Short Span Railway Bridges Research Articles

Stiffness contributions of ballast in the context of dynamic analysis of short span railway bridges

Railway bridges are subjected to intensive dynamic loading. In structural calculations, the resulting response is estimated by dynamic calculations. As the basis of these calculations, the system's natural frequencies are to be numerically determined. Various studies have shown that a discrepancy between these measured natural frequencies and the numerical determination often occurs for short-frame bridges. In common practice, the ballast is assumed to be non-contributing to the global structural stiffness. In some cases, rheological models are developed to consider possible ballast stiffnesses based on small-scale model tests. The resulting spring-damper models are based on empirical values and are not directly related to the material behaviour of ballast.This paper discusses the ballast stiffness at the material level using relationships known from soil dynamics. The research shows that the material properties of the ballast can be determined in terms of stress and shear strain dependence. An equivalent ballast stiffness can be determined and considered in dynamic calculations for a known stress state and corresponding superstructure accelerations. The paper presents an approach for determining vertical and horizontal ballast stiffness according to shear strains and superstructure accelerations.This developed approach is validated by experimental tests using a small-scale bending beam and accompanying numerical calculations.Furthermore, a parameter study shows that the stiffness of the ballast does not contribute significantly to the global bending stiffness. Thus, the first bending natural frequency is hardly influenced by the ballast track.

Stiffness Contributions of Ballast in the Context of Dynamic Analysis of Short Span Railway Bridges

Stiffness Contributions of Ballast in the Context of Dynamic Analysis of Short Span Railway Bridges

On the denoising of structural vibration response records from low-cost sensors: a critical comparison and assessment

Due to the increasing quest of adopting low-cost sensors in structural health monitoring (SHM) processes, which may lead to detecting signals contaminated by significant levels of noise, the need to devise appropriate and effective denoising strategies, at the post-processing stage, is becoming more and more essential. Among several approaches proposed in the literature, it has been demonstrated that the employment of discrete wavelet transform (DWT) as a multi-rate filter bank, as well as the use of singular value decomposition (SVD), may result to be quite effective in signal denoising within various research fields, as biological, acoustic and mechanical. Here, DWT- and SVD-based denoising techniques are first independently reconsidered and reimplemented, aiming at exploring their optimal calibration in purifying noise-corrupted vibration response signals encountered in civil engineering applications. Then, a systematic performance evaluation is provided within a comparative framework, developed at an increasing level of noise affecting the measurements, in terms of noise-to-signal (N/S) ratio. In the study, two specific classes of synthetic response signals are first considered, namely earthquake and ambient vibration signals, since they may be assumed as representative of more general non-stationary and stationary signal typologies, respectively. To achieve a complete description of the clarified signal, strengths and weaknesses of the two denoising approaches are explored, in both time and frequency domains. The results prove the effectiveness of the analyzed implementations, especially in purifying seismic response signals, while some limitations may arise concerning the treatment of ambient vibration signals, in particular for the DWT-based denoising technique. Finally, a real case study is analyzed, where both denoising approaches are adapted and employed for clarifying acceleration signals detected on a modern short-span railway bridge, with rather satisfactory results, for both techniques.

Testing of continuous shear connector

Composite steel-concrete bridges are one of the most recent additions to the bridge science. By joining the two of the most commonly used materials it is possible to use their strengths combined. This was firstly presented for composite ceilings. It was assembled by encasing the whole I-beams into the concrete deck. Nowadays, it can also be used for short span railway bridges. Ongoing research at Technical University of Košice (TUKE), Faculty of Civil Engineering, is currently trying to prove the unnecessity of usage of the entire I-beam as well as the entire block of concrete by encasing only part of the beam, the so called perforated steel strip, into the concrete.

