## 1 Introduction  
To take full advantage of materials in civil infrastructure, many steel-concrete composite structures have been proposed and investigated, such as concrete-encased composite structures [1; 2; 3], concrete-filled steel tubular (CFST) structures [4; 5; 6; 7; 8], steel-concrete composite truss structures [9; 10; 11; 12], and so on. In particular, steel-concrete composite truss structures have been widely used in bridge construction because they combine the advantages of steel truss structures [13; 14; 15; 16; 17] and reinforced concrete structures [18; 19; 20], including structural simplicity, low maintenance, high stiffness, and high bearing capacity. Many engineering examples of such bridge structures have been constructed all over the world. The Arbois Bridge in France was the first composite truss bridge in the world [9]; subsequently, many composite truss bridges (e.g., the Boulonnaia Viaduct [21; 22], the Lully Viaduct [23], Sarutagawa Bridge, Tomoegawa Bridge [24], Shitsumi Ohashi Bridge [25], and the Kinokawa Viaduct [26]) have been constructed. However, in China, research and applications of such bridges are quite a recent development. In 2005, concrete web members were replaced by steel truss webs with circular hollow sections on the Beijing-Zhenzhou highway/railway bridge. It was the first time such structures were applied to bridge constructions in China. At present, the applications of such structures are still in the early stages in China. The composite joint has been regarded as one of the most important parts for steel-concrete composite bridges, and the connection performance of the composite joint plays a vital role in the safety and serviceability of such bridges [27; 28]. Therefore, a few studies have reported on similar structures. [PERSON] et al. [29; 30] presented model tests on composite tubular K-joints to investigate their fatigue performance and ultimate strength. Furthermore, they compared the results obtained from concrete-filled joints and unfilled tubular joints. [PERSON] et al. [31] investigated the failure modes of CFST K-joints in bridges. They found that punching shear failure could typically occur in CFST K-joints. Furthermore, loading tests on CFST truss girders were carried out by [PERSON] et al. [32] to investigate the joint resistance and failure modes of such structures, and a scientific formula was recommended to evaluate the resistance of CFST joints. [PERSON] et al. [33] fabricated a sort of composite joint by inserting diagonal tubular steel into a steel box, and static destructive tests were conducted to identify the force propagation conditions and the ultimate strength of such joints. In addition, these joints were put into use in the earliest composite truss bridge (Kinokawa Viaduct) in Japan. Based on previous research results, [PERSON] et al. [34] presented various junction structures, explained their features, and conducted an experimental study to develop a design method for joint structures. [PERSON] et al. [35] conducted static loading tests on five prestressed concrete hybrid girders with steel web members. Moreover, they found that the serviceability could be enhanced by optimizing the steel web members as well as the connection joints of concrete slabs and tendons. Then, to fully evaluate the structural safety of four newly proposed connection systems, static loading tests were carried out by [PERSON] et al. [36]. The safety and serviceability of the composite truss joint were experimentally and numerically examined by [PERSON] et al. [37]. The results of the test and numerical analysis provided a reference for the first composite truss cable-stayed bridge (Minpu Bridge) in China. [PERSON] et al. [27] carried out an experimental test of the composite joint to study the safety and reliability of the proposed connection. To figure out the performance of three different composite joints, fatigue tests were conducted by [PERSON] et al. [38]. [PERSON] et al.