## 4 Conclusions  
The buckling and post-buckling behavior of the CHS T-joints under monotonic compressive loading was evaluated using more than 180 joints with various geometric properties. The joints were numerically modeled utilizing the finite element method. The brace local buckling is the most common buckling mode of the CHS T-joints in the considered range of parameters and results in a sudden significant reduction in the load carrying of the joint. Investigation of the geometric properties of the non-retrofitted CHS T-joint showed that mode in which buckling occurs most often is the brace local buckling. Generally, the chord ovalization buckling mode has the lowest critical load. For the joints with \(r/t<10\), the critical modes are the brace overall buckling or the chord ovalization, with the brace overall buckling showing more buckling load. By increasing the \(r/t\) ratio, the brace local buckling is observed such that for the joints with \(r/t>30\), the brace local buckling is the most critical mode. For the ratio of the chord's \(R/T\), if this ratio is less than 10, no chord ovalization occurs. But for a ratio of more than 15, no brace overall buckling is observed. The brace's \(l/t\) ratio can separate the brace local buckling from the other buckling modes for values over than 600 or less than 100. The brace to the chord length ratio \(l/L\) shows no clear sign of separation between the buckling modes or their critical load. But for the chord to the brace thickness ratio \(T/t\), values greater than 3 will lead to the brace local buckling. However, the values less than 1.5 will lead to the two other buckling modes. The brace's \(l/r\) ratio cannot clearly divide the buckling modes.  
The retrofit of CHS T-joints by the use of FRP composite patches may improve their load-bearing capacity. The results show good performance of the composite patches which can be concluded as follows:  
1. In the chord ovalization buckling mode, the use of FRP wrapping may improve the load bearing capacity of the retrofitted joints up to 40% in comparison with non-retrofitted ones.
2. Effectiveness of the FRP wrapping in the chord ovalization buckling mode is considerable when at least four plies with the distribution of (0/0)s and (0/90)s are utilized.
3. Investigation of the FRP wrapping performance in various buckling modes shows that this method may totally prevent the brace local buckling and modify destructive effects of the chord ovalization. This method has poor performance in the brace overall buckling.
4. The matrix tension failure is the most common composite failure in the chord ovalization and the brace overall buckling, but the matrix compressive failure is most possible in the brace local buckling.
5. The FRP wrapping can improve the load bearing capacity of the CHS T-joints; this method can increase the ultimate load-bearing capacity about 17% and 50% respectively in the overall and local brace buckling.  
Formal analysis, A.Y.; Funding acquisition, M.R.; Investigation, A.Y. and M.A.; Methodology, M.A.; Project administration, B.S.; Resources, M.R.; Software, A.Y.; Supervision, B.S.; Validation, M.A.; Visualization, B.S.; Writing--original draft, M.A.; Writing--review & editing, M.R. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
The authors declare no conflict of interest.