FEM analysis of steel-concrete composite bridges with encased perforated steel strips

One of the newest contributions to the science of bridges are steel-concrete composite bridges with encased perforated steel strips. Their main advantages are their low height, low usage of materials and therefore lower cost of the bridge. They are mainly used as short span railway bridges (under 20m of length). Main focus of today researchers and structural engineers in this area is to improve the geometry of the steel strip – and therefore find new types with lower usage of material and higher shear strength. At Technical University of Košice, Faculty of Civil Engineering, several new types have been developed and tested. In order to properly measure the shear strength, the push-out tests had to be carried. They track the dependence between load (typed by loading conditions) and slip. These experiments were performed at Laboratory of Excellent Research in Košice. In this article is closely described software modelling of a strip developed at Faculty of Civil Engineering, the experiments performed as well as the results found.

A Stochastic View on the Effect of Random Rail Irregularities on Railway Bridge Vibrations

This contribution studies dynamic amplification factors that consider the impact of random rail irregularities on the deflection and acceleration response of simple railway bridges. Response statistics for a set of plane fully coupled bridge–vehicle interaction models with random irregularity profiles are derived by Monte Carlo simulations. The underlying samples of random irregularity profiles are generated from a power spectral density function, assuming that the uneven rail surface is governed by a Gaussian stochastic process in space. A modified bootstrap method is used to identify the required sample size of profiles for sufficiently accurate description of the random response. On the basis of this pilot study, a parametric response examination of short span railway bridges with variable length, fundamental frequency, structural mass and damping, and track quality in a range commonly observed in real structures is conducted. A statistical representation of the uncertain bridge response is employed for analytical modelling of the response amplification due to random rail irregularities considering limit state-based exceedance probabilities. The results are compared to outcomes of existing simplified approaches in design codes used to take into account the effect of rail irregularities on the dynamic bridge response, showing the improvement of the developed framework.

A case study of dynamic behaviour of short span concrete slab bridge reinforced by tire-derived aggregates as sub-ballast

ABSTRACTIn the current study, the dynamic behaviour of a 6-m short span railway bridge with the concrete deck was investigated with and without 200 mm thickness tire-derived aggregate (TDA) layer as a shock absorber. The dynamic performance of the bridge was assessed under GT26 diesel loads for two moving speeds of 20 and 50 km/h in the field. The results showed an increase in track–bridge natural frequency from 20 to 23 Hz due to TDA layer installation. Moreover, maximum acceleration of sleeper and the maximum acceleration along with longitudinal strain of the bridge deck have been decreased by 71%, 40%, and 58%, respectively, for the speed of 20 km/h versus to 63%, 33%, and 58% at the speed of 50 km/h.

An equivalent additional damping approach to assess vehicle-bridge interaction for train-induced vibration of short-span railway bridges

Constant moving load models are widely adopted in the dynamic analysis of railway bridges under moving trains. However, the use of this simple model may overestimate the resonant response of simply supported bridges if the vehicle-bridge interaction (VBI) effects are neglected, particularly for short spans. To account for the VBI effects, Eurocode 1 allows engineers to consider an additional amount of structural damping which depends on the bridge span. This method is the so-called Additional Damping Method (ADM), and was formulated in order to provide a conservative prediction of the interaction benefit. Nevertheless, the Additional damping method may sometimes yield to an unsafe prediction of the bridge peak response. Considering the interaction benefits, an alternative analytical approach based on an equivalent VBI model under resonant excitations is presented in this investigation. The key parameters dominating the additional damping problem are subsequently identified. According to the numerical demonstrations supplied, the presented approach provides insights that enable an accurate prediction of the additional amount of damping needed in order to account for VBI effects on short simply-supported railway bridges.

