Design And Evaluation Methods On Dynamic Performance Of Girder Bridges Under Moving Vehicle Loads

It is bound to suffer moving loads for operation of highway bridges, and moving vehicles are main loads. It has been shown that, most of major accidents for bridges are closely related with structural strength. Forty percents of them are resulted from the vibration induced by moving vehicles. There are two types of damages of bridges, dynamic damage and destruction. As a result, not only the favourable static performance is needed for highway bridges, but also the good dynamic performance is required during the operation, which will ensure the safety of bridges and comfort of pedestrians and vehicles. Dynamic performance indexes are concerned both in the dynamic design and dynamic loading test. However, a large number of researches about evaluation of dynamic responses are mainly related with railway bridges. Actually, for highway bridges, moving vehicle load is the main factor affecting dynamic damage of structures. Therefore, it has been investigated for the design and evaluation methods on dynamic performance of highway girder bridges subjected to moving vehicle loads.Based on modal synthesis method, the programs VBCVA(Vehicle- Bridge Coupled Vibration Analysis) and VCE(Vibration Comfort Evaluation) are developed by finite element softwares ANSYS and MATLAB. Also, the programs have been verified according to compare with results calculated from the software ANSYS and existing references.Natural frequencies of bridges, impact factor of bridges, vibration acceleration of bridges, and vibration acceleration of vehicles are proposed as indexes for dynamic performance of highway bridges from aspects of dynamic characteristics of bridges, dynamic responses of bridges, and comfort of pedestrians and moving vehicles, respectively. Meanwhile, estiamtion formulas of natural frequencies for common small and middle span continuous girder bridges with uniform cross sections are obtained by both theoretical derivation and numerical simulation, which will make up the shortcoming of existing codes. Additionally, differences between impact factors for different responses of various positions are discussed. It has been found that the impact factor of displacement is largest, and that of moment is larger, and that of shear force is smallest. The impact factor of displacement can be selected as design index for conservative consideration. But for evaluation, the impact factor of displacement and that of moment should be differed.The vehicle has been assumed as the constant force, the harmonic force, and the sprung mass, respectively. Main factors affecting dynamic responses of bridges are found and their relationships are studied according to theoretical derivation and numerical simulation. At last, the design method on dynami c performance of bridges based on simplified model has been proposed. As for the simplified model without damping, three main factors affecting impact factor are mass ratio between vehicle and bridge, ratio between disturbance frequency and bridge frequenc y, frequency ration between vehicle and bridge. For cases with different speeds, the impact factor decreases with increasing of damping ratio. And the slopes of them are almost the same.As for the safety of bridges, taking impact factor and vibration acce leration of the bridge as indexes of dynamic responses, main factors affecting dynamic responses of bridges are investigated by both traditional method(single factor method) and othogonal test method. Interaction between different factors can be well considered accoridng to othogonal test method. For both impact factor and vibration acceleration of the simply supported girder bridge, the interaction between pavement roughness and other factors is significant, especially for the interaction between pavement roughness and span length of the bridge. For both impact factor and vibration acceleration of the continuous girder bridge, interactions among pavement roughness, span length, ratio between side span and middle span, and spans number are important. In addition, the deficiency of estimation fomula for impact factor in current code has been pointed out, which is the function of only one parameter(natural frequency). When the fundamental frequency of the bridge is between 2Hz and 4Hz, the error is bigger.Based on some typical girder bridges and vehicles, the riding comfort of different bridges subjected to various vehicle types is studied. The results show that the influences of speed, pavement roughness, and vehicle weight on vibation acceleration of vehicle are related with vehicle type other than bridge type. Also, the comfort of moivng vehicle affected by other vehicles located on the same lane or adjacent lane is determined by the relationship between the vibration of bridge induced by other vehicles and the pavement roughness. If the vibration of the bridge is largely smaller than the pavement roughness, the interaction between different vehicles can be ignored.According to the analysis of vehicle vibration affected by different suspension stiffness, suspension damping, tyre stiffness, and tyre damping, some suggestions on vehicle design are given at last. The vibration of small car is little influenced by suspension damping. However, for large coach, the vibration acceleration decreases with increasing of suspension damping. Also, the vibration acceleration of vehicle decreases with increasing of suspension stiffness. The vibration of large coach is little influenced by tyre stiffness and damping. But, for small car, the vibration acceleration decreases with the increasing of the tyre damping, and increases with the increasing of the tyre stiffness.Existing codes for evaluating dynamic performance are discussed in depth. Dynamic responses of common girder bridges subjected to single vehicle and multi vehicles are investigated, respectively. If there are many vehicles in the transverse direction, the reduction factor of impact factor due to phase difference of bridge vibration induced by different vehicles should be considered. And the value of reduction factor is related with the bridge type, length, and pavement roughness, etc. Finally, some advices and the operation of evaluation method for dynamic performance of bridges based on dynamic loading test are obtained.The conclusions in this paper may offer some references for design and evaluation methods on dynamic performance of bridges and offer some basis for revisions of existing related codes. Meanwhile, there are great theoretical significances and academic values in this proposal for improving the design theories, avoiding the dynamic damages and ensuring the safe operation of bridge structures.