Soil-structure interaction at railway bridges with integral abutments

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Available online at www.sciencedirect.com Procedia Engineering 00 (2017) 000–000

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Procedia Engineering 199 (2017) 2318–2323

X International Conference on Structural Dynamics, EURODYN 2017

Soil-structure interaction at railway bridges with integral abutments Hetty Bigelowa*, Daniel Pakb, Benno Hoffmeistera, Markus Feldmanna, Günter Seidlc, Thomas Petraschekd a RWTH

RWTH Aachen University| Institute of Steel Construction, Mies-van-der-Rohe-Straße 1, D-52062 Aachen, Germany Universität Siegen| Lehrstuhl für Stahlbau und Stahlverbundbau, Paul-Bonatz-Str. 9-11, D-57076 Siegen, Germny c SSF Ingenieure AG, Schönhauser Allee 149, D- 10435 Berlin, Germany d ÖBB-Infrastruktur AG, Nordbahnstraße 50, A- 1020 Vienna, Austria

b Universität

Abstract An Austrian composite single span bridge with integral abutments has been measured at different construction levels to investigate the contribution of backfill behind abutments on static and dynamic properties of the soil-structure system and their differences. Especially the determination of fundamental frequencies and corresponding damping ratios is of high interest. This paper describes the structural characteristics of the investigated bridge and the conducted measurements. As first results, the fundamental frequencies obtained at the different construction levels are presented and discussed. © 2017 The Authors. Published by Elsevier Ltd. © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the organizing committee of EURODYN 2017. Peer-review under responsibility of the organizing committee of EURODYN 2017. Keywords: backfill; composite bridge, fundamental frequencies; integral abutment bridge, railway bridge

1. Introduction Elaborate design of railway bridges involves not only consideration of dynamic loads induced by periodic axle excitation but also choice of suitable structural systems and typologies. This includes aspects of durability and maintenance, but also construction time and site conditions. Choosing bridges with integral abutments in general has two main beneficial aspects. First, there is the economic aspect, as these bridges often tend to be less expensive to build, easier to maintain and more economical to own over their life time [1] [2]. The second main aspect concerns the structural performance. When subjected to vertical loads, integral abutment bridges with stiffness properties, that

* Corresponding author. Tel.: +49 241 80 25275; fax: +49 241 80 22140. E-mail address: [email protected] 1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the organizing committee of EURODYN 2017.

1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the organizing committee of EURODYN 2017. 10.1016/j.proeng.2017.09.204

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generate load-carrying actions like structural frames, have i.a. reduced field moments, smaller deflections in mid-span and smaller abutment rotation angles than conventional bridges with bearings and joints [3]. Furthermore, due to their higher global stiffness, integral abutment bridges have higher fundamental frequencies than simply supported bridges with identical mass, stiffness and length [4], which of course is another advantage in dynamic design. Steel or steel-composite structures (especially those that are at least partially prefabricated) allow a significant reduction of construction time. Hence, the design of steel-composite railway bridges with integral abutments seems a good solution at first sight. Nonetheless, as steel-composite constructions are usually lighter than massive concrete constructions and integral abutment bridges are usually lighter than conventional bridges with bearings and joints [5], they may be more susceptible to excitation by dynamic loads and therefore require quite precise analyses. 1.1. The problem The analysis of an integral bridge differs from that of bridges with bearings and joints because the abutment is rigidly connected to the superstructure [6] [3]. Horizontal forces and displacements are transferred from the superstructure to the abutment and foundation piles. Therefore, the piles, which are interacting with the subsoil, are not only loaded by vertical forces, but also by shear forces and moments as well as forced displacements and rotations [7]. Furthermore, the abutment interacts with the backfill, resulting in an additional restraint of the superstructure [8]. For static design, the embedment of piles in the soil is represented by linear springs. The distribution of spring constants over the depth is reduced to two cases (dense soil, loose soil), which are considered separately taking into account the upper and lower limits of the static coefficient of subgrade reaction as given by the geological survey [7] [8]. However, a dynamic structural analysis of the superstructure also requires knowledge of the dynamic response of the soil, which in turn relies on dynamic soil properties measured in situ [9], which are often not known. To take the soil-abutment interaction into account for a dynamic analysis, a system of springs and dampers could be applied in that region as well, based on the dynamic soil properties of the backfill. For static analysis no explicit concepts are given in Eurocode yet, but state of the art design approaches are published and generally accepted. However, regarding the dynamic analysis, this is not the case [10]. Therefore the dynamic interaction of abutment and backfill is generally neglected, thus neglecting the stiffening effect of the backfill, which negatively affects the design of bridges with low masses such as the aforementioned steel-composite bridges. 1.2. The idea To identify the contribution of backfill to dynamic properties isolated from contributions of other structural and non-structural elements, measurements were conducted at different construction levels as shown in Fig. 1, in addition to measurements typically performed at bridges under traffic [4].