Reduction of Vehicle‐Induced Vibration of Railway Bridges due to Distribution of Axle Loads through Track

Short span railway bridges are prone to resonate caused by dynamic train axle loads, which were usually modeled as moving point loads on the bridge in many numerical studies. In reality, the axle weight of the train is a spread load for the bridge deck because of the transfer of the track structure. Previous numerical studies indicated that the spread axle load distributed through the track structure significantly reduces bridge responses compared to the point load model. In this study, the reduction effect is investigated analytically by solving the moving load problem for both the point load and the spread load cases. The analytical solution reveals that bridge responses from the spread load model can be obtained by filtering bridge responses from the point load model. The filter function is exactly the Fourier transform (FT) of the load spreading function. Based on this relationship, a reduction coefficient reflecting the load spreading effect on bridge responses is derived. Through numerical examples, the accuracy of this proposed reduction coefficient is validated not only for the moving load models but also for vehicle‐bridge interaction (VBI) problems.

Rehabilitation and Strengthening of Aged Steel Railway Bridges in Japan

In Japan, nearly a half or even more of the existing bridges with a span exceeding 15 meters were predicted to be over 50 years old in next 10 years. Appropriate repair, strengthening, or replacement work should be performed on aged steel bridge structures to ensure their performance in service condition. A novel strengthening method using rubber-latex mortar, glass fiber reinforced polymer plates, lightweight rapid hardening concrete, and reinforcement bars is proposed for strengthening short-span steel railway bridge superstructures and for improving the seismic performance of aged column structures. To confirm the effectiveness of the strengthening method, loading tests were performed on test specimens for short-span steel railway bridges, longitudinal-lateral beam connections as well as steel columns. Numerical models were also built to simulate the structural behavior of both original and strengthened steel structures. According to the obtained results, the present renovation method can significantly enhance both rigidity and load carrying capacity of short-span steel bridge superstructure, connections, and column structures, resulting in the extension of the residual fatigue service life of bridge tarp structures and improvement of seismic performance of column structures.

Vehicle–Track–Bridge Interaction Modeling and Validation for Short Span Railway Bridges

Railway bridges are critical in the transportation network and vital to the profitability of the industry. Thousands of U.S. bridge spans of more than 50 years of age are still in service. Transportation Technology Center, Inc.’s current work under the Association of American Railroads Strategic Research Initiatives Program on bridge life extension focuses on the effects of increased axle loads, on extending the safe service life of existing steel bridge spans, and on onboard inspection of bridge structural integrity. The program includes various tests at the Bridge Deflection Test Facility (BDTF) at the Transportation Technology Center, Pueblo, Colorado, as well as vehicle–track–bridge interaction modeling. This paper presents simulation and test results of a freight car and locomotive running on a railway bridge located at the BDTF. Simulation and test results of an onboard system installed on an instrumented freight car indicated that it is a useful tool for identifying some bridge condition issues. The BDTF provides adjustable bridge strength and geometry conditions. Various tests were conducted on the BDTF to investigate the potential for using onboard technology to detect bridge impairment or changes in bridge behavior. Test results were used to validate the NUCARS three-layer track model of the BDTF. Experimental and analytical case studies were conducted to develop onboard systems for dynamic inspection of bridges under varying loads.

Monitoring of strain profile variations in the RC slab of a short-span railway bridge

A novel monitoring scheme oriented towards the structural examination of reinforced concrete slabs subject to railway traffic action is presented. Monitoring is based on the measurement of the mean axial strain profile variation in the cross-section of the slab. The strain profile is reconstructed under the assumption of plane sections by measuring the variation of two distances between two pairs of points materialized at two different levels under the slab on rigid attachments fixed on the slab. The distance variations are measured by means of strain gages measuring the deformation of two very thin strips connecting the two attachments. The implementation of the above scheme on a short-span railway bridge for a period of 7 months and the processing of the acquired signals are presented in detail. Some interesting monitoring records are provided over which the efficiency of the proposed scheme is discussed.

