## Experimental Study on Hysteretic Behavior of the Overlapped K-joints with Concrete Filled in Chord  
**[PERSON] 1,*\(\copyright\)**, [PERSON] 1, [PERSON] 2, [PERSON] 3 and [PERSON] 4\(\copyright\)  
1 College of Civil and Hydraulic Engineering, Ningxia University, Yinchuan 750021, China; [EMAIL_ADDRESS]  
2 School of Civil Engineering, Southeast University, Nanjing 211100, China; [EMAIL_ADDRESS]  
3 Political and Legal Committee of Luoning County, Luoyang 471700, China; [EMAIL_ADDRESS]  
4 School of Civil Engineering, Southwest Jiaotong University, Chengdu 610031, China; [EMAIL_ADDRESS]  
* Correspondence: [EMAIL_ADDRESS]  
Received: 12 March 2019; Accepted: 2 April 2019; Published: 7 April 2019  
**Featured Application: This study is mainly aimed at the difficulty of welding hidden welds of the overlapped K-joints in engineering, and strengthening the grouting concrete in the chord to compensate for this defect. Furthermore, it can provide a technical reference for optimizing construction technology. The K-joints can be popularized and applied in civil engineering, hydraulic engineering, and even the military field in steel structures, bridge structures and airport terminals.**  
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.  
**Keywords:** strengthen overlapped K-joints; quasi-static test; hysteretic behavior; bearing capacity; ductility; finite element analysis