Abstract：To study the fatigue performance of the concrete-filled steel tubular K-joint with array-aligned internal studs (CFSK KS-joint), fatigue test and finite element study on full-scale model of CFSK KS-joint and concrete-filled steel tubular K-joint (CFST K-joint) without internal studs were conducted. The hot spot stress distribution, fatigue crack propogation and fatigue failure mode between CFST K-joint and CFST KS-joint were compared. The influences of the internal stud layout area and arrangement type on hot spot stress were studied. The S-N curves based on hot spot stress were used to discuss the fatigue life judgment criterion and fatigue life assessment method. It is found that the hot spot stress of CFST KS-joint should be calculated by the 3-point extrapolation method, and the maximum hot spot stress is located at the exterior of crown point. The fatigue crack evolution of CFST KS-joints can be divided into three stages, namely fatigue crack initiation, propagation and failure-CFST KS-joint has a higher number of loading cycles during the crack growth stage and a lower fatigue crack growth rate during the crack failure stage. Due to the use of internal studs, the stiffness near the saddle point of the weld is higher than that of the tube. Hence, after the fatigue crack propagates to the saddle point, the fatigue crack does not continue to propagate along the direction of the intersecting line, but develops at a certain angle to the tube axis. It is recommended that the number of loading cycles corresponding to 15% of the peak strain at the crown point can be taken as the fatigue life judgment criterion. Based on the hot spot stress, the S-N curve for the fatigue design of CFST K-joint is proposed, and it is predicted that under the same hot-spot stress amplitude, the use of studs can increase the fatigue life of CFST K-joint by 45%. By reducing the axial and radial spacing of studs in the side branch and increasing the axial arrangement range, the fatigue performance of the joint can be improved. The layout of studs in the off-side branch and the size of studs have little effect on the fatigue performance. The axial arrangement range of the stud has an impact on the stress state of the studs themself to some degree. Based on the static and fatigue properties of CFST KS-joint, the reasonable arrangement of internal studs in the tube is proposed.