Efficient methodology for the probabilistic safety assessment of high-speed railway bridges

The efficiency of different probabilistic methodologies for the safety assessment of short span railway bridges is compared in the current paper. Two different simulation methods, namely the Monte Carlo and the Latin Hypercube, are combined with two different procedures to enhance the efficiency of the assessment. One of the methods is a tail modelling approach based on the extreme value theory that uses the Generalized Pareto Distribution to model the tail of the obtained distribution. The other one is an Enhanced Simulation procedure which uses an approximation procedure based on the estimates of the failure probabilities at moderate levels for the prediction of the far tail failure probabilities by extrapolation. A composite bridge with six simply supported spans of 12m and ballasted track is selected as case study for the crossing of the TGV-double high-speed train. The variability of parameters related to the bridge, the track and the train is taken into account along with the existence of track irregularities. The running safety of trains due to loss of contact between the wheel and the rail and the track instability due to excessive deck vibrations are the two safety criteria analyzed, providing examples of limit state functions with different degrees of complexity. The obtained results are extremely promising and indicate the feasibility of the application of this type of methodology in common practice due to the very reasonable computational costs that are required.

Model Updating of Railway Bridge Using In Situ Dynamic Displacement Measurement under Trainloads

To address the need for monitoring aging railway bridges, this paper presents a finite-element (FE) model updating approach using time-domain optimization based on in situ measurement of the bridge’s dynamic displacement histories under trainloads. Field tests are performed on a short-span plate girder railway bridge to measure its displacement response to freight trainloads with different speeds using a low-cost remote vision sensor recently developed by the authors’ group. A FE model of the bridge is developed considering train–track–bridge dynamic interactions. Sensitivity analysis is carried out to investigate the intrinsic effects of parameters of the train, track, and bridge subsystems on the dynamic response of the bridge. It is found that the bridge displacement response is primarily sensitive to the stiffness of the bridge, whereas the acceleration response is affected by many other parameters. This justifies that the bridge dynamic displacement is more suited than acceleration for updating the railway bridge stiffness. Consequently, a model updating approach is proposed using the measurement of bridge displacement response to trainloads. This approach is applied to the short-span bridge to first identify the train speed, followed by the bridge stiffness by minimizing the error between the measured and computed displacement time histories. Furthermore, this paper investigates the frequency characteristics from both the measured and computed displacements. The analysis shows that the trainload does not excite the bridge’s natural modes of vibration because the frequency is much lower than the bridge’s natural frequencies. Therefore, it is difficult to use the modal identification-based FE model updating methods based on the dynamic response measurement. This is commonly the case for short-span railway bridges, which tend to have high natural frequencies. The proposed periodic displacement measurement and time-domain FE model updating can be developed into an effective tool for long-term structural health monitoring of short-span railway bridges.

Effect of Axle Load Spreading and Support Stiffness on the Dynamic Response of Short Span Railway Bridges

In dynamic analyses of railway bridges, the train axle loads are often modeled as moving point forces. However, these point forces are further distributed to the bridge structures through the ballast, which can lead to large reductions in the bridge response, especially for short span bridges. For this reason, the Eurocode prescribes distribution of the axle loads over three adjacent sleepers. In this paper, the axle load distribution is first studied using a plane finite element analysis, and based on this, a triangular load distribution is proposed. Then, numerical simulations are performed to compare the effect of this load distribution with the one prescribed by the Eurocode. Both simply supported bridges and bridges with integrated backwalls, all with span lengths less than 10 m, are studied. For the latter bridges, the effect of the stiffness of the foundation has been studied by adding springs at the supports.

Probabilistic safety assessment of a short span high-speed railway bridge

A probabilistic methodology for the safety assessment of short span railway bridges for high-speed traffic is presented. The purpose is to create a simple, efficient and automatic procedure that allows identifying the critical train speeds over the bridge and assessing the safety of the train–bridge system. The methodology combines simulation techniques with the extreme value theory in order to minimise the required computational time and guarantee accurate results. Stochastic simulation is employed as it allows reflecting the real variability of the parameters that characterise the dynamic response of the train–bridge system. As a case study the safety of a short span filler beam railway bridge crossed by a TGV double train is assessed. The behaviour of short span railway bridges is known to be particularly difficult to predict due to the complexity of the coupled train–track–bridge system, as well as for being particularly sensitive to resonant phenomena. The variability of the bridge, the track and the train was accounted for, as well as the existence of track irregularities. The proposed probabilistic methodology proved to be efficient as it allowed identifying the critical train speeds with a reduced computational cost. Furthermore, for these critical train speeds two different methods were used to estimate the probability of failure. The obtained results showed a good agreement guaranteeing the accuracy of the methodology.

