## 3 Parametric Study
### Results and Discussion  
The results for axial force are displayed in Figures 4 and 5, for the direct and iterative methods, respectively. The observation of these results prompts the following remarks:  
1. The errors are in the approximate range of \(-600\%<e<20\%\). The shaded \(\pm 10\%\) regions are located in the lower left corner of the maps--closer to the corner as \(b/t\) increases--even if, for some cases, they extend up to the right side. These regions are quite small with respect to the parameter ranges considered. 2. A comparison between the direct and iterative methods reveals that they (I) coincide for \(b/t=20+f_{yb}=200,700\) MPa and \(b/t=35+f_{yb}=200\) MPa, (II) are very similar for \(b/t=50+f_{yb}=200\) MPa, and (III) are somewhat different in the two remaining cases (\(b/t=35,50+f_{yb}=700\) MPa), with the shaded \(\pm 10\%\) regions being quite smaller for the iterative method. 3. The yield stress has no effect for \(b/t=20\) (and a very small effect for \(b/t=35\), iterative method) since, for these cases, there is no reduction in the stiffener wall widths due to local buckling. For the remaining cases, the isoline slopes are slightly changed and the differences observed in the direct method maps can be somewhat significant. Figure 3: Comparison of the GBT symmetric distortional deformation mode and the simplified method for uniform compression. Figure 4: Error isoline maps for axial force, direct method. Figure 5: Error isoline maps for axial force, iterative method. For major axis bending, the results are presented in Figures 6 and 7 (direct and iterative methods, respectively), and deserve the following comments:  
1. The errors are smaller than for axial force, falling approximately between \(-70\%\) and \(45\%\). The shaded \(\pm 10\%\) regions are wider than for axial force, and are now located diagonally or horizontally, in the middle of the maps. 2. As for axial force, the direct and iterative methods (I) coincide for \(b/t=20+f_{yb}=200,700\) MPa and \(b/t=35+f_{yb}=200\) MPa, and (II) are very similar for \(b/t=50+f_{yb}=200\) MPa. However, they are now remarkably different in the two remaining cases (\(b/t=35,50+f_{yb}=700\) MPa), even if both methods yield shaded \(\pm 10\%\) regions with similar areas and therefore have a similar accuracy (but obviously for different parameter ranges). 3. Also, as for axial force, the yield stress has no effect for \(b/t=20\). For the remaining cases, the changes in the maps can be quite significant, particularly for the direct method. It is of interest to examine the differences in the error maps when the numerical distortional stress is calculated using the distance from the neutral axis to the most compressed fiber (\(h/2+t/2\)), instead of the distance to the stiffener centroid, a procedure that avoids the calculation of the stiffener effective section. The results are shown in Figures 8 and 9, where the shaded areas correspond to the \(\pm 10\%\) regions of the corresponding graphs in Figures 6 and 7, to facilitate the comparison. In general terms, the differences in each map obviously increase as the differences between the two distances become more relevant (i) as \(h/b\) decreases, and (ii) as \(c/b\) increases. Thus, the differences are lower at the top-left corners and higher at the bottom-right corners, never exceeding \(20\%\)--the maximum difference occurs for \(b/t=20\), regardless of the method or yield stress considered, and the differences decrease with increasing (i) \(b/t\) and (ii) yield stress. Concerning the \(\pm 10\%\) regions, it is observed that using the most compressed fiber leads to a small decrease in the error on the negative side, but a significant increase in the error on the positive side. Figures 10 and 11 display the direct and iterative method results for minor axis bending, making it possible to draw the following remarks:  
1.