## 3 Model Definition and Methodology  
In this section, the modelling strategy adopted for the realization of the F.E. model as well as the material properties and geometric features of the composite connection are described. The composite joint is modeled considering the behavior of the different materials using the finite element software ABAQUS (rel. 2022.0). The model is built aiming to replicate the connection tested in the laboratory by [PERSON] et al. [13] in order to validate the reliability of numerical results. Figure 4 shows the steel plates of the joint. Information regarding the geometry of each member composing the entire connection as well as material properties are obtained from the original technical drawings and are reported in Table 1.  
Figure 4: Steel plates within the ABAQUS model of the composite connection. Figure 3: Maximum loads applied to the joint during laboratory tests. According to the data provided by laboratory tests done by [PERSON] et al., the authors chose C50 concrete class and ST52 steel. In Tables 2 and 3, the characteristics of concrete and steel assumed as input of the software model have been reported, respectively. Well-known constitutive laws, widely adopted in the literature, are employed for the characterization of the nonlinear behavior of the materials. The modified version of the Ramberg-Osgood relationship [26] is used for steel (see Figure 5). The modified version of the Popovics relationship [27] is used for the compressive behavior of the concrete (see Figure 6) and the Belarbi-Hsu relationship [28] is implemented to reproduce the tensile behavior. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Member’s Name** & **No. for the Assembly** & **Number Used in the Model** &
\begin{tabular}{c} **Dimensional Specifications mm** \\ **(Length \(\times\) Width \(\times\) Thickness)** \\ \end{tabular} \\ \hline Main connector plate & 1 & 2 & \(1500\times 1087\times 20\) \\ \hline Perforated vertical connection plate & 2 & 2 & \(757\times 310\times 20\) \\ \hline Horizontal middle connector plate & 3 & 1 & \(1500\times 310\times 20\) \\ \hline Vertical Box section column & 4 & 1 & \(1000\times 350\times 350\) thickness 20 mm \\ \hline Diagonal Box section beam & 5 & 1 & \(1200\times 450\times 350\) thickness 20 mm \\ \hline I-beams & 6 & 1 & \(1190\times 350\times 250\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Geometrical characteristics of the members of the steel truss joint. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Density** & \begin{tabular}{c} **Young’s** \\ **Modulus** \\ \(\rho\) **[kg/m\({}^{3}\)]** \\ \end{tabular} & \begin{tabular}{c} **Poisson’s Ratio** \\ **Ec** \\ **[GPa]** \\ \end{tabular} & \
\begin{tabular}{c} **Poisson’s Ratio** \\ **\(\
u\)** \\ **[-]** \\ \end{tabular} & \
\begin{tabular}{c} **Tensile Strength** \\ **fct [MPa]** \\ \end{tabular} &
\begin{tabular}{c} **Compressive** \\ **Strength** \\ **fc** \\ **[MPa]** \\ \end{tabular} \\ \hline
2500 & 34.5 & 0.15 & 3.19 & 50.6 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Material properties of C50 concrete. Figure 5: Constitutive relation of ST52 steel. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Density** & \begin{tabular}{c} **Young’s** \\ **Modulus** \\ \(\rho\) **[kg/m\({}^{3}\)]** \\ \end{tabular} & \begin{tabular}{c} **Poisson’s** \\ **Modulus** \\ **Es** \\ **[GPa]** \\ \end{tabular} & \begin{tabular}{c} **Poisson’s** \\ **Ratio** \\ **[-]** \\ \end{tabular} & \
\begin{tabular}{c} **Tensile Strength** \\ **ft** \\ **[MPa]** \\ \end{tabular} &
\begin{tabular}{c} **Yielding Stress** \\ **fy** \\ **[MPa]** \\ \end{tabular} \\ \hline
7850 & 210 & 0.30 & 540 & 410 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Material properties of ST52 steel. According to the loading process performed during the laboratory tests, the incremental analysis has been subdivided into 17 steps by assigning 10% of the final design force at each step.