## 3 Image Feature-Based Extraction of Damage Indicators
### Residual Bearing Capacity Extraction Algorithm
#### 3.1.2 Residual Bearing Capacity Extraction  
The residual bearing capacity (RBC) of the bridge is extracted from the image using template matching. According to the static analysis of the bridge pier and bridge deck, it is known that the bridge residual bearing capacity is affected by the combined effect of the two and will be affected by three factors: the strike location, damage status, and the strike size. Taking the impact crater size as the basis for the establishment of the template, ten types of residual bearing capacity after impact are established, and the residual bearing capacity of the bridge is calculated for the single-crater case. For the case of a multi-bomb crater, the strike location and state of the damage are the influencing parameters according to the weight value of the influence of the impact on the size of the bearing capacity. The weighted coupling is used to calculate the residual bearing capacity under the combined impact of each crater, and finally the calculation results of the pier and bridge deck are coupled. The specific calculation steps are as follows:  
1. The results of the abutment and bridge deck image change detection are utilized to mark each strike zone. Figure 3: Simplified diagram of bridge deck. Figure 2: Simplified diagram of bridge abutment. 2. The state of destruction of each strike region is identified to distinguish between penetration and absence, and the location of their destruction is determined; the abutment is divided into the upper and lower halves, and the bridge deck is divided into two parts, the end of the inward third of the bridge and the remaining middle region. 3. For the screened damaged area, the relative damaged area ratio is calculated, which is defined as the ratio of the area of the damaged area (crater) to the area of the smallest bridge lateral area containing the damaged area, as shown in Figure 4. When the relative damage area ratio exceeds 50%, the crater area is too large, and the bridge may collapse directly, which divides the relative damage area ratio into 0-0.05, 0.05-0.1, 0.1-0.15, 0.15-0.2, 0.2-0.25, 0.25-0.3, 0.3-0.35, 0.35-0.4, 0.4-0.45, and 0.45-0.5. The ten intervals correspond to residual bearing capacity classes 1-10, respectively. 4. According to the matching rule, the higher bearing capacity grade of the bridge abutment indicates that its residual bearing capacity is smaller and the degree of destruction is more serious according to the results of the third step of the calculation matching the residual bearing capacity grade of each strike position. 5. Combined with the damage state of each damaged area and the location of the damage, the results of the residual bearing capacity level of each damaged area are a weighted coupling in order to ensure that the coupling is positively correlated and the range of values is unchanged; the calculation method is given in the following equation: \[RBC=\frac{10}{\frac{\sum_{i}^{T}(10\times 0.35)}{T}}\times\frac{\sum_{i}^{T}( RBC_{i}\times\text{ Weight })}{T},\] (3) where \[RBC\] denotes the final residual bearing capacity index value of the bridge abutment or bridge deck, \[T\] indicates the total number of damaged areas, \[RBC_{i}\] indicates the residual bearing capacity index of the ith damaged area, and weight indicates the weight value of a certain type of damage. The functional destruction of the bridge abutment is equivalent to the structural destruction, and for the bridge abutment as a whole, its structural strength is consistent. According to the results of the static analysis, the size of the residual bearing capacity of the bridge abutment is negatively correlated with the strike height. Analyzed from the loading point of view, the stress underneath the bridge abutment is greater and the damage is more likely to cause collapse. It is assumed that the bridge deck and abutment cross-sectional area are of the same size.