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This research innovatively presents a multi-parameter seismic fragility analysis methodology anchored on Blending ensemble learning tailored for the high-speed railway track-bridge (HSRTB) system. Leveraging ensemble machine learning models, a seismic demand model encapsulating multiple earthquake intensity measures (IMs) is formulated. The research progresses to deduce the mean seismic fragility representations for single, dual, and composite earthquake parameters. Key findings of this investigation underscore that the advanced methodology aligns well with the Monte Carlo (MC) fragility, signifying its robustness. Crucially, earthquake IM parameters, Sa(0.5) and Sa(0.3)_ASI, emerged as the optimal choices for singular and dual-parameter seismic fragility delineations. Further enriching this research, a composite parameter optimization technique rooted in multi-CPU genetic algorithms is proposed. This avant-garde method remarkably minimizes the fragility estimation error, emphasizing its unparalleled efficacy in enhancing the precision of fragility evaluations. [ABSTRACT FROM AUTHOR]

High-speed railways are “lifeline projects” that shoulder the heavy responsibility of transporting relief supplies and medical forces for the first time after earthquakes. To ensure the train’s safety after earthquakes, it is of great urgency to ascertain a post-earthquake speed threshold. To that end, a target model for seismic irregularity emerges as a key parameter. In this paper, a null phase assumption-based technique for the mutual conversion between evolutionary power spectral density and non-stationary signal was proposed. Taking a high-speed railway track-bridge system as the research object, the target model of seismic irregularities was constructed based on the proposed technique. The rationality of the target model of seismic irregularities was verified, and the construction parameter settings were discussed. Moreover, a simplified frequency-domain fitting method for the target model of seismic irregularities was proposed based on the spectral decomposition theory. According to the research findings, the null phase assumption-based technique is capable of performing interconversion between seismic irregularity and its evolutionary power spectral density with satisfactory accuracy. It is recommended to set the minimum number of seismic irregularities, spatial sampling interval, hop size, and length of window function as 50, 0.25m, 1, and 40 to 100, respectively. [ABSTRACT FROM AUTHOR]

Track irregularity is an important factor affecting the train-track-bridge system interaction, and for the seismic design of the track-bridge system, the study of the law of the influence of track irregularity on the train dynamic effect is of great theoretical significance. Based on this, the finite element models of China railway track structure (CRTS) II plate high-speed railway simple-supported girder bridge and CRH2C high-speed train model are developed and compared with the shaking table test results to verify their validity. Considering track irregularity and earthquake randomness, a sample library of the train dynamic effect of the track-bridge system under earthquake is established, the probabilistic statistical characteristics of the train dynamic effect are analyzed, and the influence level of track irregularity on the train dynamic effect of the track-bridge system under earthquake is studied, and the standard value of track irregularity influence coefficient of train dynamic effect based on probability guarantee rate is proposed. The results indicate that: the influence of track irregularity on the seismic response of bridge structure is slight, with their greatest impact not exceeding 1%. However, it has a significant impact on the seismic response of track structures. The standard values of the sliding and mortar layers' track irregularity influence coefficients are 1.49 and 1.71, respectively, at 0.1 g. Under various peak ground acceleration (PGA) settings, the fasteners' standard values for the track irregularity influence coefficient are less than 1.1. Additionally, the degree of influence of track irregularity on the seismic response of track structure decreases significantly as the PGA increases. [ABSTRACT FROM AUTHOR]

The void of cement asphalt (CA) mortar layer will aggravate the abnormal vibration of high-speed trains at the void position and threaten the running safety. Few studies have been conducted on the effect of mortar layer void on the dynamic performance of slab track on bridges and the damage limits. Aiming at the practical problem of CA mortar layer void in track-bridge systems, a dynamic response calculation program for the train-track-bridge system is prepared based on the train-track-bridge interaction theory. A sample library of high-speed trains running performance indicators considering random initial track irregularity under CA mortar layer void conditions is constructed. The influence of void length on the dynamic performance of high-speed trains under two different CA mortar layer void conditions is investigated by means of sample upper bounds based on probability guarantee rate. The results show that the wheel load reduction rate and vertical acceleration of high-speed trains are more sensitive to changes in void length than the derailment coefficient. The effect of CA mortar layer void on the running performance index of high-speed trains is significantly enhanced after a certain critical void length value, railway maintenance should pay more attention to critical thresholds. The mid-span void and beam-end void length thresholds of CA mortar layer based on the probability guarantee rate decrease significantly with the increase in running speed while utilizing the wheel load reduction rate as the control index. However, the void position has little influence on the void length threshold. The void length threshold results can provide a basis for the CA mortar void limits. [ABSTRACT FROM AUTHOR]

China's high-speed railway (HSR) construction has gradually extended to earthquake-vulnerable areas in recent years. However, there is currently a lack of studies on the characteristics of earthquake-induced rail irregularities, making it impossible to ascertain the threshold of post-earthquake train running speed. Taking the multi-span HSR simply supported girder bridge of a CRTS II-type track system as the research focus, the distribution models for residual rail irregularity under random seismic actions have been analyzed in this study. The statistical distribution of the evolutionary power spectrum density (EPSD) of the residual rail irregularity set is also explored. Based on the obtained results, the EPSD of earthquake-induced rail geometric irregularity was calculated, and a time-domain sample for earthquake-induced rail geometric irregularity was developed. Finally, the equating method of earthquake-induced rail geometric irregularity under different span numbers was discussed, and a method for fitting earthquake-induced rail geometric irregularity was proposed. [ABSTRACT FROM AUTHOR]

