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Essential modelling details in dynamic FE-analyses of railway bridges
KTH, School of Architecture and the Built Environment (ABE), Civil and Architectural Engineering, Structural Engineering and Bridges.
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The increased need to reduce the use of fossil fuels imposes higher demands on the efficiency of rail transportation. Therefore, an improved knowledge regarding the dynamic properties of railway bridges and infrastructure for railway traffic in general is required. Typically, increased train speed, longer trains and increased axle loads increase the dynamic response in railway bridges. Modelling details for bridge structures such as the flexibility of the foundations, radiation damping in the subsoil and the embankments as well as hysteretic effects in bridge bearings and the track superstructure are typically neglected. The reason for this is that suitable models which consider the influence of such effects in engineering calculations have not yet been implemented in the effectual design codes. This thesis is mainly based on a case study of a ballasted, simply supported steel-concrete composite bridge, which shows a considerable variation in the natural frequencies and damping ratios depending on the amplitude of vibration. Furthermore, the natural frequencies were found to increase significantly during the winter. It is well known that the dynamic properties of typical civil engineering structures are dependent on the amplitude of vibration. However, the fact that certain railway bridges exhibit such non-linear behaviour also for very small amplitudes of vibration has been shown only during later years. This has been verified by means of measurements of the free vibrations after train passages on three typical Swedish beam bridges for railway traffic. Possible sources to this amplitude dependency have been identified primarily in the bridge bearings and the track superstructure. Models of these structural components, based on the so called Bouc-Wen model, have been implemented in a commercial finite element program and was used in a preliminary study. The results indicate that roller bearings and pot bearings can give rise to a non-linear mode of vibration, characterised by two different states. At very small amplitudes of vibration (. 0:1m=s2), no movement over such bearings occur (state 1) since their initial resistance to motion is not overcome. Depending on parameters such as the longitudinal stiffness of the foundations and substructures, the beam height over the supports as well as the bearing type, there is an amplitude of vibration at which the initial resistance to motion is completely overcome (state 2). The bearings are then free to move, with a resistance characterised by the kinematic friction (pot bearings) or the rolling resistance (roller bearings). During the transition from state 1 to state 2, the frequency decreases continuously towards an asymptotic value and the damping initially grows considerably, from a value which corresponds quite well to the recommendations of the Eurocodes and then returns to a value similar to that in state 1. The preliminary study indicates that it is possible to design certain bridges so that this increase in damping is optimal over the relevant range of amplitudes of vibration.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. , x, 58 p.
Series
Trita-BKN. Bulletin, ISSN 1103-4270 ; 120
National Category
Infrastructure Engineering
Research subject
Järnvägsgruppen - Infrastruktur
Identifiers
URN: urn:nbn:se:kth:diva-132298OAI: oai:DiVA.org:kth-132298DiVA: diva2:659471
Public defence
2013-11-01, Kollegiesalen, Brinellvägen 8, KTH, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

QC 20131025

Available from: 2013-10-25 Created: 2013-10-25 Last updated: 2013-10-25Bibliographically approved
List of papers
1. Application of the continuous wavelet transform on the free vibrations of a steel-concrete composite railway bridge
Open this publication in new window or tab >>Application of the continuous wavelet transform on the free vibrations of a steel-concrete composite railway bridge
2011 (English)In: Engineering structures, ISSN 0141-0296, E-ISSN 1873-7323, Vol. 33, no 3, 911-919 p.Article in journal (Refereed) Published
Abstract [en]

In this article, the Continuous Wavelet Transform (CWT) is used to study the amplitude dependency of the natural frequency and the equivalent viscous modal damping ratio of the first vertical bending mode of a ballasted, single span, concrete steel composite railway bridge. It is shown that for the observed range of acceleration amplitudes, a linear relation exists between both the natural frequency and the equivalent viscous modal damping ratio and the amplitude of vibration. This result was obtained by an analysis based on the CWT of the free vibrations after the passage of a number of freight trains. The natural frequency was found to decrease with increasing amplitude of vibration and the corresponding damping ratio increased with increasing amplitude of vibration. This may, given that further research efforts have been made, have implications on the choice of damping ratios for theoretical studies aiming at upgrading existing bridges and in the design of new bridges for high speed trains. The analysis procedure is validated by means of an alternative analysis technique using the least squares method to fit a linear oscillator to consecutive, windowed parts of the studied signals. In this particular case, the two analysis procedures produce essentially the same result.

Keyword
Railway bridges, Steel concrete composite bridge, Train induced vibrations, Continuous wavelet transform, System identification
National Category
Civil Engineering
Identifiers
urn:nbn:se:kth:diva-31633 (URN)10.1016/j.engstruct.2010.12.012 (DOI)000287901300021 ()2-s2.0-79251597214 (Scopus ID)
Note
QC 20110322Available from: 2011-03-22 Created: 2011-03-21 Last updated: 2017-12-11Bibliographically approved
2. Influence of non-linear stiffness and damping on the train-bridge resonance of a simply supported railway bridge
Open this publication in new window or tab >>Influence of non-linear stiffness and damping on the train-bridge resonance of a simply supported railway bridge
2012 (English)In: Engineering structures, ISSN 0141-0296, E-ISSN 1873-7323, Vol. 41, 350-355 p.Article in journal (Refereed) Published
Abstract [en]

