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The track structure has an effect on the dynamic characteristics of the bridge structure, which makes the seismic response of the bridge structure complicated. In this paper, an assessment method is proposed to evaluate the seismic risk of bridge structure, including over-limit risk and over-limit scale. Based on a newly proposed simplified model with high accuracy and high computational efficiency, a total of 3000 calculation conditions are analyzed and the sample stability of response is validated to ensure the reliability of the seismic risk analysis. The seismic risks in the longitudinal direction of the track-bridge model are compared with that of the trackless model, and the influence of the constraint effect of China Railway Track System II ballastless track structure on the seismic risk of the structure is obtained. The influence of the number of spans and PGA on track constraints is also revealed. The analysis results, based on a large number of samples, show that although track constraints can reduce the seismic risk of some phenomenon, such as girders' collision, the expected enhancement of longitudinal seismic safety of the system cannot be guaranteed in general, especially when the number of spans is large. However, it is noted that the track constraint can effectively control the over-limit scale of the system, which can reduce the expected repair cost after an earthquake. Therefore, it can be considered that the positive effect of the track constraints is more significant in economy than in structural safety when the bridge is subjected to a longitudinal earthquake excitation. [ABSTRACT FROM AUTHOR]

The structure of a high-speed railway (HSR) bridge with a track system is complex, and the computational efficiency of seismic analysis for a refine HSR bridge with a track system model (BM) is low. This study considers an HSR bridge with CRTS II slab ballastless track system as a case study and develops a simplified model (SM) based on a method for simulating track constraints with equivalent springs. To obtain the optimal equivalent springs stiffness of SM, a finite element structural parameters updating framework utilizing an improved genetic algorithm is established, and the updated SM's dynamic characteristics and seismic response are compared to BM's. In addition, it uses SM to analyze the over-limit state and applicability of the bridge structural essential components. The results indicated that the model simplification ratio of SM reaches 95%. The seismic response results of SM and BM accurately agree with an average relative error of 3% and a maximum relative error of less than 5%. The over-limit state of the structure seismic response tends to be stable as the number of ground motion samples increases. The shear and bending capacity of the pier and the deformation of the fixed support have a high over-limit risk under longitudinal seismic, which is worth noting in the engineering design. [ABSTRACT FROM AUTHOR]

Near-fault earthquakes can cause more intricate dynamic responses and potentially lead to severe damage to bridge structures compared to far-field earthquakes. In January 8, 2022, a 6.9-magnitude earthquake struck Menyuan, Qinghai Province, resulting in the disruption of the Lanzhou-Urumqi high-speed railway. The Liuhuanggou Bridge is the first high-speed railway bridge in the world to have undergone severe earthquake damage. In order to investigate the reasons for the seismic damage and girder falling mechanism, this study established a comprehensive nonlinear analysis model that considers the entire process from mild damage to complete failure of bridge components. Based on the LS-DYNA explicit dynamic analysis software, the model accurately simulates the collision, girder falling, and bearing damage. Based on limited field investigation data and combined with theoretical methods, reasonable seismic inputs are constructed, which accurately reproduce the bridge damage. The analysis results indicate that the girders and piers of high-speed railway simply supported bridges exhibit good seismic performance under seismic, while the bearings and shear keys are damaged. The seismic intensity (IX degree) significantly exceeded the fortification intensity (VII degree) of the bridge. The near-fault earthquake velocity pulse, inappropriate seismic measures, and nonuniform excitation of cross-fault earthquake are significant factors leading to the collapse of the Liuhuanggou Bridge. Further research is needed for the rational selection of bridge types in fault zones. The numerical results and conclusions of this study can provide a reference for the seismic design of high-speed railway simply supported bridges built near faults. [ABSTRACT FROM AUTHOR]

This paper proposes a modified method to select the optimal intensity measure (IM) in the seismic fragility analysis by the incremental dynamic analysis (IDA). The performance of an IM is evaluated by its efficiency, the efforts on fragility estimation, and sufficiency, the ground motion information represented by the IM. The results by IDA in various IMs are generally obtained by the conversion from that with the scaling factor IM using a given selected ground motions suite. The present study shows that the inter-IM conversion leads to an incompatibility of IM value range at different IDA curves, hindering the normalization for eliminating the difference of unit and range in various IMs. To tackle this issue, a normalization procedure is developed specifically for IDA, employed in the definition of a normalized and relative efficiency metric. Regarding the sufficiency of IM, the information-theory-based metric, the approximate relative sufficiency measure (aRSM), is adopted. It is calculated by the EDP-based approach, consistent with its original definition, rather than the IM-based approach in the literature. By the Bayes' theorem, the prerequisite to neglecting this inconsistence of the IM-based aRSM calculation is derived. It is the approximated equality in the occurrence probabilities of the two compared IMs. A case study is performed in a typical simply-supported high-speed railway bridge and track system to explore the IM performance in different structural components by local EDPs. The feasibility and validity of the proposed method is demonstrated, particularly for the normalization procedure and the EDP-based aRSM calculation. [ABSTRACT FROM AUTHOR]