## 4 Model Validation  
In this section, the numerical results are presented and the accuracy of the F.E. model is be discussed. The stresses in concrete and steel at the final load step are pictured in Figure 8: (a) shows the maximum tensile principal stress (\(\sigma_{11}\)) in concrete, whereas (b) shows the minimum compressive principal stress (\(\sigma_{33}\)) in concrete, (c) shows maximum tensile principal stress (\(\sigma_{11}\)) in steel and (d) shows the minimum compressive principal stress (\(\sigma_{33}\)) in concrete.  
The comparison between numerical output and the stresses measured in the laboratory is drawn in correspondence with the position of the strain gauges placed in the specimen.  
As shown in Figure 7, strain gauges from S4 to S13 are placed on steel plates along the axis of the longitudinal element, whereas gauges from S23 to S29 are placed along the diagonal member's axis.  
Figure 6: Compressive constitutive relation of C50 concrete.  
Horizontal gauges are compressed with stresses ranging between 30 and 55 MPa, whereas diagonal ones are tensed in the upper part (S23 to S26) and almost unloaded in the lower part (S27 to S29) as shown in Table 4.  
The same process is repeated to determine the stress state within the concrete encasing the steel plates. Nine strain gauges (from R3 to R11, see Figure 7) placed on horizontal rebars within concrete are considered for the comparison in this study.  
The resulting stresses from the laboratory tests, [PERSON]'s ANSYS model and the ABAQUS model developed by the authors are presented in Tables 4 and 5 for steel and concrete rebars of the composite joint, respectively.  
Figure 8: Results of numerical simulation of laboratory test in term of stresses [kN/m2].  
Figure 7: Strain gauges’ positions on steel surfaces (S) and concrete reinforcement bars (R).  
A good level of match between the experimental results and the adopted numerical model as well as fair agreement with the numerical results obtained in [13] can be appreciated from the comparison; nevertheless, the better performance of the ABAQUS model is clear.  
For the steel plates, which are subjected to stresses ranging from 0 to 60 MPa, the average difference between the laboratory results and the finite element models drops from 6.8 MPa ([PERSON] data) to 2.6 MPa in the present research. For concrete rebars, which are subjected to stresses ranging from \(-6\) to \(-11\) MPa, the average difference between the laboratory results and the finite element models drops from 1.2 MPa ([PERSON] data) to 0.6 MPa in the present research.  
In conclusion, given that the behavior and the numerical results of the built model are in good agreement with the results obtained from the previous research, it can be affirmed that the 3D finite elements model proposed by the authors is validated and can be adopted for further investigations.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Strain Gauge**} & \multicolumn{5}{c}{**Stress (MPa)**} \\ \cline{2-6}  & \multirow{2}{*}{**Experimental**} & **[PERSON] et al. [13]** & **[PERSON] et al.** & \multirow{2}{*}{**\(\mathbf{|ANSYS--Exp|}\)**} & \multirow{2}{*}{**\(\mathbf{|ABAQUS--Exp|}\)**} \\ \cline{1-1} \cline{5-5} \cline{7-7}