Study on fatigue performance of concrete-filled steel tubular K-joints with internal studs

To study the fatigue performance of concrete-filled steel tubular K-joint (CFST K-joint) with internal studs, fatigue tests on CFST K-joint without internal studs, CFST K-joint with array arrangement internal studs (CFST KSA-joint), and CFST K-joint with circular arrangement internal studs (CFST KSC-joint) were conducted. The finite element models of CFST K-joints with studs were established. The hot-spot stress distributions, fatigue crack growth processes, and fatigue failure modes of the joints mentioned above were comparatively studied. The study showed that the maximum hot-spot stress of the CFST K-and KSA-joints appeared at the crown toe, while that of the CFST KSC-joint occured at the position of 45 degrees. As compared with CFST K-joints, the CFST KSA-and KSC-joint specimens had 34% and 12% reductions in hot-spot stress, respectively. At the location of the internal studs, the hot-spot stress decrease significantly. By reducing the chord deformation and peeling arc length, the internal studs reduce the hot spot stress and increase the fatigue life. The fatigue life of the CFST KSA-joint specimen with array internal studs was 101% higher than that of the CFST K-joint specimen. By applying the circular internal studs, the fatigue life of the CFST KSC-joint specimen was 97% higher than the CFST KSA-joint specimen with array internal studs. The internal studs delay the occurrence of fatigue cracks, and change the propagation path of fatigue cracks along the length of intersecting welds into the direction of steel tube wall thickness, therefore, improving the fatigue life of K-joints.