## 4 Discussion
The vertical ribbing of the lateral walls (models B15-B19) had the positive effect of reducing the maximum stress in critical area B (for example by \(\approx\)1.43% in model B17 with \(g=20\) mm and vertical ribs spaced at 75 mm), and the negative effect of the undesired increase of maximum stress in critical area A (for example by more than 7% in the same model B17), as shown in Figure 20. The (nearly linear) decrease of stress in critical areas A and B with the increase of lateral wall thickness is evident. The same tendency of the maximum stress to increase in critical area A and slightly decrease in critical area B is identified in the models with vertical ribs spaced at 100 mm (models B20-B24), Figure 21. In model B22 the maximum stress in critical areas A and B  
Figure 19: Deviation of maximum stresses in critical areas for frames with vertical ribs spaced at 50 mm and different thicknesses of the side walls (model code B10–B14), reported at the reference model B0. Figure 20: Deviation of maximum stresses in critical areas for frames with vertical ribs spaced at 75 mm and different thicknesses of the side walls (model code B15–B19), reported at the reference model B0. (the most important ones) are simultaneously smaller than the corresponding ones in the frame used for reference, but the gain is, however, insignificant. Rib density has an evident influence on the maximum stress in the critical areas, it is significantly more visible if expressed in percentages, as shown in Figure 22. In relation to the reference model, an increase of more than 7% of the maximum stress in critical area A is noticed in the model with ribs spaced at 75 mm. In critical area B, there is a slight decrease (0.9-3.2)% of the maximum stress at the models with ribs spaced at 50 and 100 mm, and increases by (2-3.69)% of the maximum stress in critical area C, but reduced as absolute values. The high density of ribs seems to be favourable. \"Left tilted ribbing\" definitely has a positive effect on the maximum stress in critical area B and a definitely unfavourable effect on the maximum stress in critical areas A and C, Figure 23.  
Figure 17:0 Deviation of maximum stresses in critical areas for frames with left tilted ribs spaced at 75 mm and normal thicknesses (20 mm) of the side walls (model code B25–B32), reported at the reference model B0. Figure 17:1 Deviation of maximum stresses in critical areas for frames with vertical ribs spaced at 100 mm and different thicknesses of the side walls (model code B20–B24), reported at the reference model B0. Figure 17:2 Deviation of maximum stresses in critical areas for frames with vertical ribs spaced at 50, 75 and 100 mm, and normal thicknesses (20 mm) of the side walls (model code B12, B17 and B22), reported at the reference model B0. The maximum stress in area B decreased by \(\approx\)3% in the majority of studied cases, with a maximum of 4.49% in model B27 with ribs tilted at 30\({}^{\circ}\). It needs be pointed out that model B27 is one of the \"left tilted\" ribbed models that ensures one of the best increases in stiffness [16]. The maximum stress in critical area A increases, with a single exception (model B26), by (0.6 to 1.6)%, the minimum pertaining to model B28, with ribs tilted at 40\({}^{\circ}\). The maximum stress in critical area C in all cases is greater than in the reference model, the increase typically being smaller than 2% and, in any case, small as absolute value. The favourable models are those with ribs tilted at 30\({}^{\circ}\) and 40\({}^{\circ}\), where the maximum stress in critical areas A and C increases slightly, and the stress in critical area B decreases significantly. The improvement of the stress state is evident in critical area B, Figure 24, also in the \"right tilted\" rib models, the maximum stress values are smaller than those in the reference model by \(\approx\)2% in most of the studied cases.