# Ultimate Compressive Strength of H-Section Stub Columns Subject to Random Fitting Corrosion Damage
## 4 Discussion  
The compressive behavior and ultimate strength of randomly pitted H-section stub columns have been studied under various corrosion patterns, pitting intensities and sizes of plate elements. The strength reduction in columns with multiple corroded plate elements can be linearly superposed by the individual plates, being highly related to the pitting intensity. The method to equalize the thickness loss due to pitting corrosion was regarded to be closely correlated with the standard deviation of depth of random corrosion pits. The columns with section sizes of H11.5 \(\times\) 7.5 and H7.5 \(\times\) 7.5 were utilized herein to further study on the failure mechanism of randomly pitted columns. Cross-sections of H11.5 \(\times\) 7.5 along the column length were segmented by 1 mm intervals, and the area of each cross-section, with random corrosion pits, was calculated upon its irregular section. Figure 18 shows the variation in the cross-sectional area along the column length under two corrosion patterns. The minimum cross-sectional area, \(A_{min}\), the average cross-sectional area, \(A_{ave}\), and the cross-sectional area, \(A_{ver}\), at the vertex of the buckling half-wave were found. Note that the average cross-sectional area, \(A_{ave}\), was determined by tackling random pitting corrosion as general corrosion based on the same \(DOV\), represented by the red dashed lines in Figure 18.  
It is proven in Figure 18 that the local buckling of H-section columns tended to occur at the smallest or relatively smaller cross-section [12]. For the double-flange corrosion shown in Figure 18a, under the \(DOV\) 4.8%, the minimum cross-sectional area was 4668 mm\({}^{2}\) at \(l_{min}\) = 224 mm. The local buckling occurred at \(l_{ver}\) = 228 mm, with the cross-sectional area of 4712 mm\({}^{2}\) being very close to the minimum cross-sectional area. Similarly, under the \(DOV\) 13.8%, the minimum cross-sectional area was 4237 mm\({}^{2}\) at \(l_{min}\) = 38 mm, but the local buckling arose at \(l_{ver}\) = 269 mm, with the relatively smaller cross-sectional area of 4268 mm\({}^{2}\), which was also close to the minimum cross-sectional area. As for double-flange combined corrosion in Figure 18b, under the \(DOV\) 4.8%, the minimum cross-sectional area was 4533 mm\({}^{2}\) at \(l_{min}\) = 313 mm, while the local buckling occurred at \(l_{ver}\) = 220 mm with the cross-sectional area of 4577 mm\({}^{2}\). Under the \(DOV\) 13.8%, the minimum cross-sectional area was 4084 mm\({}^{2}\) at \(l_{min}\) = 269 mm, while the local buckling took place at \(l_{ver}\) = 270 mm, with a cross-sectional area of 4095 mm\({}^{2}\) which was close to the minimum cross-section. This was due to the fact that multiple corrosion pits with larger  
Figure 18: Variation of cross-sectional area of a corroded column (H11.5 \(\times\) 7.5) along its length: (a) double-flange corrosion; (b) double-flange combined corrosion. sizes clustered at the relatively smaller cross-section, leading to serious stress concentration and then causing local buckling on such a relatively smaller cross-section rather than the minimum cross-section, as shown in Figure 10.  
Obviously, it is not rational to simplify random pitting corrosion into general corrosion based on the same \(DOV\), as the variation in cross-sectional area along the column length became more pronounced with the increase in \(DOV\), leading to an increasing COV, as shown in Figure 18. In fact, the failure of randomly pitted columns always arose at the minimum or relatively smaller cross-section, but not the average cross-section. On the other hand, the random pitting corrosion significantly differs from regular pitting corrosion, since the latter caused less strength reduction in the columns with regularly distributed pits less than those with uniform thickness loss [12].