## 1 Introduction  
Truss structures are popularly used in various engineering fields [1; 2; 3; 4], and among them, steel tubular trusses have been widely applied in large engineering structures, such as stadiums, bridges, concert halls, and offshore platforms, because of their architecturally attractive shapes and favorable structural properties [5]. Many investigations have been carried out related to steel tubular truss structures [6; 7; 8]. In addition, a hybrid structural analysis and procedure were developed by [PERSON] [9; 10] to enhance the load rating of railway truss bridges. The common steel tubular truss types are Warren trusses, Pratt truss, Fink truss, and Vierendeel truss, among which Warren trusses generally provide the most economical solution due to their reasonable mechanical performance and effective structural layout [11]. The joint form of Warren truss is usually K-joint and KK-joint, however,for these two types of joints, the overlapped condition arises when one brace (overlapping brace) intersects the other brace (overlapped brace), requiring a weld between the braces. In addition, part of the overlapped brace is hidden within the overlapping brace, and the hidden toe of the overlapped brace may or may not be welded to the chord [12]. Nevertheless, the question whether hidden weld of overlapped joints (K-joint and KK-joint) welded or unwelded is not clearly mentioned in the current Chinese Standard for Design of Steel Structures (GB 50017-2017) [13]. The joints with hidden welds welded may have an increased strength, but with reduction in joint ductility and hysteretic performance [12; 14; 15; 16; 17]. It should be noted that the aforementioned studies mainly focus on single joint, while the research on the performance of overall truss is very limited on joints with and without hidden welds. On other hand, steel tubular trusses made of circular hollow section (CHS) members usually fail due to surface plasticity or punching shear of the chord members due to low stiffness of the chord members. One of the commonly used reinforcing methods is to fill the chord members with grout or concrete. Concrete-filled steel tubular (CFST) members, as compared to their pure steel or concrete counterparts, have improved stiffness, load bearing capacity, buckling resistance [18; 19; 20; 21], and even improved impact resistance [22; 23]. In addition, structural health monitoring of CFST members [24; 25; 26; 27] and assemblies [28; 29] have attracted much attention. For CHS joints, extensive research works have been carried out on the static performance [30; 31], hysteretic performance [32; 33; 34], impact performance [35; 36], fatigue performance [37; 38], and mechanical properties at elevated temperature [39; 40; 41], among others. However, compared with CHS joints, little investigation has been carried out on the performance of CFST joints. [PERSON] et al. [42] analyzed the different failure modes of CFST joints and compared them to CHS joints, indicating the increased resistance attained by adding concrete in the chords of tubular joints. [PERSON] et al. [43], [PERSON] et al. [44], and [PERSON] et al. [45; 46] investigated the performance of CFST joints under static loading. An experimental investigation was conducted by [PERSON] et al. [47] on hysteretic behavior of tubular N-joints. [PERSON] et al. [48] presented an experimental and numerical investigation on double-skin circular CHS tubular X-joints under axial compression. The failure mechanism was analyzed and new design equations were proposed. [PERSON] et al. [49] 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 experiment results. Moreover, [PERSON] et al. [50], [PERSON] et al. [51], and [PERSON] et al. [52] carried out studies on the fatigue behavior of CFST joints. The CFST structures have also been applied to arch trusses in bridges, as well as the truss girders in steel structures [53; 54; 55].