## 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  
The primary girder of a bridge generally consists of a roadway, non-motorized lanes, bike paths, and pedestrian walkways. Considering the randomness of vehicle movement, this study examines the dynamic response and damage patterns of the primary girder in a bridge when subjected to explosive forces at various lateral positions. Detonation points along the width of the girder span include critical areas such as the roadway centerline, the connection between box chamber 1 and T-beam, and the web regions between box chambers. Additional blast locations involve the central points of the top plates in chambers 1, 2, and 3, along with the cantilever section. The blast conditions, with a TNT equivalent of 200 kg and the blast center placed 1 m above the top plate, are visually presented in Figure 28. Full details can be found in Table 8.  
The damage patterns of specimens PC and PPCG under loading conditions 6 through 14 are shown in Figure 29. The failure mode of specimen PC primarily consists of full penetration damage to the deck plate. In contrast, the failure of specimen PPCG shows \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Specimen**} & \multirow{2}{*}{**Condition**} & \multirow{2}{*}{**TNT/kg**} & \multicolumn{3}{c}{**Blast Charge Position**} & \multirow{2}{*}{**Blast Height/m**} & \multirow{2}{*}{**Scaled Standoff Distance/m/kg\({}^{10}\)**} \\ \cline{5-6}  & & & & **Transverse Direction** & & & \\ \hline \multirow{6}{*}{PG/PPCG} & 6 & \multicolumn{3}{c}{Centerline of the roadway} & \multirow{6}{*}{} & \multirow{6}{*}{} & \multirow{6}{*}{} \\  & 7 & & Quarter-span of the T-beam & & & \\ \cline{1-1} \cline{5-6}  & 8 & & Web-connecting box chamber 1 and T-beam. & & & \\ \cline{1-1} \cline{5-6}  & 9 & & Top plate center in box chamber 1 & & & \\ \cline{1-1} \cline{5-6}  & 10 & 200 & Web section between box chambers 1 and 2 & & & \\ \cline{1-1} \cline{5-6}  & 11 & & Top plate center in box chamber 2 & & & \\ \cline{1-1} \cline{5-6}  & 12 & & Web section between box chambers 2 and 3 & & & \\ \cline{1-1} \cline{5-6}  & 13 & & Top plate center in box chamber 3 & & & \\ \cline{1-1} \cline{5-6}  & 14 & & Web-connecting box chamber 3 to the cantilever. & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Blast-response calculation conditions for prototype box-grider segments under different transverse blast locations. Figure 27: Vertical displacement time histories and peak curves of the baseplate under different TNT equivalents. localized damage to the upper deck surface without penetration to the bottom surface. In specimen PC, the concrete at the back side of the deck plate suffers spalling, which is caused by the interaction of the blast wave on the front side and tensile stresses on the rear, resulting in a funnel-shaped breach. For specimen PPCG, the polyurea coating dissipates the blast energy, preventing spalling on the back-blast side and demonstrating protective efficacy. Damage to the deck plate is more significant when the explosion occurs above the web compared to the box chamber or T-beam sections. This is because when the explosion occurs above the web, the deck plate in specimen PC undergoes constrained deformation due to the web's reinforcing effect, which reduces its energy-absorption capacity. As a result, the deck plate experiences greater deformation and a subsequently larger damage area under blast loading. Compared to detonations in the center of the box chamber and the T-beam section, the constraint effect from a detonation above the web makes the deck plate more susceptible to impact and damage. In conclusion, the polyurea provides effective protection for the underside of the deck plate, mitigating damage to the bottom surface. Based on the classification of ultra-wide box girder structures, the configurations are categorized into three types: box chamber center, T-beam sections, and web.