## 5 Conclusions and Future Work  
The overlapped K-joints were strengthened by grouting concrete in the chord. Six K-joints including the joint with a circular chord and brace and the joint with a square chord and circular brace, were designed to carry out a quasi-static test and numerical simulation. The failure modes, hysteretic curves, skeleton curves, ductility coefficient, and energy dissipation ratio of the joints were obtained. The hysteretic performance, ductility, and energy dissipation capacity of the joints were evaluated quantitatively. The following conclusions were drawn from this research:  
(1) Under the cyclic loading, the major failure mode of the overlapped K-joints without strengthening was plastic failure of the chord; while the failure mode of the joints strengthened with concrete in the chord was tension crack failure of the toe weld of the brace. The compressive brace had slight buckling, which could dissipate some energy. The failure modes of strengthened K-joints with ordinary concrete and fly ash concrete were basically the same.  
(2) Compared with the unstrengthened K-joints, the ductility and energy dissipation ratio of the strengthened K-joints were reduced to a certain extent, however the strengthening measure with concrete significantly enhanced the bearing capacity of the K-joints, delayed the plastic development of the chord and improved the mechanical properties of the K-joints.  
(3) The hysteretic behavior of the K-joints with the circular chord and brace was slightly better than that of the K-joints with a square chord and circular brace. The hysteretic behavior of the K-joints strengthened with fly ash concrete was better than that of the K-joints strengthened with ordinary concrete. The ductility and energy dissipation ratio of the K-joints strengthened with fly ash concrete were superior to that of the K-joints strengthened with ordinary concrete. Based on the above conclusions, it was feasible to use fly ash instead of ordinary concrete to strengthen the K-joints partly.  
(4) The numerical simulation of six K-joints specimens under reciprocating loading was carried out by ANSYS, and the results were in good agreement with the test results, which showed that the numerical simulation analysis model and method established in this paper was feasible.  
In the current research, the effect of corrosion was not considered. It is well known that chloride diffusion plays an import role in corrosion in both steel and concrete structures [49; 50; 51]. One area of our future work is to consider corrosion in the study of hysteretic behavior of the overlapped K-joints with the concrete filled in chord. Using piezoceramic transducers, scholars have conducted research on damage detection of concrete structures [52; 53], especially in concrete filled tubular structures [54; 55; 56]. In future work, the authors will explore the health monitoring of the K-joints with concrete filled in chord by using piezoceramic transducers.  
[PERSON] accepted the original idea and designed the experiment. [PERSON] analyzed the data. [PERSON], [PERSON], and [PERSON] wrote the paper. [PERSON] revised the paper.  
This research was supported in part by the National Natural Science Foundation of China (Nos. 51468054) and in part by the Funding Project of First-Class Discipline Construction of Universities in Ningxia (Domestic First-Class Discipline Construction) under Grant NXYLX2017A03.  
The authors declare no conflicts of interest.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Specimens** & **Test Result (kN)** & **FEA Result (kN)** & **Error** \\ \hline K-CC & 232.38 & 228.65 & 1.61\% \\ K-CCC1 & 398.72 & 405.93 & 1.82\% \\ K-CCC2 & 389.36 & 424.73 & 9.08\% \\ K-RC & 235.36 & 230.28 & 2.16\% \\ K-RCC1 & 408.69 & 412.27 & 0.88\% \\ K-RCC2 & 404.37 & 413.48 & 2.25\% \\ \hline \hline \end{tabular}
\end{table}
Table 5: The bearing capacity comparison of joints.