## 7 Conclusions  
In this paper, the composite joint of the Minpu bridge is selected as a case study for the assessment of the structural behavior of strengthened composite joints. An ABAQUS model of the truss connection has been realized and a comparison with the experimental test of a previous study has been conducted successfully. The validation of the numerical model and its reliability was demonstrated.  
The authors then performed a sensitivity analysis in order to assess the impact of the concrete grade on the structural response of the composite joint. A predictable influence of this parameter with slight differences on the concrete stresses but stronger variations on the concentration of stresses in steel for the different concrete grades can be observed.  
Finally, a retrofitting approach with post-tensioned metal straps (PTMSs) is proposed on a specimen modeled using C25/30 concrete.  
The cracking load of the member realized with C25/30 concrete corresponds to 35% of the load applied in the laboratory test.  
Different layouts of straps were analyzed. The number of the straps as well as their position was studied.  
The introduction of PTMSs reduces both the principal tensile stress and the principal compressive stress in concrete caused by the applied external load.  
The principal tensile stress can be reduced up to 17%, and the principal compressive stress up to 14%.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Number of Used**} & **Maximum** & **Minimum** & **Maximum** \\  & **Tensile Stress** & **Compressive Stress** & **Tensile Stress** \\
**Belts** & **in Concrete \(\sigma_{11}\)** & **in Concrete \(\sigma_{33}\)** & **in PTMS** \\  & **[MPa]** & **[MPa]** & **[MPa]** \\ \hline
0 & \(1.32\) & \(-11.4\) & n.a. \\
4 & \(1.29\) & \(-10.9\) & 156 \\
4+1 & \(1.23\) & \(-9.98\) & 155 \\
4+2 & \(1.22\) & \(-9.95\) & 148 \\
7 & \(1.23\) & \(-9.83\) & 132 \\
11 & \(1.10\) & \(-9.77\) & 112 \\ \hline \hline \end{tabular}
\end{table}
Table 9: Maximum stresses in concrete and in the belts at final load in function of the number of PTMSs.  
Two PTMS layouts show better cost/benefit characteristics: the eleven-strap solution provides the best absolute results, whereas the four-plus-one solution evidences the best gain per unit weight of PTMS introduced.  
A smaller number of PTMSs leads to higher stresses at cracking (155 MPa for four-plus-one vs. 112 MPa for eleven). Therefore, the performance after cracking is better for the eleven PTMS configuration as they remain in elastic field for higher applied loads.  
In conclusion, as a result of this research, post-tensioned metal straps can be assumed as a cheap and effective retrofitting approach to increase the cracking load of composite joints, control the crack formation and reduce the opening of the cracks.  
Better confinement and higher stiffness of the concrete strengthening reduce the stress concentration in the steel elements of the joint, reducing the dangerous effect of fatigue phenomena.  
Further studies may analyze the effect of construction and retrofitting phases to assess the real load effectively transferred to concrete encasing and PTMSs when they are realized after the opening to traffic of the bridge as a retrofitting technique.  
Conceptualization, A.H.; methodology, A.H. and G.B.; software, A.H.; validation, R.C. and G.B.; formal analysis A.H.; investigation, A.H.; resources, G.B.; data curation, A.H.; writing--original draft preparation, R.C. and A.H.; writing--review and editing, R.C. and G.B.; visualization, A.H.; supervision R.C.; project administration, A.H. and G.B. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The raw data supporting the conclusions of this article will be made available by the authors on request.  
The authors declare no conflicts of interest.