## 6 Conclusions  
The main findings of the present study concerning the effects of vehicle speed on the vehicle-induced dynamic behaviors of a concrete bridge with smooth and rough road surfaces are summarized below:  
1. It was observed via an on-location inspection in Ref. [4] that a concrete bridge supporting vehicles running at lower speeds suffers from more severe apparent damages compared with one supporting vehicles of higher speeds. However, with a smooth road surface (as assumed by the numerical simulations presented in Ref. [4]), the vehicle-induced structural response of a bridge was found to be greater for a high-vehicle-speed case than for a low-vehicle-speed case in this article. The present numerical simulations suggest that with a sufficient increase in road surface roughness, the resonant responses are significantly amplified for the low-vehicle-speed cases, and the vehicle-induced tensile stresses are therefore calculated to be close to the tensile strength of the concrete material for both the low-vehicle-speed case and the high-vehicle-speed case. Actual bridges are usually associated with high degrees of road surface roughness. Therefore, a bridge supporting vehicles of lower speeds should suffer from more severe damages, as the vehicle load holding duration is longer for the low-vehicle-speed case than for the high-vehicle-speed case. The practical implication of the present findings on concrete bridge maintenance and monitoring is that for bridges with a smooth road surface (newly built bridges), maintenance and monitoring should focus on supporting high-speed vehicles, while for bridges with sufficiently rough road surfaces (bridges in operation for years), maintenance and monitoring should focus on supporting low-speed vehicles. 2. The field experiments undertaken by the present study further reveal the mechanism behind the observed phenomenon. It was found that merely considering the effects of road surface roughness, the frequency of vehicle excitation for a bridge with sufficient  
Figure 19: Variation in frequency difference with vehicle speed. Figure 20: Maximum dynamic stresses measured in different vehicle speed cases. road surface roughness should markedly decrease with the decrease in vehicle speed and becomes closer to the low-order natural frequencies of the bridge. Therefore, the resonant responses ought to be more significantly amplified for the lower-vehicle-speed case after the increase in road surface roughness. 3. The existing literature separately considers the effects of road surface roughness and vehicle speed on vehicle-bridge coupling dynamics. However, no study has well revealed significant interactions between vehicle speed and road surface roughness in the context of vehicle-bridge coupling dynamics. This scientific gap is well filled in this article. Since only nine cases were considered for the present numerical simulations assuming a 30-ton vehicle running through Renyihe Bridge at three vehicle speeds (20 km/h, 50 km/h, and 90 km/h) with three road surface roughness levels (smooth, slightly rough, and severely rough), mathematical models accounting for the coupled road surface roughness and vehicle speed effects can hardly be established at present due to sample scarcity. However, after more cases have been studied on Renyihe Bridge or other bridges, we can formulate useful empirical formulae to quantitatively guide concrete bridge maintenance and monitoring in the future. Although the present study indicates that the resonant responses are supposed to be more significantly amplified for the lower-vehicle-speed case after the increase in road surface roughness, it should be noted that for the rough road surface case, although the nature of vehicle excitation is influenced by the effects of road surface roughness when the vehicle speed is decreased, this influence is sometimes not sufficient to bring about the reduction in frequency of the vehicle's excitation, as the automobile engine frequency is markedly increased.