Vehicle Response And Derailment Assessment On Bridges Considering Seismic Uncertainty

China now has the largest high-speed rail network in the world.Inevitably,a number of constructed and being planned high-speed railway lines cross seismic zones.The seismic design of bridge structures in railway engineering should not only meet the seismic requirements of the bridge itself,but also ensure the safety of the vehicles on the bridge during earthquakes.Therefore,the dynamic response of the vehicle bridge interaction(VBI)system and the running safety of the vehicles on the bridge have become the focus of seismic research in high-speed railway systems.This dissertation focuses on the mechanism of vehicle derailment during earthquakes,the vehicle response under composite random excitation and the simplified assessment of the running safety of vehicles on bridges during earthquakes.Firstly,the vehicle response characteristics and derailment behavior process under different frequency excitations are analyzed,then the vehicle running reliability under random composite excitation and the combination theory of extreme values of derailment index(wheel unloading)are studied.Finally,considering the uncertainty of ground motion,based on the concept of performance seismic design,intensity measures are introduced into railway engineering.The selection of optimal intensity measure is conducted.Based on the selected optimal intensity measure,a formula is proposed to evaluate the safety of vehicle running on bridges during earthquakes.The main tasks are as follows:(1)The effects of different frequency excitations on the vehicle dynamic response and derailment are studied.A derailment criterion based on relative wheel-rail displacement is used to assess the running safety of vehicle under earthquakes.Based on the seismic performance of the vehicle,the wheel-rail relative displacement derailment criterion is divided into three classes.The displacement amplitude limits of vehicle running under harmonic excitations of different frequencies are given.The differences of vehicle response and derailment mechanism under different frequencies are systematically studied.The results show that: under the low frequency excitation,the vehicle response is mainly in the lower center rolling mode,and the vehicle derailment is mainly in the rocking derailment;under the high frequency excitation,the vehicle response is mainly in the upper center rolling mode,and the vehicle derailment is mainly caused by the collision between the wheel and rail.Finally,the differences of vehicle response and derailment mechanism on bridges with different fundamental periods under earthquakes are compared.(2)A spectral representation-random function model is used to generate the random excitation.Uniform point-set selection is carried out in five dimensions using the number theoretic method.The probability density evolution method is used to analyze the vehicle running safety under the random excitations.The distribution of the extreme values of derailment indicators in different seismic intensity zones and the running safety of vehicles on the bridge are given.The analysis also focuses on the combination of extreme values of vehicle derailment indicators under random track irregularities and ground motions.Finally,the effect of random track irregularities on vehicle derailment under earthquakes is investigated at the probability level when evaluating vehicle derailment with wheel-rail displacement.(3)Probabilistic methods based on fragility analysis have been commonly applied in structural engineering,but it has not been applied to the evaluation of railway vehicle running safety.The selection of optimal ground motion intensity measures(IMs)is particularly important in fragility analysis.In this dissertation,eight typical IMs are chosen.Using the 22 far-field and28 near-field ground motion records recommended in FEMA P695 as input,the response of railway vehicles was analyzed using fully non-linear wheel-rail contact simulations.The correlation between eight intensity measures and vehicle derailment was evaluated based on two types of optimal IMs evaluation criteria,PSDM and IDA method,respectively.The two types of IM selection methods are mainly compared and analyzed in terms of selection results and calculation workload to select a better method.(4)A simplified method for the excitation of bridges to vehicles during earthquakes is presented.This simplified method decouples the vehicle-track subsystem from the bridge subsystem.The decoupling method divides the problem of vehicle derailment on bridges under earthquakes into two problems:(a)bridge vibrations due to the earthquake;(b)vehicle derailment due to bridge vibration excitations.The former is a structural vibration problem;the latter is a vehicle-track dynamic problem.This decoupled method is convenient for engineering applications.The fundamental modes and frequencies of the vehicles are analyzed.A comparative study of the vehicle response on bridges with different frequencies during earthquakes is presented.The correlation between vehicle derailment and IMs of ground motions or bridge deck motions is analyzed for vehicles running on bridges under seismic conditions.Optimal IM for predicting vehicle derailments on bridges of different frequencies are selected based on the IMs evaluation criteria of the IDA method.(5)Based on the vehicle-bridge decoupling method under earthquakes,an excitation model that considers both the lateral and torsional responses of the bridge deck is proposed.This excitation model(LTE model)is an improvement of the excitation model(LE model)recommended in the Japanese specification.Using the safety limits of the optimal IM for bridge deck vibrations obtained by the VBI model during earthquake as the benchmark,the safety limits obtained by the LTE model and the LE model were compared.The safety limits obtained by the LE model are larger than those obtained by the VBI model.That is,ignoring the torsional excitation of the bridge deck will underestimate the derailment risk of vehicle running on bridges during earthquakes.The safety limits obtained by the LTE and VBI models are in good agreement,demonstrating the applicability of the LTE model.Subsequently,the physical mechanism of vehicle derailment caused by the combined effect of lateral and torsional excitation of the bridge deck is investigated.The results show that the contribution of the two excitations to vehicle derailment during earthquakes is related to the bridge frequency,and a functional expression for the correlation between the contributions of the two excitations to vehicle derailment is given.Based on the selected optimal intensity measure,a formula is proposed to evaluate the running safety of vehicles on bridges during earthquakes,which takes into account both the lateral and torsional excitations of the bridge deck to the vehicle.The accuracy of the formula is verified compared with the results of vehicle-bridge coupling simulation during earthquakes.