Seismic duration significantly impacts the structural seismic response. In order to obtain a correct time history analysis result of structural seismic design, determining a reasonable seismic duration scheme is imperative. In this paper, a five-span simply supported beam bridge and China Railway Track System (CRTS) II slab ballastless track system is taken as the research focus. A finite-element model for a high-speed railway track-bridge system was developed. The influence mechanism of starting and end threshold on seismic response was analyzed. Different bracketed duration schemes were put forward, and their applicability was assessed. The optimal bracketed duration scheme of time history analysis for a high-speed railway track-bridge system was determined. The results indicate a response curve deviation between the full duration and partial duration in the starting part due to the inconsistent structural movement trends and a deviation in the end part because of the structure entering a free vibration state in advance. When calculating the seismic peak response, the starting threshold A and end threshold B at both ends should be taken as5% of peak acceleration. When calculating seismic residual response, the starting threshold C at the left end should be taken as 25% of peak acceleration. [ABSTRACT FROM AUTHOR]

Time-history analysis is currently the main approach to the aseismic design of high-speed railway, and the time step significantly affects the calculation efficiency and accuracy. A 5-span high-speed railway bridge-track system was used as an example in this study. The effect of different fixed time steps on maximum seismic response was analyzed. An approach of improved optimal fixed time step was proposed by introducing maximum frequency and sampling frequency multiple. According to the research findings, for a specified ground motion record, the calculation error and efficiency of the improved fixed time step decrease with the increasing sampling frequency multiple. For a specified sampling frequency multiple, the calculation efficiency decreases with the increasing high-frequency components in ground motion record. The calculation error of secondary sampling signal for random ground motion records approximately obeys the normal distribution, and a stable and deterministic upper bound can be constructed based on the random sample of calculation error. The sampling frequency of secondary sampling signal in aseismic analysis for high-speed railway bridge-track system should be set as 4fmax. The improved optimal fixed time step should be set as (4fmax)−1 to simultaneously satisfy both the calculation error and efficiency requirements. [ABSTRACT FROM AUTHOR]

The longitudinal seismic response reduction in railway bridges is worthy of attention because it relates to the engineering cost. It is difficult to further explore the reduction effect because the complexity of traditional analysis model leads to the low efficiency in parametric analysis. In this paper, the track structure was simplified, and a simplified calculation model was established. The parametric analysis considering the span number and pier height on the simplified model seismic responses was carried out. The results showed that the reduction effect was extensive and there was a great internal force transferred to the subgrade-track structure. [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]

Bridge dislocations are hard to be voided in the operation of high-speed trains, which will lead to abnormal vibration of trains. In this study, the train–track–bridge interaction theory was adopted to develop and validate a three-dimensional coupled train–track–bridge vibration model using the finite element method. Thereafter, the influence of the dislocation amplitude on the dynamic performance of high-speed trains under five typical bridge additional transverse dislocation modes was investigated utilizing the overrun probability analysis approach. The results showed that the dislocation amplitude significantly affects the transverse dynamic behavior of high-speed trains, but it has a considerably smaller impact in the vertical direction when exposed to five typical bridge transverse dislocation modes. Moreover, under constant dislocation amplitude, the transverse parallel misalignment of the girders has the greatest influence on the running performance of high-speed trains. Although all the running performance indicators did not exceed the limit in the transverse unilateral cornering condition of the girder, speed control was suggested to guarantee operational stability. At various vehicle speeds, dislocation thresholds for the transverse symmetrical cornering, transverse parallel cornering, transverse misalignment, and transverse parallel misalignment were provided based on probabilistic guarantee rates utilizing the transverse Sperling index as the control index. [ABSTRACT FROM AUTHOR]

Based on the finite Fourier series and the energy variation principle, an analytical calculation method for the dynamic response of multi-span bridge-track structure systems under the action of moving load series is proposed. The applicability of this method is verified by the comparison of the results with the analytical and numerical approaches. Based on the proposed method, the dynamic response of double-layer and four-layer beam models, commonly used in railway bridge systems is solved, and the calculation formulas of resonance and vibration cancelation velocity of multi-layer beam systems are proposed. The influence of the span number of simply supported beams and the length of subgrade sections on the dynamic response of the four-layer beam system is investigated. The results show that the deflection of the dynamic response of double-layer and four-layer beam systems reaches the maximum value near the mid-span. These two systems have multiple critical speeds under moving loads. When the resonance and vibration cancelation of the multi-beam system occur simultaneously, the vibration cancelation plays a leading role. The span number of simply supported beams and length of subgrade sections influence the dynamic response of the four-layer beam system but have no significant effect on the critical velocity of resonance and vibration cancelation. At present, there is a lack of research on analytical or semi-analytical methods for the dynamic response of multi-span bridge-track structure systems. This research may provide a theoretical basis for the planning, design, operation, and maintenance of bridge-track structure systems to a certain extent. [ABSTRACT FROM AUTHOR]