Finite Element Multi-scale Time-varying Boundary Approximation Method And Its Application To Local Dynamic Analysis For Steel Railway Bridges

Steel bridges have been applied in infrastructure constructions of railway transportation in China,due to their excellent material properties,high strength-weight ratio,excellent spanning capacity and easy construction,etc.Especially with the drops of steel prices and the occurrence of new structural forms,such as orthotropic steel deck system and overall node plate,steel bridges have good application prospects.The loads and environmental factors causing local vibrations usually maintain in large scales,while the damage or cracks generally occur in micro scales.Traditional train-bridge coupling vibration analysis pay close attentions to dynamic responses of the integral bridge.The significant local dynamic effects cannot be precisely taken into account because of excessive simplifications on local complex details like orthotropic steel deck system.In addition,the components connections and the welds of steel bridges are complex and numerous,local vibrations induced by the high-speed and heavy loads may easily bring fatigue damage to the mentioned key parts,which eventually cause cracks and lead to the losses of their service function.Thus,multi-scale synchronization analysis and precise calculation of dynamic responses and fatigue damages for steel railway bridges are of great significance for monitoring the bridge health status and ensuring their service safety.Therefore,the multi-scale time-varying boundary approximation method is proposed in this paper.Meanwhile,an analytical framework of dynamic responses calculation and fatigue performance evaluation for the local structural details of steel bridge is constructed.Moreover,the proposed method is verified by the experimental data and the effectiveness of this method is discussed as well.The main work and innovations in this paper are as follows:(1)A simplified model of a long-span cable-stayed steel bridge for train-bridge dynamic analysis and its engineering applicationA three-dimensional vehicle model is established using multi-body dynamics,and a bridge model is set up by finite element method,in which the quality and stiffness of bridge deck system is simplified with proper equivalence.Combining the assumed wheel and rail interaction relationship,the dynamic motion equation of the train-bridge system is built and solved through the iteration method.Taking the Pingtan Highway-railway Bridge in the Fuzhou-Pingtan Railway Line as an example,dynamic responses of the bridge are calculated when the train passing through and the whole process of the interacted system is simulated.(2)Multi-scale time-varying boundary approximation method.Based on the structure dynamics and finite element method,a multi-scale time-varying boundary approximation method,which is used to solve local dynamic effects of complex structure systems in steel bridges,is proposed.The methodology is validated through calculations of spatial beam and shell element models.Taking the of orthotropic steel bridge deck system of the Pingtan Bridge as an example,local dynamic responses are analyzed using the proposed method.(3)Experimental validation of the multi-scale dynamic boundary approximation method and effectiveness study.An experiment using a steel beam in structure laboratory is carried out and dynamic responses of the beam induced by a force-hammer are measured.Meanwhile,based on the proposed methodology,dynamic responses at specific locations of the identical finite element beam model are calculated.Comparisons between the calculated and the experimental dynamic responses illustrate that the proposed method has the characteristics of high precision and less calculation.Moreover,the effectiveness of the method is discussed in the end.(4)Validation by field tests and calculation of bridge dynamic stress.The measured data from dynamic and strain-stress tests of the Baihe Bridge in Beijing-Tongliao Railway Line are used for validations of the proposed method.Dynamic stresses of local key components of the bridge are calculated based conversions between different element types and corresponding relationships of displacements.The calculation method for local dynamic stress with various element types is proposed.(5)Mechanical properties and fatigue performance evaluation of orthotropic steel bridge deck system.Design parameters and mechanical characteristics of orthotropic steel bridge deck system are summarized.Taking the aforementioned system of the Pingtan Bridge as an example,fatigue loads models and operation conditions are predicted according to the existing other railway lines.Considering the train-bridge interacted vibration,fatigue dynamic stresses of different structural details of the segment are identified using the proposed method.According to the recommended S-N nominal-stress approach in China Design Code,equivalent stress,which aims for calculation the fatigue damage accumulation,is applied for fatigue damage analysis.Subsequently,the remaining service life of the bridge is estimated.