Study Of Dynamic Responses And Impact Factors Of Girder Bridges Due To Vehicle Braking

Vehicle load is one of the important loads for the bridge structures and needs to be considered in the design of bridges. The problem of bridge vibration caused by a moving vehicle at a constant speed has been studied extensively in the past. Besides, the methods of calculating impact factors in the bridge design specifications are generally based on the case of the vehicle with constant speeds. However, under the actual traffic conditions, vehicles moving on the bridge will inevitably make the emergency brake and other non-uniform motions in case of unexpected situations. The resulting dynamic responses of the bridge will be significantly greater than those due to vehicles traveling at constant speeds, and the dynamic impact factors of bridges caused by vehicle braking may exceed those prescribed in the bridge design codes. In this way, it is necessary to consider the effect of vehicle braking in the study of vehicle-bridge coupled vibration.In this thesis, a three-dimensional vehicle-bridge coupled model was developed to study the dynamic responses and impact factors of bridges with different span lengths and cross sections due to vehicle braking. The main works were summarized as follows:(1) The method of considering the problem of vehicle traveling with variable speed was first proposed in this thesis, and it is verified by the vibration of the contact forces. Then the study was conducted on a two-axle truck model and a typically simply-supported concrete girder bridge. The bridge responses at the midspan, including deflection, strain, and vertical acceleration were obtained as well as impact factors calculated from the increment of the deflection and strain at the midspan. Then, the effects of several important parameters on the dynamic impact factors were studied. In addition, a comparison was made between the impact factors caused by vehicle braking and acceleration.(2) Impact factors for different bridge responses, including deflection, bending moment, and shear of five simply-supported concrete girder bridges with different span lengths under the braking of a of a typical three-axle vehicle model HS20-44 were obtained. The similarities and differences between the impact factors of different bridges under different conditions were investigated. In addition, a comparison was made between the calculated dynamic impact factors and those specified in the current AASHTO bridge design code and Chinese bridge design code. The results show that the impact factors due to vehicle braking may exceed those specified in the current bridge design codes.(3) Impact factors of six concrete girde r bridges including both simply-supported and continuous with different cross-sections and span lengths under the braking of the vehicle model HS20-44 were obtained. A parametric study was also performed to investigate the effect of different parameters, including the vehicle braking position, deceleration rate, initial vehicle speed, road surface condition, on the impact factors. Impact factors of bridges with different cross sections were compared. A comparison was also made between the calculated dynamic impact factors and those specified in the current Chinese bridge design code.