## 1 Introduction  
Steel tube truss structure has become one of the most common building structures in large public buildings at home and abroad due to its large torsional stiffness, excellent seismic performance [1; 2; 3; 4; 5; 6], easy installation and easily meets the requirements of modern buildings for the sense of view. The joints do not need auxiliary components such as joint plates, which is beneficial to the improvement of the overall performance of the structure. However, due to the intersection of the members in the truss structure in the form of intersecting lines at the joints, the shape of the end section of the members was complex, and the joints were the major energy-dissipating parts under seismic loads. The seismic performance of the joints and the corresponding strengthening measures could not be ignored. As one of the most widely used steel tubular truss joints in the field of building structure, the overlapped K-joints are generated in the actual construction process of the steel tubular truss when two braces (overlap brace and through brace) intersect on the surface of the chord. In overlapped K-joints, part of the through brace is hidden within the overlap brace, and the hidden toe of the through brace may or may not be welded to the chord [7]. Additionally, the bearing capacity of joints with hidden welds that are welded is higher than the joints with hidden welds unwelded, but with a reduction in ductility and hysteretic behavior [8; 9; 10; 11]. What's more, due to the insufficient radial stiffness of the chord, the steel truss composed of circular hollow section (CHS) members often undergoes plastic deformation or punching of the chord surface. Therefore, it is imperative to study the reinforcement measures for the overlapped K-joints. In China, CFST (Concrete Filled Steel Tube) truss girder was first applied in 1997 in the deck of the Zidong cable-stayed bridge in Nanhai City (Guangdong Province, China) [12]. Usually, only concrete is grouted into the chords of CFST truss structures, while no concrete was added to the braces [13]. Compared with the unstrengthened steel tube truss structure or concrete counter parts, the mechanical properties of steel tubular members grouting concrete have been effectively improved, such as the stiffness, the bearing capacity, buckling resistance [14; 15; 16; 17; 18; 19], and impact resistance [20; 21; 22]. Welded steel tubular joints are usually used to connect the brace and chord of the Circular Hollow Section (CHS) structures and CFST structures, which includes K-joints, KK-joints, KT-joints, T-joints, N-joints, Y-joints, X-joints and so on. The International Institute of Welding (IIW) [23] proposed an ultimate strength formula for CHS overlapped K-joints considering the local yielding failure of the braces. [PERSON] et al. [24] researched the static behavior of overlapped CHS K-joints and the impact of the hidden weld. The results showed that the influence of hidden welds on the static strength of overlapped K-joints was not obvious, while it may have influenced the failure mode. [PERSON] et al. [25] carried out static and fatigue testing on K-joints in truss girders strengthened with grouting concrete in the chord, and found that the fatigue strength of CFST joints was higher than that of CHS joints. [PERSON] et al. [26] investigated the behavior of CFST trusses, obtaining the different failure modes of CFST trusses, and concluded that the joints in tensile showed weld failure and chord punching shear failure. Additionally, [PERSON] et al. [27] presented the experimental and numerical research on double-skin circular CHS tubular X-joints under axial compression, and new design equations were proposed according to the failure mechanism on different joints. [PERSON] et al. [28] studied the static behavior of concrete filled double skin steel tubular (CFDST) chord and CHS brace composite K-joints, and the effects of important parameters on joint strength were discussed based on experimental results. [PERSON] et al.