Due to the insufficient radial stiffness of the steel tube, the cracking of the weld and the plastic deformation of the string often occur under the cyclic loading of the hollow section pipe joint. In order to avoid such a failure, the overlapped K-joints were strengthened by pouring different concrete into the chords. Furthermore, to explore the detailed effect of filling different concrete in a chord on the hysteretic behavior of the overlapped K-joints, six full-scale specimens were fabricated by two forms, which included the circular chord and braces, the square chord and circular braces, and the low cyclic loading tests, which were carried out. The failure modes, hysteretic curves and skeleton curves of the joints were obtained, and the bearing capacity, ductility and energy dissipation of the joints were evaluated quantitatively. The results showed that plastic failure occurs on the surface of the chord of the joints without filling concrete, while the failure mode of the joints filled with concrete in the chords was the tensile failure of the chords at the weld of the brace toe, and the compressive braces had a certain buckling deformation; The strengthening measures of concrete filled with chord can effectively improve the mechanical properties of the K-joints, the delay of the plastic deformation of the chord, and improve the bearing capacity of the K-joints. Contrarily, the ductility coefficient and the energy dissipation ratio of K-joints decreased with the concrete filled in the chord. The hysteretic behavior of the K-joints with a circular chord and brace was slightly better than that of the K-joints with a square chord and circular brace, and the hysteretic behavior of the K-joints strengthened with fly ash concrete, which was better than that of the K-joints strengthened with ordinary concrete. The results of ANSYS (a large general finite element analysis software developed by ANSYS Company in the United States) analysis agreed well with the experimental results.

For truss structures, the question of whether to weld hidden welds or not has been controversial. In the actual construction process of truss structures, the members are usually spot welded in place on the assembly platform, and then welded as a whole, while the hidden welds of the truss are not welded, especially for small pipe diameter trusses. Furthermore, in this study, under hidden weld unwelded conditions, two kinds of reinforcing method (adding a half outer sleeve on each joint and filling concrete into the chord members) are adopted to achieve the purpose of strengthening the truss. Therefore, this paper presents an experimental study on the static behavior of four types of Warren tubular trusses made of CHS members. These four types are (1) T-HW: The truss with hidden welds welded; (2) T-HN: The truss with hidden welds unwelded; (3) TS-AS: The truss strengthened on the basis of T-HN by adding a half outer sleeve on each joint; (4) TS-FC: The truss strengthened on the basis of T-HN by filling concrete into the top and bottom chord members. The mechanical behavior, failure mode, bearing capacity, and load-displacement of all specimens were investigated. The surface plasticity of the bottom chord member, the weld fracture around tubular joints at the bottom chord member, and the bending deformation of the bottom chord member were observed in the tests. Compared with the T-HW specimen, the load carrying capacity of the T-HN specimen decreased by 18%. On the other hand, the T-HN specimen has better deformability than the T-HW specimen. The reinforcing method of adding a half outer sleeve on each joint and filling concrete into the chord members can effectively improve the load carrying capacity and stiffness of the truss, thus reducing the overall deformation of the truss, but the reinforcing method of filling concrete into the chord members is more efficient. of 22trusses under static loading. The failure modes, load carrying capacity, overall deflection, and strain intensity of all test specimens were discussed. Finite element analysis (FEA) software was used for numerical modelling of Warren CHS tubular trusses and the results obtained by numerical analysis were compared with those of the experimental results. Experimental Study Test SpecimensA total of four types of Warren CHS tubular trusses, including the truss with hidden welding welded (T-HW), the truss with hidden welding unwelded (T-HN), the truss strengthened on the basis of T-HN by adding a half outer sleeve on each joint (TS-AS), and the truss strengthened on the basis of T-HN by filling concrete in top and bottom chord members (TS-FC), were designed according to the design guidelines given in the Chinese Standard for Design of Steel Structures (GB50017-2017) [13]. The truss configuration of all specimens is Warren truss with symmetric geometry, loading application, and boundary conditions.The nominal dimensions of CHS members, including chord members, brace members, and lateral bracings of all types of multiplanar tubular trusses, are identical ...

The lifetime of hollow section tubular joints frequently can be shortened owing to the occurrence of the welded cracks and the plastic deformation of chords under the cyclic loading, because of the deficient radial bearing capacity of the steel tube. To avoid such failures, this paper proposes a novel method to strengthen the chord with double plates at the intersection of the chord and braces. To further investigate the efficiency of this strengthening method on hysteretic performance and energy depletion ability of the overlapped K-joints with hollow sections, two unreinforced K-joints and two reinforced K-joints were fabricated. By loading on the braces with collaborative cyclic loading, the joints failure modes, hysteresis curve, and skeleton curve were obtained. The bearing capacity, ductility, and energy depletion of the joints were assessed and the restoring force model of joints was proposed. The results show that the failure mode of the unreinforced joint is the plastic failure of the surface of the chord. For the K-RC1 (double-plate reinforced square hollow section tubular K-joints), cracks appeared at the junction weld between the through brace and the overlapped brace. However, cracks extended along the weld at the intersection of the chord and the through brace for K-CC1 (double-plate reinforced circular hollow section tubular K-joints). There is no obvious deformation on the chord surface of reinforced joints. Experimental results reveal that the mechanical properties of the joints can be improved effectively by such reinforcement measures and that the plastic deformation of the chord can also be restrained. Meanwhile, the reinforcement measures demonstrate the ability to avoid the risk of large stress concentration of the chord in the area where the braces and chords are intersected. The bearing capacity of the joint was increased; however, the ductility of the joint was weakened.