Residual life of historic riveted steel bridges: an analytical approach

As a consequence of the increasing traffic demand, the life of a large stock of riveted bridges that still exist throughout the world must be extended, ensuring at the same time that safety is not compromised for economic reasons. As fatigue life estimation of metal historical bridges is a key issue for managing cost-effective decisions regarding rehabilitation or replacement of existing infrastructure, there is a need to estimate how long these structures could remain in service. In this paper, a fatigue assessment of a common short-span railway bridge is analysed and assessed. Based on analytical and parametric studies, the remaining life of the bridge is presented and discussed.

Measuring Deflections of a Short-Span Railway Bridge Using a Robotic Total Station

AbstractA robotic total station (RTS) [or robotic theodolite or theodolite positioning system (TPS)] was used for the first time to measure the deflections of a short-span bridge in response to passing trains. The RTS measurements aimed to a reflector which was set on one of the midspans of the historical Gorgopotamos Bridge in Greece permitted to identify the measurement noise (apparent displacements) up to ±1.3 mm when no trains were passing, and deflections with peaks of 2.5–6 mm during intervals when small or larger trains were passing. These results confirm previous experiments and indicate that, under certain conditions (mostly favorable atmospheric conditions), an RTS can be used for monitoring dynamic displacements of relatively stiff bridges, and as a useful tool for structural health monitoring.

Response of a historical short-span railway bridge to passing trains: 3-D deflections and dominant frequencies derived from Robotic Total Station (RTS) measurements

Based on measurements with a Robotic Total Station (RTS), we computed the displacement characteristics (pattern, amplitude, frequency) of the Gorgopotamos Bridge, a short-span, historical bridge in central Greece, in response to passing trains. Vertical deflections of selected points of the midspan were found to range up to 6mm for heavy cargo trains, exceeding the noise level of 1–2mm, and were analyzed in nearly semi-static and dynamic components. Dominant frequencies of the bridge span, in the range 3.18–3.63Hz, were also computed. Evidence of possible lateral and longitudinal deflections was also found. Measurement noise and quality of results were controlled on the basis of the analysis of apparent displacements of reference points during excitation and of non-excitation intervals, as well as on the basis of structural constraints. The amplitude of measured deflections was found to correlate with the load, and a somewhat distinct oscillation waveform was observed in the cases of large oscillations, despite clipping affecting highly dynamic RTS recordings. Observed deflections are large for modern standards, but reasonable for a >100years old bridge partly destroyed and rebuilt twice during World War II.

Safety assessment of a short span railway bridge for high-speed traffic using simulation techniques

The dynamic behaviour of railway bridges in high-speed lines has been the subject of several studies in recent years. Nevertheless, most of the work was developed assuming a deterministic perspective. In this paper a probabilistic approach is used to analyse the sensitivity of the dynamic response of a small span bridge due to the variability of the main structural parameters. A filler beam railway bridge was selected as case study and random variables were identified. A variable screening procedure was performed in order to determine which variables had a higher influence on the dynamic response of the bridge. Simulation techniques were applied to analyse the variability of the dynamic response of the bridge, as these methods allow an accurate consideration of the randomness of the main structural parameters. Based on the simulation results, a traffic safety assessment of the bridge was performed. As conclusion, the safety assessment results are discussed, with special focus on the comparison between the results obtained when adopting the European standards approach and when considering a probabilistic approach.