Dynamic behaviour of a short span soil–steel composite bridge for high-speed railways – Field measurements and FE-analysis
Introduction
Soil–steel composite bridges (SSCB) refer to structures where a buried flexible corrugated steel pipe works in composite action with the surrounding soil. The section of the steel pipe is often rather slender and the load bearing capacity as well as the stability relies on the confinement from the backfill. These structures are being increasingly used in road and railway projects as an alternative to standard type bridges, e.g. short- and medium span concrete beam- and portal frame bridges. They are often found to be cost effective, both due to less material consumption and reduced construction times. The development during recent years has resulted in longer spans, more efficient arch geometries and less height of fill cover.
The Swedish regulations for design of bridges [1] refer to the design method described in [2] for design of SSCB. This method was developed by [3] that also presented validation with full-scale testing. Additional field tests are reported in [4], [5], [6], [7], [8], [9], [10], comprising static full-scale models, in situ tests and dynamic measurements of passing trains. In [6], the dynamic response of a soil steel composite bridge with a span of 11 m is measured during passages of a locomotive at speeds from 10 km/h to 125 km/h. Dynamic amplification factors (DAF) of 1.2 for displacements and thrusts and 1.45 for moments are found. The highest vertical acceleration in the ballast is found to be 0.45 m/s2. Field tests of the dynamic response from passing trains at moderate speeds are reported by [12], [13], [14], [15], generally showing small displacements and accelerations. In [16], the static and dynamic responses of four steel culverts are measured under passages of truckloads at speeds from 10 km/h to 70 km/h. The DAFs are found to be in the range of 1.12–1.26 for displacements, and 1.11–1.29 for strains.
The behaviour of soil–steel composite bridges under live loads has been studied in a number of papers such as [17], [18], [19], [20]. In these papers, the static behaviour of these structures is studied due to the truck loads through FE-analysis. However no research has been found regarding dynamic FE-analysis of these structures due to the train loads.
For railway bridges on high-speed railway lines, extensive dynamic design checks are usually needed. The main dynamic criterion is the vertical deck acceleration, a measure to prevent ballast instability during resonance. For such analyses [1], refers to the methods in Eurocode [11], which is mainly valid for beam like structures. The dynamic behaviour of SSCB may be significantly different than beam like structures due to the composite action with the surrounding soil. To the authors’ knowledge, no design recommendations for SSCB on high-speed railway lines currently exist.
Section snippets
The bridge
The studied bridge is named ‘’Rörbro i Märsta’’ in Swedish which is a closed shaped corrugated steel culvert (km 43 + 765). It is located in Sweden, about 40 km North of Stockholm. The bridge was built in 1995 during an extension of the North junction of the Arlanda Express line, a link between the East Coast railway line and Arlanda Airport. It has elliptic cross section with a horizontal and vertical diameter of 3.75 m and 4.15 m, respectively. The total length transverse to the tracks
Response to train X52
Table 1, Table 2 contain a summary of bridge responses under 11 passages of train X52. The entire measured responses are filtered by low-pass Butterworth filter [22], with a cut-off frequency of 30 Hz and an order of 8th. Maximum differences are calculated as (max–min)/mean. The lowest scattering belongs to deflections which makes it the most reliable parameter for calibrating the finite element models. The critical section in terms of stress is at the haunch which shows the lowest scattering
Procedure
To model these structures, there is a need to adopt a suitable geometry and appropriate material properties. The geometry is defined based on the available technical drawings. However, there are no certain values for static and dynamic properties of the soil material, which is determined through parametric studies and the calibration against the field measurements (Sections 5.3 Influence of the soil stiffness, 5.4 Influence of the material damping, 5.5 Influence of the soil density). These
Comparison of FEM results and field measurements
Fig. 17 presents the responses of the bridge calculated by FE-analysis in comparison with the field measurements. Entire results are filtered by low-pass Butterworth filter, with a cut-off frequency of 30 Hz and order of 8th. In the 3D model, two middle bogie loads are applied in order to decrease the cpu-time. Generally, there is a fair agreement between the field measurements and the FEM results. The main noticeable discrepancy is observed for the stress at the top fibre of the crown.
Conclusions
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Maximum deflection and stress at the crown is 0.4 mm and 6 MPa, respectively. Maximum acceleration at the crown of the culvert is 0.8 m/s2, and, in the ballast above the crown between two sleepers is 1.5 m/s2.
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The damping properties of the soil material can be modelled using mass proportional Rayleigh damping curves with an alpha value of 3.9 in average.
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Calculated effective widths from the 3D model show higher values than those calculated based on the assumption of 2:1 stress distribution in soil.
Acknowledgements
The field testing was funded by Viacon and performed by the technician at the Division of Structural Engineering and Bridges, KTH.
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