ANDREATT

AbstractIn many countries sustainable slab track systems have been established as standard solutions for railway tunnels. As a result of safety requirements, the construction of drivable slab track systems for tunnels represents a new challenge, since until now satisfactory solutions hardly exist.A research project of the University of Innsbruck, Austria, in collaboration with RTE Technologie GmbH deals with the development of a drivable slab track cover system. A first proposal for a drivable slab track cover system is presented in this paper, based on the results of experimental and numerical investigations.Keywords: slab track system, cover element, concrete, load arrangement, numerical simulation, experiments.1 IntroductionAs a result of their stability, durability and low maintenance effort slab track systems are increasingly applied for high performance railway lines all over the world. Particularly in tunnels slab track systems represent the standard solution by now. Modern tunnels require complex safety concepts regarding escape, rescue, fire protection and fire fighting. One important aspect is the possibility of driving along the tracks with road vehicles in case of emergency. In this context slab tracks offer the best prerequisites due to structural stability and low maintenance effort.However, up to now existing solutions for drivable slab tracks do not meet all demands. Thus, the University of Innsbruck in collaboration with RTE Technologie GmbH started a research project dealing with the development of a drivable slab track cover system, meeting all technical and economical requirements concerning fabrication, installation, function and maintenance.The research work started with tests for determining realistic load arrangements. These load assumptions were used to develop a concept for a drivable slab trackPaper 142Design of a Cover Element forSlab Track SystemsA. Andreatta 1, Y. Theiner 1, J. Feix 1G. Hofstetter 1, M. Kowalski 2 and S. Knittel 2 1 Department of Civil Engineering, University of Innsbruck, Austria 2 RTE Technolgie GmbH, Rhomberg Bahntechnik GruppeBregenz, Austria©Civil-Comp Press, 2012 Proceedings of the First International Conference on Railway Technology: Research, Development and Maintenance,J. Pombo, (Editor),Civil-Comp Press, Stirlingshire, Scotlandcover system consisting of two element types. Furthermore, investigations on the entire production and installation process of these elements will be carried out in order to obtain an economic solution.2 RequirementsAt the start of the project it was necessary to collect all functional, operational and economic requirements concerning the construction of drivable slab track cover systems. In Germany the current requirements were compiled by the Department for Railway Systems of the Karlsruhe Institute of Technology on behalf of the German Slab Track Research Group (AFF) [1]. These requirements were provided as a basis for all further research steps and can be summarized briefly as follows: • functional requirements: adequate geometry, structural stability and safety, position stability, position accuracy, sufficient roughness;• operational requirements: unrestricted train operation, no negative effects on the control and safety systems, quick and easy maintenance, quick and easy repair and replacement;• economic requirements: low material costs, low installation costs, short installation time, low maintenance costs, low repair and replacement costs.3 First solution proposalDifferent types of slab track systems have been developed. Hence, in course of designing a drivable slab track cover system it is necessary to consider the respective type of the slab track, as only the combination of slab track and cover system offers the possibility to drive along railway tracks with road vehicles. Due to different geometries of currently used slab track systems the upper side of these systems has to be adapted slightly.The solution presented in this contribution is based on the slab track system “ÖBB-Porr” (Figure 1).The research work conducted until now resulted in a concept for a drivable slab track cover system consisting of two element types shown in Figure 2.3.1 Side elementsThe side elements, situated on both sides of the rails (Figure 2) are relatively small, with low weight to ease the installation and the maintenance works. The gap between the side elements and the rails represents an important design aspect. The shape and size of the side elements have to be determined such that vehicle wheels (car or truck) cannot enter the gap. Furthermore, service inspections, small works on rails and rail fastening elements as well as replacement of rails must be feasible without removing the side elements.Figure 1: Slab Track System “ÖBB-Porr”.Figure 2: The entire slab track cover set for a stretch of four elements.As a result of economical aspects the side elements are made of plain concrete, in order to reduce the material and labour costs. In addition, possible adverse effects on the train signalling and control system can be avoided. Elements without reinforcement require a concrete composition with high tensile strength. According to the European Standard ÖNORM EN 1992-1-1 [2] the concrete used for the side elements can be classified as C45/55 (Table 1).mean value characteristic value design valueN/mm² modulus of elasticity E cm36000 cylinder strength f c53 N/mm² 45 N/mm² 24 N/mm²axial tensile strength f ct 3.8 N/mm² 2.7 N/mm² 1.44 N/mm²Table 1: Material properties C45/55 (plain concrete) according to ÖNORM EN1992-1-1.The side elements were developed by an iterative optimization process, consisting of experimental and numerical investigations which are described in detail in sections 5 and 6.3.2 Middle elementAs it is not necessary to remove the middle elements for inspection