# Study on the Load-Bearing Behavior of Underground Steel Structures with Pitting Corrosion Damage
## 5 Results of Simulated Pitting Damage Three-Point Bending Test
### Analysis of Bearing Capacity and Deformation Capacity of Three-Point Bending Test  
Figure 13 shows the load mid-span displacement relationship curves of three-point bending tests; Figure 14 shows the comparison of load-mid-span deflection curves of three-point bending specimens. It can be seen that with the addition of pitting damage, the ultimate bearing capacity of the steel beam gradually decreases. In addition, the slope of the linear elastic stage and the final vertical displacement at the mid-span of the specimens all show varying degrees of reduction, among which the sample with the inclined pitting damage shows the most significant decrease. For the undamaged intact components, after the full-section yields, a distinct strengthening phenomenon occurs, during which the load increase rate slows down while the deformation continues to increase until the final failure. For the components with pre-existing damage, the ultimate bearing capacity is reached shortly after the full-section yields, and the strengthening stage becomes shorter or even almost disappears, leading to the final failure of the component. Meanwhile, based on the  
Figure 11: Failure mode of three-point bending specimen with pitting damage (BEND-PIT-IV).  
Figure 12: Failure mode of three-point bending specimen with pitting damage (BEND-PIT-I).  
Figure 10: Failure mode of three-point bending specimen with pitting damage (BEND-PIT-III).  
data from these five groups of curves, the relevant mechanical performance index of the beams with and without pitting damage can be determined. The yield load (_P_y) is taken as the load value corresponding to the lowest point of the load-mid-span displacement curve at the yield stage, and the yield displacement (\(\Delta\)y) is taken as the mid-span vertical displacement when the member reaches the yield load. The ultimate load (_P_u) is taken as the load value corresponding to the failure of the beam in the test, and the ultimate displacement (\(\Delta\)u) is taken as the mid-span vertical displacement when the component reaches the ultimate load. Table 3 presents the statistical results of the bearing performance of all components in these tests.  
Figure 13: Load–mid-span deflection curves of five specimens in three-point bending: (**a**) intact specimen BEND-S; (**b**) specimen BEND-PIT-I; (**c**) specimen BEND-PIT-II; (**d**) specimen BEND-PIT-III; (**e**) specimen BEND-PIT-IV.  
From the test results in Table 3, when comparing specimen BEND-PIT-I with the intact specimen BEND-S under the same working conditions, it is found that its yield displacement \(\Delta\)y and ultimate displacement \(\Delta\)u hardly decrease, while the yield load \(P\)y and ultimate load \(P\)u decrease by 0.47% and 2.41%, respectively. Similarly, when comparing specimen BEND-PIT-II with the intact specimen BEND-S under the same working conditions, it can be found that its yield displacement \(\Delta\)y does not decrease, but the ultimate displacement \(\Delta\)u decreases by 22.7%, and the ultimate load \(P\)u and yield load \(P\)y decrease by 12.8% and 2.72%, respectively. For specimen BEND-PIT-III, the ultimate load \(P\)u and ultimate displacement \(\Delta\)u decreased by 15.54% and 14.86%, respectively, and the yield load \(P\)y and yield displacement \(\Delta\)y decrease by 14.0% and 10.35%, respectively. For specimen BEND-PIT-IV, the ultimate load \(P\)u and ultimate displacement \(\Delta\)u decrease by 12.31% and 32.41%, respectively, and the yield load \(P\)y and yield displacement \(\Delta\)y decrease by 19.74% and 30.01%, respectively.