# Study on the Load-Bearing Behavior of Underground Steel Structures with Pitting Corrosion Damage
## 4 Results of Simulated Pitting Damage Compression Test
### Analysis of Bearing Capacity and Deformation Capacity of Compression Test  
Under axial compression loads, it can be seen from the load-displacement curves characteristics in Figure 7 that compressive specimens C-S, C-PIT-I, C-PIT-II, C-PIT-III, and C-PIT-IV all undergo elastic, plastic, and failure stages, and Figure 8 shows compares the load-displacement curves of five compression specimens. At the beginning of loading, the curve is linear, and the slope of the load displacement curve indicates the initial stiffness of the specimen. The initial stiffness of the four groups of specimens (C-S, C-PIT-I, C-PIT-II and C-PIT-III) is similar, and the stiffness is significantly higher than that of C-PIT-IV, indicating that the inclined preform pitting damage will significantly reduce the stiffness of the \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Sample Number** & **Specimen Size** & **Loading Condition** & **Pitting Type** & **Test Quantity** \\ \hline C-S & NO.20b \(\times\) 600 mm & uniaxial compression & intact & 3 \\ C-PIT-I & NO.20b \(\times\) 600 mm & uniaxial compression & vertical row & 3 \\ C-PIT-II & NO.20b \(\times\) 600 mm & uniaxial compression & horizontal row & 3 \\ C-PIT-III & NO.20b \(\times\) 600 mm & uniaxial compression & triangular row & 3 \\ C-PIT-IV & NO.20b \(\times\) 600 mm & uniaxial compression & inclined row & 3 \\ BEND-S & NO.20b \(\times\) 1200 mm & three-point bending & intact & 3 \\ BEND-PIT-I & NO.20b \(\times\) 1200 mm & three-point bending & vertical row & 3 \\ BEND-PIT-II & NO.20b \(\times\) 1200 mm & three-point bending & horizontal row & 3 \\ BEND-PIT-III & NO.20b \(\times\) 1200 mm & three-point bending & triangular row & 3 \\ BEND -PIT-IV & NO.20b \(\times\) 1200 mm & three-point bending & inclined row & 3 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Loading information of specimens. Figure 4: Three-point bending specimen with prefabricated pitting damage. specimen in the elastic stage. The horizontal row, vertical row, and triangular distribution of prefabricated pitting damage have little influence on the compressive stiffness of the sample at the elastic stage. When the load continues to increase, the load and displacement curve of the specimen is nonlinear, and the specimen is in the elastoplastic stage. The load continues to increase, and the specimen reaches the yield stage until the ultimate load of the specimen is loaded. Comparing the ultimate load of the five groups of specimens, it is found that the ultimate loads of C-PIT-I, C-PIT-II, C-PIT-III and C-PIT-IV are significantly lower than that of C-S.  
Table 2 shows the statistical results of the bearing performance of all types of compressed samples in this compression test. According to the test results in Table 2, comparing specimen C-PIT-I with complete specimen C-S without pitting damage under the same working condition, it can be found that the ultimate load \(Pu\) and ultimate displacement \(\Delta u\) decrease by 1.8% and 10.2%, respectively, and the yield load \(Py\) and yield displacement \(\Delta y\) decrease by 1.78% and 33.3%, respectively. Comparing specimens C-PIT-II, C-PIT-III, and C-PIT-IV with complete specimen C-S without pitting damage under the same working condition, it can be found that the ultimate load \(Pu\) and ultimate displacement \(\Delta u\) of specimen C-PIT-II decrease by 2.04% and 4.7%, respectively, and the yield load \(Py\) and yield displacement \(\Delta y\) decrease by 1.8% and 36.95%, respectively. The ultimate load \(Pu\) and ultimate displacement \(\Delta u\) of specimen C-PIT-III decrease by 3.55% and 7.35%, respectively, and the yield load \(Py\) and yield displacement \(\Delta y\) decrease by 2.67% and 34.40%, respectively. The ultimate load \(Pu\) and ultimate displacement \(\Delta u\) of specimen C-PIT-IV decrease by 8.82% and 3.18%, respectively, and the yield load \(Py\) and yield displacement \(\Delta y\) decrease by 7.19% and 14.46%, respectively. Figure 5: Loading arrangements in uniaxial compression test.