of stainless-clad (SC) bimetallic circular hollow section (CHS) T- and gapped K-joints at elevated temperatures. The investigated SCCHS joints were fabricated using chord and brace members made of SC bimetallic steel tubes, which were cold-formed from 8 mm thick plates composed of a 2 mm 304L stainless-steel cladding and a 6 mm Q235 carbon steel substrate. Elevated-temperature material tests were first conducted to characterize the tensile and interfacial properties of the SC bimetallic steel utilized in the joint fabrication. Six SCCHS joints, including three T-joints and three K-joints, were subsequently tested using the steady-state test method. Axial tension was applied to the brace member of the T-joints, whereas axial compression was applied to the chord member of the K-joints. Three target temperatures, i.e., 20°C, 450°C, and 650°C, were selected to investigate the behavior of the SCCHS T- and K-joints. The failure modes, load-deformation responses, and joint strengths at ambient and elevated temperatures are reported and analyzed. A comparison between the reduction factors for the joint strengths and those of the SC bimetallic steel at elevated temperatures, as well as the composite reduction factors calculated from the code-recommended values of the stainless and carbon steels, revealed that the degradation of the T-joint strength with increasing temperature was slower than that of the elastic modulus and yield strength of the SC bimetallic steel, whereas the strength reduction of K-joints closely matched the deterioration trends of the yield strength. Finally, the experimental joint strengths were used to assess the applicability of current design provisions for carbon steel joints (ANSI/AISC 360-22, EN 1993-1-8, and GB 51249-2017) to the SCCHS T- and K-joints at elevated temperatures. The results demonstrate that using the strength equations in these provisions with the elevated-temperature yield strength of the SC bimetallic steel incorporated leads to conservative strength predictions for the SCCHS T- and K-joints.