Previous experimental work has identified Variations in the natural frequency and the modal damping ratio of the first vertical bending mode of vibration of a simply supported, single span steel-concrete composite bridge. It was found that the natural frequency decreased and the modal damping ratio increased with increasing amplitudes of vibration. This paper illustrates the influence of these variations on the train-bridge resonance of this particular bridge by means of a non-linear single degree of freedom system, based on the previously mentioned experimental results. As one might expect, the results indicate that the influence of the increasing damping ratio leads to a considerable decrease in the resonant amplitude whilst the decreasing natural frequency decreases the critical train speed at which resonance occurs. Further studies along this line of research may help us reduce the uncertainties in dynamic assessments of existing bridges based, on dynamic measurements and improve our understanding of the dynamic properties of railway bridges in general.

Keyword
Railway bridges, Dynamics, Non-linear stiffness and damping, Train-bridge resonance
National Category
Infrastructure Engineering
Research subject
Järnvägsgruppen - Infrastruktur
Identifiers
urn:nbn:se:kth:diva-70019 (URN)10.1016/j.engstruct.2012.03.060 (DOI)000306200200031 ()2-s2.0-84860510290 (Scopus ID)
Note

QS 20120328. Updated from submitted to published.

Available from: 2012-01-30 Created: 2012-01-30 Last updated: 2017-12-08Bibliographically approved
3. Seasonal effects on the stiffness properties of a ballasted railway bridge
Open this publication in new window or tab >>Seasonal effects on the stiffness properties of a ballasted railway bridge
2013 (English)In: Engineering structures, ISSN 0141-0296, E-ISSN 1873-7323, Vol. 57, 63-72 p.Article in journal (Refereed) Published
Abstract [en]

In this article it is shown empirically that ballasted bridges in cold climates can exhibit a step-like variation of their natural frequencies as the yearly season changes. The bridge under study was observed to have significantly higher natural frequencies (as much as 35%) during the winter months compared to the summer. This variation was rather discrete in nature and not proportional to temperature. Furthermore the increase in natural frequencies took place only after the temperatures had dropped below 0 °C for a number of days. It was thus hypothesized that this change in natural frequencies was due to changes in the stiffness parameters of some materials with the onset of frost. In low temperature conditions not only the mean value of the measured frequencies increased, but also their variance increased considerably. Given the large spread of the measured natural frequencies, the stiffness parameters were assumed to be stochastic variables with an unknown multivariate distribution, rather than fixed values. A Bayesian updating scheme was implemented to determine this distribution from measurements. Data gathered during one annum of monitoring was used in conjunction with a finite element model and a meta model, resulting in an estimation of the relevant stiffness parameters for both the cold and the warm condition.

Keyword
Railway bridges, Dynamics, Ballasted track, Seasonal effects, Bayesian updating, Markov-Chain Monte-Carlo Sampling
National Category
Infrastructure Engineering
Research subject
Järnvägsgruppen - Infrastruktur
Identifiers
urn:nbn:se:kth:diva-132254 (URN)10.1016/j.engstruct.2013.09.010 (DOI)000330488800006 ()2-s2.0-84885204016 (Scopus ID)
Funder
EU, FP7, Seventh Framework Programme, 286276
Note

QC 20131205

Available from: 2013-10-25 Created: 2013-10-25 Last updated: 2017-12-06Bibliographically approved
4. Influence of rate-independent hysteresis on the dynamic response of a railway bridge
Open this publication in new window or tab >>Influence of rate-independent hysteresis on the dynamic response of a railway bridge
2013 (English)In: International Journal of Rail Transportation, ISSN 2324-8386, Vol. 1, no 4, 237-257 p.Article in journal (Refereed) Published
Abstract [en]

It is well known that the dynamic properties of civil engineering structures have a more or less pronounced amplitude dependency. However, it is rather difficult to quantify this both experimentally and theoretically. This paper describes an attempt to identify the sources of the amplitude dependent variation of the natural frequency and the modal damping ratio of the first vertical bending mode of a simply supported, ballasted steel-concrete composite railway bridge. It is proposed that the most likely sources to the non-linear properties of this mode of vibration are the ballasted track, the foundations and the roller bearings used mainly to relive constraint forces due to changes in temperature. The non-linear influence of the suggested sources were modelled in a 2D finite element model using the classical univariate Bouc–Wen model which was implemented as a user-defined element in ABAQUS. The results suggest that the roller bearings alone can give account for the variation in the dynamic properties observed in experimental data from the bridge and that the combination with a simple model of the track superstructure gives the most realistic result. A tremendous increase in the dissipation of energy was found as the amplitude of vibration was increased beyond that available in the experimental data, thus motivating further research within this field.

Keyword
railway bridges, non-linear dynamics, soil–structure interaction, ballast, roller bearing, Bouc–Wen models
National Category
Infrastructure Engineering
Research subject
Järnvägsgruppen - Infrastruktur
Identifiers
urn:nbn:se:kth:diva-132257 (URN)10.1080/23248378.2013.835129 (DOI)
Note

QC 20131025

Available from: 2013-10-25 Created: 2013-10-25 Last updated: 2013-10-25Bibliographically approved

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