## 4 Discussion  
The thickness modification of the lateral walls in the vicinity of the 20 mm value causes an increase of the maximum stress in critical area A. The increase is significant if the wall thickness is smaller than 20 mm, and less significant if the wall thickness is greater than 20 mm. The modification of the lateral wall thickness has a minor, even insignificant effect on the maximum stress in critical areas B and C. The variations, expressed as percentages compared to the reference model, are more evident than those in absolute values, as shown in Figure 17.  
Adopting a 20 mm lateral wall thickness is a good decision. Considering only the maximum values in the critical areas, a 21 mm lateral wall thickness may represent an alternative but would cause an important increase in the volume of metal required for the manufacturing of the frame. Models with lateral walls with horizontal ribs spaced at 75 mm, as shown in Figure 18, reveal a small but interesting (as a percentage) variation of the maximum stress values in the critical areas. An important decrease (by more than 20%) of the maximum stress in critical area C of the model of small wall thickness (\(g=18\) mm) could be noticed, most probably due to the decrease in stiffness of the frame in the respective area, and a considerable decrease (by almost 9%) of the maximum stress in critical area A of the model with large wall thickness, \(g=22\) mm, represents a good evolution in relation to model B4 (Table 2), which is un-ribbed and of the same lateral wall thickness. In the model of the same lateral wall thickness as that of the reference model, the horizontal ribbing spaced at 75 mm (model B8) determines an increase of the maximum stress by more than 5% in critical area A, its decrease by \(\approx\)1.77% in critical area B and increase by 3.76% in critical area C. As a conclusion, the model with horizontal ribs spaced at 75 mm is not recommended, as the maximum stress in the critical areas is generally greater than in the reference model. Figure 17: Deviation of maximum stresses in critical areas for ribless frames and different thicknesses of the side walls (model code B0–B4), reported at the reference model B0. Figure 18: Deviation of maximum stresses in critical areas for frames with horizontal ribs spaced at 75 mm and different thicknesses of the side walls (model code B5–B9), reported at the reference model B0. In frame models B10-B14 with high density vertical ribs spaced at 50 mm, the increase of lateral wall thickness determines an alternating evolution of the maximum stress in critical area A, however with a reduced variation ranging from 11.2 to 11.7 MPa, that is in the interval (\(-\)1 to +3)% in relation to the corresponding value determined for reference model B0, Figure 19.  
The maximum stress in critical area B decreases continuously with the increase of the lateral wall thickness, although not considerably, with the tendency of stopping slightly under 3% in relation to the corresponding value of the reference model. The maximum stress in critical area C, although relatively small as absolute values, vary slightly within a narrow range (4.06 to 4.25) MPa, that is (\(-\)1.5 to +2.98)% around the corresponding value of the reference model, but alternatingly with the increase in lateral wall thickness. Model B12, of 20 mm lateral wall thickness and vertical ribs spaced at 50 mm is the only one where the maximum stress in critical areas A and B is simultaneously smaller than in the reference model, what renders this solution worth considering.