| Because of the high cost when building them and their long period of use, the security of bridges deserves great caution. The performance and durability of the bridges are mainly determined by the design, construction, the quality of materials used and other factors. Because of the defects that may exist in the design, construction and materials, there maybe inherent weaknesses in the bridges built. In addition, inevitable human injury, various natural erosion and acquired diseases may happen to the bridges. Congenital defects and acquired diseases, together with the possible heavy loads and outside forces, may cause unforeseeable damage to the bridges. In the past, the damage and sudden collapse of bridges caused huge catastrophes all over the world. According to statistics, over one-third of the major existing highway bridges of China have structural defects, various degrees of hidden troubles of functional failure in them. The same problems are also frequently seen in the United States, Japan, Western Europe and other developed countries. Therefore, the inspection and assessment of the structure, detection of the status of them, assesses of the sustainability performance, and immediate maintenance and reinforcement are becoming more and more important.Dynamic load test has the following advantages: (1) fully reflection of the health status of the bridge; (2)discovering injury or defects of the structure, or inside the inaccessible parts; (3)The results are seldom affected by man-made factors ,its higher accuracy and objectivity. Therefore, dynamic load test can be an effective detection method of the existing reinforced concrete girder bridges.There are three major methods in the bridge dynamic load tests: (1) Resonance test; (2) Microseism test; (3) Self-vibration test. As dynamic characteristics of the different types and spans of the bridges are quite different, besides, if the energy of excitation is too tiny, it will not be sufficient to evoke the needed modal. However, nonlinear or partial destruction of the structure may be caused if the input energy is too much. Therefore, it is necessary to choose reasonable excitation methods and exciting position according to the type and span of the reinforced concrete girder bridges.For short-span reinforced concrete girder bridges, bump excitation is generally used. The height of the bump is a key issue and also difficulty in the bump test. Some relevant articles have given some definitions about the bump height, but none has given the theoretical calculation method of the bump height.This paper presents a theoretical method which can calculate the bump height of reinforced concrete simply-supported girder bridges. This method simplifies the vehicle into two pieces of irrelevant mass. Based on Euler-Bernoulli model, the analytical result of dynamic response in each section is gained. Correspondingly, the analytical result of internal response in each section is also obtained. The comparison is made between the internal force responses and the internal force responses brought by the design load. The maximum height allowed is determined taking the maximum inner force in each section not exceeded as presume in the condition that there are no damages to the bridges. Finally, the finite element method is used to derive the bump height, dynamic response and internal force. And the calculating result is compared with the result obtained with the methodproposed in the paper. The analysis result shows that there is a good correlation with each other. The method presented in the paper is proved to be correct and effective. Still, this paper provides the basic theory for the selection of bump height in bump test during bridge detection.
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