Fig. 1. Measurement concept

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Mainly accelerations were monitored, evaluated to determine natural frequencies and damping properties of the structural system [4]. The first construction stage investigated was the bare bridge without backfill behind its abutments and without finishing elements such as ballast, sleepers, rails, etc. In the second construction stage investigated, the backfill had been added. The third measurement took place after the bridge was completed, shortly before its final acceptance and opening for traffic. The fourth measurement took place five months later. 1.3. The bridge An Austrian railway bridge, crossing the river Salzach near Neumarkt/St. Veit im Pongau, was chosen as a reference-bridge for the monitoring campaign. The bridge, which is designed as a frame bridge constisting of prefabricated composite girders (VFT) has a span of 46 m. Fig. 2 shows blue prints of the structure. The Austrian Federal Railways (ÖBB) built the double-tracked bridge in the years 2015 and 2016 as a so-called twin bridge. This means that it consists of two decks (one for each track), which are separated by a longitudinal gap but are covered by a joint ballast layer, see Fig. 2. The abutments are also separated. Twin bridges are known for the interaction effect between the two decks caused by the ballast layer transferring forces/deformations between the separated decks [11] [12]. As there was no ballast bed on the investigated deck (deck 1, traffic from Wörgel to Salzburg) during the two first monitoring sessions, no interaction with the already completed second deck (deck 2, traffic from Salzburg to Wörgel) (which was already open to traffic) took place.

Fig. 2. Salzach bridge

2. Measurements 2.1. Positioning of equipment and conduction of experiments During the first three measurements, four accelerometers were placed on top of deck 1 and relative vertical displacements were measured between the decks (see Fig. 3). The accelerometers in use had a measurement range of 0.1-100 Hz and ± 0.5 g respectively with an accuracy of ± 0.4 dB max over the bandwidth. To determine the frequencies of the actual structure, the bridge was targetedly excited with a hydraulic actuator (see also Fig. 3) at frequencies in a total range between 0.5 and 30 Hz by IBW Ingenieurbüro für Bauwerkserhaltung Weimar GmbH. First, the frequencies were steadily increased according to the concept given in Table 1. These so-called sweeps were evaluated on-site to determine natural frequencies by means of Fast Fourier Transformations (FFT) using the software

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Flexpro [13]. A cosine Hamming window was applied during the FFT but no further filtering was performed. From the very distinctive peaks in the frequency domain, the fundamental frequencies of the bridge could be easily identified (see Fig. 4). The bridge was then excited in the determined frequencies for a duration of 30 seconds each, which was sufficient to create steady-state oscillations. This test procedure was performed twice at each of the first three measurements: once in mid-span and once at around one sixth of the span (see Fig. 3). Table 1. Excitation concept “Sweeps” for measurements 1-3 Sweep No.

Frequencies [Hz]

Duration [s]

Force [kN]

Excitation mass in basket [kg]

1

0.5-1

300

2

1-3

360

(600 s/Hz)

0.25

430

(180s/Hz)

1

430

3

3-10

4

10-20

300

(43 s/Hz)

7

430

300

(30 s/Hz)

7

430

5

20-30

300

(30 s/Hz)

7

124

After the third measurement was conducted, deck 1 was opened to railway traffic as well. Both decks were under railway traffic during the fourth measurement, thus the hydraulic actuator was not used anymore to excite the bridge. Furthermore, accelerometer no. 4 had to be repositioned outside the minimum clearance outline. As a strong interaction between the two decks was observed at measurement no. 3, additional accelerometers were placed underneath the bridge at measurement no. 4, see Fig. 4.

Fig. 3. Positioning of equipment and hydraulic actuator (here: at Pos. 1 at measurement no. 1)

2.2. Examples and results Fig. 4 shows exemplary acceleration time-histories obtained at logger no. 3 (see position marked in Fig. 3) and the respective FFT analyses. The accelerations from the first three measurements result from the hydraulic actuator excitation, where frequencies were constantly increased from 3 to 10 Hz (43s/Hz, see Table 1, Sweep no. 3). The hydraulic actuator was positioned in mid-span. The presented frequencies from measurement no. 4 were obtained analyzing the free decay process subsequent to a local train crossing deck 1.

