## 4 Study on the Blast Resistance of Ultra-Wide Concrete Twin-Edge Box Girders Coated with Polyurea
### Parametric Analysis
#### 4.2.3 Analysis of the Impact of Different Repeated Explosion Patterns
The design of multiple blast scenarios is presented in Table 10.  
Damage patterns of ultra-wide box girders under continuous repeated blasts, both inside and outside the box, are depicted in Figure 38. When the blast occurred above the top plate center of each compartment, both specimens PG and PPCG experienced local perforation damage to the top plate, while the web and baseplates showed no significant damage, with the steel bars at the rupture area bent and yielded. The second explosion occurred inside the box compartment, where the shock wave impacted the top, bottom, web plates, and diaphragm beam. In specimen PG, both the top plate and baseplate had perforations, and concrete spalling occurred on the rear-blast surfaces of both side webs and diaphragm beams. In specimen PPCG, only the top plate was perforated, with the baseplate remaining intact, and no significant spalling was observed on the side webs or the rear surface of the diaphragm beam. Compared to an explosion above the box girder, the internal explosion resulted in more extensive damage. The reason for this is that during the blast above the box girder, the top plate directly absorbs the blast wave, leading to localized damage, whereas during the internal explosion, the shock wave is confined by the walls, leading to multiple reflections and superimpositions, resulting in multiple impacts on the box, thus amplifying the damaging effect. The results show that polyurea coating significantly improved the blast resistance of the box girder, with the most noticeable improvement in chamber 3 at the boundary, where only minor concrete spalling occurred on the end face. (3) Simultaneous detonation at multiple points  
As analyzed in Section 4.2.2, the central section of the T-beam, the quarter-span points, and the outermost web are considered vulnerable areas. To investigate the impact of simultaneous detonation at various points on the blast resistance of the ultra-wide box girder coated with polyurea, explosive loads were applied at these three weak positions simultaneously, with a TNT charge of 500 kg and a detonation height of 1 m. The finite-element model for simultaneous detonation at multiple points of the ultra-wide box girder is shown in Figure 39. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Specimen**} & \multirow{2}{*}{**Condition**} & \multirow{2}{*}{**Number of Explosions**} & \multicolumn{2}{c}{**Blast Charge Position**} & \multirow{2}{*}{**Blast Height/m**} & \multirow{2}{*}{**TNT Charge/kg**} & \multirow{2}{*}{**Scaled Distance/(m/kg\({}^{13}\))**} \\ \cline{5-6}  & & & **Transverse Direction** & & & **Insight/m** & **Charge/kg** & **Distance/(m/kg\({}^{13}\))** \\ \hline \multirow{6}{*}{PG/PPCG} & \multirow{2}{*}{17} & First & Top plate center of box chamber 1 & \multirow{2}{*}{Mid-span} & \multirow{2}{*}{1} & \multirow{2}{*}{200} & \multirow{2}{*}{0.171} \\  & & Second & Center of the interior of box chamber 1 & & & & \\ \cline{1-1} \cline{5-6}  & \multirow{2}{*}{18} & First & Top plate center in box chamber 2 & \multirow{2}{*}{Mid-span} & \multirow{2}{*}{1} & \multirow{2}{*}{200} & \multirow{2}{*}{0.171} \\  & & Second & Center of the interior of box chamber 2 & & & & \\ \cline{1-1} \cline{5-6}  & \multirow{2}{*}{19} & First & Top plate center in box chamber 3 & \multirow{2}{*}{Mid-span} & \multirow{2}{*}{1} & \multirow{2}{*}{200} & \multirow{2}{*}{0.171} \\  & & Second & Center of the interior of box chamber 3 & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Design of multiple blast scenarios. Figure 38: \(Cont\). Figure 38: Comparison of the damage morphology of ultra-wide box girders coated with polyurea before and after repeated external and internal explosions. Figure 40 illustrates the damage characteristics of the polyurea-coated box girder under simultaneous detonation at multiple points. It can be observed from the figure that local perforations appeared on the top plate below the explosion centers in specimen PG, and the plastic strain was greatest near the perforations.