## 5 Conclusions and Prospect  
This study investigated the dynamic response of an in-service continuous rigid-frame bridge under vehicle load based on the crack damage report, the finite element software ANSYS 17.0, and custom software. The main findings and conclusions are as follows:  
1. The crack damage of continuous rigid-frame bridges in Shaanxi Province was investigated, and it was found that the positive cracks of the structure floor were mostly concentrated in the midspan, the one-fourth section, and the one-third section of the middle span. The characteristic parameters and damage model of the cracks were established, and the simplified crack zone unit was obtained. The stress state of the crack element area was analyzed using a numerical method, and the total bending moment of the section and the resultant force of the reinforcement were solved. The process was iterated until convergence was achieved, and the resulting coefficient of the sectional flexural rigidity reduction for the cracked area was obtained. Based on this parameter, the in-service damage to the bridge was simulated. 2. The investigation of traffic flow and vehicle types on highway bridges concluded that the largest proportion of the highway traffic consisted of dual-axle vehicles; therefore, the dual-axle vehicle model was adopted in this study. According to the analysis of different vehicle parameters, the results showed that under different speed conditions, the undamaged bridge was more sensitive to the speed of 80 km/h, and the damaged  
Figure 23: Impact coefficient of each section under different damage degrees. bridge was more sensitive to the speed of 90 km/h at the midspan and one-fourth span sections. For the one-third section, the sensitive speed of the damaged bridge was still 80 km/h, but the difference between the stress peak and the value at 90 km/h was very small. Therefore, it is generally recommended to focus on a speed limit of 90 km/h for damaged bridges. 3. In the investigation of the dynamic responses under different vehicle masses, the results showed that the dynamic response was obvious for the damaged bridge and that the increase in the concrete stress at the lower edge of each section was prominent. Therefore, it is recommended to strengthen the middle span of the bridge in the crack disaster area in time to prevent further cracking. 4. The results of the analysis of the dynamic displacement and impact coefficients of the bridge showed that the dynamic response of vehicle mass to bridge structure was more obvious. With the increase in vehicle mass, the displacement response and impact coefficient response of each section continued to increase. With the continuous damage to the structure, the dynamic response and impact coefficient of each section continued to increase, and the growth rate of the impact coefficient increased. This indicates that the dynamic performance of the in-service bridge will continue to decline with the aggravation of the damage. Therefore, the influence of vehicle-bridge coupling should be emphasized in the maintenance of the in-service bridge. In this study, only the factors of speed, vehicle weight, and damage degree were considered. In the subsequent research, more factors can be introduced for analysis, and the superposition of multiple factors can also be considered. In addition, this study investigated the dynamic response of damaged bridges based on single-vehicle crossing conditions; thus, the influence of multi-vehicle conditions and random traffic flow can be considered in the future. Conceptualization, L.W. ; methodology, L.W. ; software, Y.L. and C.J. ; validation, L.W., Y.L. and H.S. ; formal analysis, Y.L. ; investigation, C.J. ; resources, L.W. ; writing--original draft preparation, Y.L. and C.J. ; writing--review and editing, Y.L. ; visualization, H.S. ; supervision, L.W. ; project administration, L.W. ; funding acquisition, L.W. All authors have read and agreed to the published version of the manuscript.