## 4 Analysis of Factors Affecting the Impact Factor
### Effect of Box Girder Configurations  
To study the effect of box girder configurations on the impact factor, this paper examines the impact factors of box girders with flat steel webs and steel bottom plates. The flat steel web box girder replaces the corrugated steel web with a flat steel web of equivalent thickness. The steel bottom plate box girder replaces the concrete bottom plate with a steel bottom plate on the corrugated steel web box girder. The equivalency criterion is based on equal mid-span static deflection under live load. Following this equivalency principle, a 20 mm thick steel plate replaces the concrete bottom plate in the corrugated steel web box girder model to obtain the steel bottom plate box girder, and the beam height is adjusted to 2.6 m.  
As shown in Figure 15, a comparison is made between the dynamic responses of flat steel web box girder bridges and corrugated steel web box girder bridges on grade A pavement. The comparison considers vehicle speeds of 20 km/h, 60 km/h, and 120 km/h. The results reveal that the maximum dynamic deflection of the flat steel web box girder decreases by 4.2%, 4.7%, and 3.7%, respectively, compared to the corrugated steel web box girder bridge, which aligns with expectations. According to the research in paper [2], the effective elastic modulus of corrugated steel webs is approximately 1/700 of the original elastic modulus, meaning that they bear minimal axial force and bending moment in the box girder. However, as the static deflection of the flat web box girder is also relatively reduced, the impact factor between the two is not significantly different, as shown in Table 7.  
As shown in Figure 16, under pavement grade A, the dynamic response of the steel bottom plate box girder bridge is compared with that of the concrete bottom plate box girder bridge. This comparison is made at vehicle speeds of 20 km/h, 60 km/h, and 120 km/h. It can be observed that although the static deflections under live load are nearly identical for both bridges, the dynamic deflection of the steel bottom plate box girder bridge increased. The increases were 21.7%, 30.3%, and 33.4% relative to the concrete bottom plate  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Box Girder Type**} & \multicolumn{3}{c}{**Working Conditions**} \\ \cline{2-4}  & **20 km/h** & **60 km/h** & **120 km/h** \\ \hline Corrugated steel web box girder & 0.066 & 0.154 & 0.310 \\ Flat web box girder & 0.061 & 0.158 & 0.311 \\ Steel bottom plate box girder & 0.247 & 0.457 & 0.672 \\ \hline \hline \end{tabular}
\end{table}
Table 7: Impact factor of each box girder.  
Figure 15: Mid-span vertical displacement of flat steel web and corrugated steel web box girder bridges: (**a**) 20 km/h; (**b**) 60 km/h; (**c**) 120 km/h.  
box girder bridge. As shown in Table 7, the impact factor of the steel bottom plate box girder bridge increased by 274.2%, 196.8%, and 116.8% compared to the concrete bottom plate box girder bridge. This shows that, while replacing the concrete bottom plate with a steel bottom plate can further reduce the self-weight of the box girder while maintaining static load-bearing capacity, it also introduces vibration issues for the bridge.