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Fig. 4: Analyses of fundamental frequencies, all obtained at logger no. 3

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3. Assessment of results and future work The given example reveals only a negligible influence of the backfill on the fundamental frequencies (when comparing the first two measurements in particular). Nonetheless, previous studies emphasized that dynamic behavior of structures can depend significantly on the soil-structure interaction [14] and so further research i.a. based on numerical models is required. The Salzach bridge already exhibits a relatively high equivalent rotational stiffness of the pile foundation, which means that the additional backfill restraint does not influence the frequencies significantly. However, this foundation type with stiff concrete piles is typical for Germany and Austria, but flexible steel piles (subjected to bending about the weak axis) are usually used in other countries such as USA and Sweden [8] [3]. Using calibrated finite element (FE) models parametric studies will be performed to reveal circumstances, under which soilstructure-interaction either has always to be considered or improves the quality of calculation results noteworthy or simply can be fully neglected. These future parametric studies will include:  foundation type / stiffness  soil parameters subsoil (stiff / weak)  span and type of superstructure (mass, stiffness) Furthermore damping characteristics will be identified from the measurements using e.g. the logarithmic decrement to analyze free decay processes subsequent to individual excitations with the hydraulic actuator or train crossings. Acknowledgements The project is financially supported by the Research Association for Steel Application (FOSTA) which is gratefully acknowledged. Furthermore the authors wish to thank their partners from P1099 (Lehrstuhl für Geotechnik im Bauwesen, RWTH Aachen University, Institut für Massivbau, Leibniz Universität Hannover, IBW Ingenieurbüro für Bauwerkserhaltung Weimar GmbH), Strabag SE and the railway companies ÖBB and DB. References [1] Iqbal, N., Gervasio, H., Eriksen, J., Veljkovic, M., da Silva, L., “Sustainable construction - a life cycle approach in engineering”, Proceedings, Intern. Symposium Malta, 23.-25.07.2010. COST Action C25 [2] Feldmann, M., Pak, D. (2009), Zu Verbundbrücken mit integralen Widerlagern. Stahlbau, Jg. 78, Nr.12. Berlin (D): Ernst und Sohn Verlag, p..907-915. doi 10.1002/stab.200910106 [3] Feldmann, M., Naumes, J., Pak, D., Veljkovic, M., Eriksen, J., Hechler, O., et al.: Handbuch INTAB - Wirtschaftliche und dauerhafte Bemessung von Verbundbrücken mit Integralen Widerlagern. RWTH Aachen University, Aachen, 2010, ISBN 978-3-00-032871-8 [4] Marx, S.; Geißler, K.: Erfahrungen zur Modellierung und Bewertung von Eisenbahnbrücken mit Resonanzrisiko. in: Stahlbau 79 (2010), Heft 3, S. 188-198; Ernst & Sohn, Berlin - doi: 10.1002/stab.201090009 [5] Seidl, G., Vogel, C., Schmitt, V.: P692 - Untersuchungen zum verstärkten Einsatz von Stahlverbund-konstruktionen bei Brücken kleiner und mittlerer Stützweiten. FOSTA - Forschungsvereinigung Stahlanwendung e.V., Düsseldorf, 2005. [6] Berger, D., Graubner, C.-A., Pelke, E., Zink, M.: Fugenloses Bauen - Entwurfshilfen für integrale Widerlagerbrücken. Hessisches Landesamt für Straßen- und Verkehrswesen, Wiesbanden, 2003. [7] Pak, D., Bigelow, H., Feldmann, M. (2017),·Design of composite bridges with integral abutments, Steel Construction 1/2017, Berlin (D): Ernst und Sohn Verlag, p. 23-30. doi: 10.1002/stco.201710006 [8] Pak, D.: Zu Stahl-Verbundbrücken mit integralen Widerlagern“, Dissertation, Shaker Verlag (2012), ISBN 978-3-8440-0362-8 [9] Luna, R.; Jadi, H.: Determination of Dynamic Soil Properties Using Geophysical Methods. Proc. of 1st International Conference on the Application of Geophysical & NDT Methodologies to Transportation Facilities & Infrastructure, St. Louis, MO, Dec 2000. [10]Marx, S.; Herrmann, R.; Wenner, M.; Schenkel, M.; Curbach, M. (Hrsg.): Monitoring an Talbrücken im Eisenbahnhochgeschwindigkeitsverkehr, Tagungsband des 23. Dresdner Brückenbausymposiums. Planung, Bauausführung, Instandsetzung und Ertüchtigung von Brücken. Dresden, 12. März 2013. Technische Universität Dresden, S. 131-152. [11] Rauert, T., Bigelow, H., Hoffmeister, B., Feldmann, M., Patz, R. and Lippert, P. (2010), Zum Einfluss baulicher Randbedingungen auf das dynamische Verhalten von WIB-Eisenbahnbrücken. Bautechnik, 87: 751–760. doi:10.1002/bate.201010049 [12] Rauert, T.; Bigelow, H.; Hoffmeister, B.; Feldmann, M.: On the prediction of the interaction effect caused by continuous ballast on filler beam railway bridges by experimentally supported numerical studies, Engineering Structures, issue 12, p. 3981-3988, 2010. [13] Weisang GmbH, FlexPro Developer Suite, Version 10.0.20 [14] Glitsch, W., „Richtlinie „Integrale Bauwerke“ – Sachstandsbericht“, Stahlbau 82 (2013), Heft 10, Ernst und Sohn Verlag, Berlin.