Fatigue Performance Test and Finite-Element Analysis of Concrete-Filled Steel-Tube K-Joints

To avoid fatigue cracking of K-joints in concrete-filled steel-tube (CFST) truss arch bridges, fatigue performance tests and finite-element analysis were conducted to investigate the hot spot stress distribution, fatigue performance evolution, and final fatigue failure pattern of the CFST K-joints. The results of a CFST K-joint were compared with those of a steel tube (ST) K-joint, to reveal the influence of concrete filled in a chord on the fatigue performance of the CFST K-joint. The results show that the hot spot stress of a CFST K-joint can be obtained by quadratic extrapolation. The stress concentration factor (SCF) on the chord side of the intersecting weld of CFST K-joints is larger than those on the brace side, and the maximum SCF is located at the crown point on the chord side of the intersecting weld connecting the tensioned brace to the chord. Because the constraint action of the core concrete improves the radial stiffness of the chord, making the section radial deformation of the CFST K-joint much smaller than that of the ST K-joint, the in-chord concrete could relieve the SCF of CFST K-joints by 19.5%-49.7%. Since the existing SCF calculation methods of ST K-joints and CFST K-joints do not consider the effect of in-chord concrete or the coupling effect of geometric parameters, they are not applicable to the SCF calculation of CFST K-joints. The fatigue crack growth process of a CFST K-joint can be divided into four stages: crack initiation, slow propagation, rapid propagation, and sharp fracture. The fatigue crack propagation speed of the CFST K-joint is obviously slower than that of the ST K-joint due to the effect of the in-chord concrete. The number of cycles corresponding to the 15% change of the peak dynamic strain is recommended as the fatigue life of the CFST K-joint. An S-N curve for fatigue life evaluation of CFST K-joints is proposed. Comparing with the fatigue test in this paper and existing fatigue test of other CFST joints, the proposed S-N curve for CFST K-joints has high precision and safety.