## 3 Results and Analysis
### Effect of Steel Grade on Ultimate Strength  
As shown in the tables in Appendix B, the ultimate strength with and without accounting for the welding residual stress of the stiffened plates, regardless of the structural forms (flat-bat stiffened, or angle-bar stiffened, or tee-bar stiffened plates), increase with the yield stress of the material. The numerical results for the ultimate strengths of the stiffened plates accounting for the welding residual stress are shown in Figure 11, where the pattern of ultimate strength versus the column slenderness ratio is plotted for each type of stiffened plate. It can be seen that the larger the column slenderness ratios are, the slower the increase in the ultimate strength of the stiffened plates with the yield strength. For instance, the increases in the ultimate strength of the stiffened plates F1520, A1520 and T1520 are very small. This is because linear elastic buckling usually occurs in stiffened plates with large column slenderness ratios, and the increase in yield strength does not significantly improve the ultimate strength. Taking the example of the  
Figure 10: Ultimate strength of tee-bar stiffened plates versus column slenderness ratio. stiffened plate F1520, the column slenderness ratios corresponding to steels S235, S315, S350 and S390 are 1.273, 1.47, 1.564 and 1.639, respectively, and the numerical results for the ultimate strength are 85.1 MPa, 91.6 MPa, 96.1 MPa, and 98 MPa, respectively. While the Euler stress \(\sigma_{E}\) of F1520, according to the dimensions of the cross-section and the formula for calculating the Euler stress of single spanned beam with simple support at both ends \(\sigma_{E}=\pi^{2}EI/Al^{3}\), is 117.6 MPa. The numerical results for the ultimate strength obtained in the present study are even smaller than the Euler stress, which means it is likely that linear elastic buckling is present, and consequently the effect of increasing the yield strength is insignificant and negligible. In contrast with the stiffened plates with large column slenderness ratios, increasing the yield strength can significantly improve the ultimate strength of stiffened plates with small column slenderness ratios, for instance, F3320, A3320 and T3320. This is because the stiffened plates with small column slenderness ratios have elastoplastic buckling, and the increase in the yield strength effectively improves the ability of the structure to resist plastic deformation under compressive load. If the column slenderness ratio is sufficiently low, buckling damage can even occur. Taking the example of T3520, the column slenderness ratio corresponding to S235 is 0.314, and the ultimate strengths with and without accounting for the welding residual stress are 227.2 MPa and 217.7 MPa, respectively, both of which are very close to the yield strength of 235 MPa. This indicates that buckling damage is present. Figure 11: The trend of ultimate strength of the stiffened plates with various built-up steels. To investigate the influence of the material on the ultimate strength of stiffened plates of various column slenderness ratios, a ratio of the ultimate strength to the yield strength ratio \(\Delta\sigma_{U}/\Delta\sigma_{Y}\) is defined as follows  
\[\left(\Delta\sigma_{U}/\Delta\sigma_{Y}\right)_{S_{i}=S_{j}}=\frac{\sigma_{U_{S _{i}}}-\sigma_{U_{S_{j}}}}{\sigma_{Y_{S_{i}}}-\sigma_{Y_{S_{j}}}} \tag{8}\]  
where the subscripts \(S_{i}\) and \(S_{j}\) denote two different steels; \(\sigma_{Y_{S_{i}}}\) and \(\sigma_{Y_{S_{j}}}\) are the yield stresses of the two steels; and \(\sigma_{U_{S_{i}}}\) and \(\sigma_{U_{S_{j}}}\) are the ultimate strengths of stiffened plates built in these steels. Figure 12 shows the ratio \(\Delta\sigma_{U}/\Delta\sigma_{Y}\) versus the column slenderness ratio \(\lambda\) of the three types of stiffened plates.