Strength performance of <scp>CFDST</scp> K‐joints after and before elevated temperatures: Experimental and numerical investigation

Abstract Punching shear and stress concentration at the weld point at brace‐to‐chord intersection in concrete‐filled double steel tubular (CFDST) joints subjected to mechanical and thermal loads play critical importance in the overall capacity of the joint. However, the existing design codes and regulations fail to address such issues. The present study proposes an alternative but practical approach of casting self‐consolidating concrete (SCC) in the space between inner and outer chords as well as inside the circular hollow section (CHS) brace members. In this regard, laboratory and finite element analyses were carried out to assess the load‐bearing capacity of CFDST K‐joints under static and thermal loads. First, a number of three laboratory joints with SCC were fabricated and exposed to temperatures of 20, 500, and 750°C. The joints were then subjected to static loading in order to evaluate the effect of various temperatures on the ultimate strength capacity of the joints. After successful verification of the proposed numerical model, a set of 96 finite element models was created in ABAQUS so as to investigate the influence of mechanical and geometrical parameters on the capacity of K‐joints in terms of load‐deformation curves, failure pattern, ductility, and buckling mode at different temperatures. Based on the obtained experimental and numerical results, buckling is transferred from the main chord to the brace element for CFDST joints at ambient temperature (20°C). This finding is of high significance because it indicates a more favorable load path and enhanced ductility. As such, when buckling shifts to the brace element, the main chord remains elastic and continues to carry the majority of the axial load, while the brace deforms more gradually. Such redistribution spreads strains over a larger portion of the joint, reducing peak stresses at the weld. It also allows larger rotations of the brace before loss of load‐carrying capacity, which is characteristic of a ductile behavior. Furthermore, additional energy‐absorbing mechanisms are engaged that can dissipate load over a longer deformation path. In another finding, the load capacity of CFDST joints is approximately twice that of hollow double steel tube (DST) companions, indicating the significant contribution of confined concrete. This finding can be incorporated into future design guidelines to achieve higher capacity and better ductility of steel‐concrete CFDST joints. Although higher temperatures decrease the ultimate strength, the core concrete not only prevents the loss of joint strength at 500°C but also increases its capacity by 20%.