## 4 Study on the Blast Resistance of Ultra-Wide Concrete Twin-Edge Box Girders Coated with Polyurea
### Parametric Analysis
#### 4.2.2 Analysis of the Impact of Different External Detonation Positions on the Box Girder
A comparative analysis of these categories is provided below:  
Based on the classification of ultra-wide box girder structures, the configurations can be categorized into three types: box chamber center, T-beam sections, and web. A comparative analysis of these classifications is provided below:  
(1) Detonation above the center of the box chamber  
When the blast is located above the midpoint of each box chamber, the temporal evolution curves illustrating the vertical shift of the baseplate in chambers 1 to 3 are shown in Figures 30-32. When the explosion originates at the box chamber center, the maximum vertical displacement values recorded at the baseplate just below the explosion point for specimens PG and PPCG are the highest. Specifically, under condition 9, the vertical shift measured at the center of the baseplate in chamber 1 of specimen PG reaches 9.15 mm; under condition 11, the peak in chamber 2 is 8.67 mm; and under condition 13, the peak in chamber 3 is 12.69 mm. For specimen PPCG, under the same conditions, the peak vertical displacement values are 8.72 mm for chamber 1 (condition 9), 8.45 mm for chamber 2 (condition 11), and 12.21 mm for chamber 3 (condition 13), indicating reductions of 4.70%, 2.54%, and 3.78%, respectively, compared to specimen PG. These results demonstrate that for both specimens PG and PPCG, as the distance from the explosion center shortens, the peak vertical displacement grows. The region immediately beneath the explosion point experiences the most intense impact from the blast wave, and, due to boundary effects, the vertical displacement response in chamber 3, located at the end of the main girder, is more pronounced. Following the application of polyurea coating, the vertical displacement measured at the baseplate right beneath the explosion point slightly decreases under identical conditions. Figure 29: _Cont._  
Figure 29: Failure modes in conditions 6–14. (2) Blast detonation above the webs  
The temporal evolution curves of vertical displacement for the baseplates in chambers 1 to 3 subjected to blast conditions, with the detonation point above the web separating  
Figure 31: TNT equivalent of vertical displacement at the center of the baseplate in box chamber 2 under conditions 9, 11, and 13. Figure 30: Temporal evolution curves of vertical displacement at the center of the baseplate in box chamber 1 under conditions 9, 11, and 13. Figure 32: TNT equivalent of vertical displacement at the center of the baseplate of box chamber 3 under conditions 9, 11, and 13.  
the chambers, are shown in Figures 33 and 34. As observed, during condition 10, the highest vertical displacements of the baseplate in chambers 1 and 2 of specimen PG are 14.45 mm and 12.56 mm, respectively, while in specimen PPCG, the corresponding values are 13.43 mm and 11.03 mm. The peak displacements of specimen PPCG are reduced by 7.06% and 12.18%, respectively, compared to specimen PG. The vertical displacement patterns at the baseplate centers on either side of the web are consistent for specimens PG and PPCG, showing similar peak displacements. Notably, the application of polyurea significantly reduces the displacement in the longitudinal direction near the boundary. During condition 12, the maximum vertical shifts of the baseplate of chambers 2 and 3 of specimen PG are 11.21 mm and 17.36 mm, respectively, while in specimen PPCG, the corresponding values are 10.20 mm and 14.27 mm. Compared to specimen PG, the peak displacements of specimen PPCG are reduced by 9.01% and 17.80%, respectively. Both specimens PG and PPCG exhibit similar trends in the vertical shift at the baseplate centers on either side of the web. However, the displacement in the baseplate of chamber 3 exceeds that in chamber 2, likely because chamber 3 is located at the end of the main girder's cross-section, where the constraining effect is weaker. These results indicate that the response in each chamber is closely linked to the cross-sectional design and the boundary conditions of the ultra-wide box girder.