works, they are designed with dimensions much larger than the side elements. For this reason and because of transportation aspects they are made of reinforced concrete. Since the middle elements are acting as single-span girders reinforcement bars are providedonly in the lower part, reducing thereby adverse effects on the train signalling and control system.The middle elements are situated between the rails in the middle of the track (see Figure 2). Because of their higher weight, they can be placed and removed only by appropriate lifting equipment. Moreover, the middle elements are easily adaptable toany irregularity of the track such as ducts or train control devices.4 Load assumptionsAs the load assumptions according to the European standard differ by far from thereal loads acting on drivable slab track cover systems, a considerable part of the research work was related to the determination of realistic loads. Two representativeload types were considered. On the one hand the loading by fire trucks represents an extraordinary load case and on the other hand the loading caused by maintenance vehicles represents a permanent load case.The force transmitted from a tire to the surface of the cover elements consists of a vertical and a horizontal component. The vertical component on the one hand depends on the static tire-load (maximum laden weight) and on the other hand on the moment, resulting from the braking force of the respective vehicle. The breakingforce represents the horizontal component. In order to determine the contact area between tire and surface of the cover system a particular test arrangement was developed (Figure 3a). In Figure 3b exemplarily the contact area of a tire with the dimension 315/70 R 22.5 is shown. The experimentally determined contact areas areused in the numerical simulation of the side elements, described in section 6.a)b)Figure 3: (a) Experimental determination of the contact area between tire and coversystem and (b) contact area.According to [1] the gap between the drivable surface of the side elements and the rail is 180 mm in the area of the rail support and 100 mm in the area between the crossties (Figure 4). Therefore, additionally to the determined contact areas (see Figure 3b), also load arrangements were considered, assuming only parts of the tire treads for load transmission.Figure 4: Gap between side element and rail in the area of support and between thecrossties.Finally in Figure 5 the respective load arrangement of a fire truck, causing maximum tensile stress within the side element, is shown. The respective vertical component of the tire load amounts to 55 kN and the horizontal one to 34 kN.Figure 5: Load arrangement for a single tire of a fire truck.5 Experimental investigationIn order to obtain a technical certification for a slab track cover system experimental investigations are required.Concerning the side element different load arrangements, described in chapter 4, were considered. Figure 6 shows the experimental set-up allowing the transmission of vertical and horizontal forces.Figure 6: Experimental set-up. 100115195955.2 Experimental resultsHere for brevity, only the experimental results for the load arrangement shown in Figure 5 are described.In order to fulfil the code requirements regarding the ultimate limit state the following equation has to be fulfilled:d d S R ≤, (1)where d S denotes the design value of action and d R denotes the design value ofresistance. In the present case equation (1) yields,10.7S k m ct pl R S R γαγ⋅≤⋅⋅⋅, (2)where k S denotes the characteristic value of action and m R denotes the mean valueof resistance. Considering the partial safety factor for action S γ = 1.0, the one formaterial R γ = 1.5, the factor ,0.8ct pl α= for plain concrete and a factor of 0.7describing the ratio between characteristic value and mean value of the axial tensile strength, equation (2) results in:1.00.373k m S R ⋅≤⋅. (3)The calculated design value of the resistance capacity 0.373d m R R =⋅, has to becompared with the respective design value of action, 1.0d k S S =⋅, described insection 4 (vertical and horizontal component of the tire load of 55 kN and 34 kN, respectively). In order to fulfil equation (3) the mean value of resistance m R , represented by the vertical and horizontal component of the tire load ,V m R and ,H m R , has to achieve the following minimum values:, 1.055 kN 147.45 kN 0.373v m R ⋅==, (4), 1.034 kN 91.15 kN 0.373H mR ⋅==. (5)Collapse of the prototype of the side element occurred at a vertical load of 60 kN and a horizontal load of 37 kN. Figure 7 shows the respective fracture pattern.Figure 7: Fracture pattern of the side element.The comparison of the experimentally determined load components with equation (4) and (5) shows, that the required safety level according to the European Standard is not fulfilled for the presented proposal. Therefore, the further development of the side element is based on an iterative optimization process consisting of numerical simulations and experimental investigations.6 Numerical investigationBeside the experimental investigation also a three-dimensional numerical simulation of the above described prototype of the side element was carried out, assuming linear elastic material behaviour but taking into account a nonlinear contact relationship between the side element and the surface of the slab track system. The Finite Element (FE)-Model of the side element consists of about 1.2 Mio. degrees of freedom. The contact areas (support) between side element and slab track system are shown in Figure 8. The contact relationship is characterized by the ability to transmit shear stresses and compressive stresses, whereas tensile stresses cannot be transferred across the interface.Figure 8: Geometry model of the side element with contact areas (red coloured).In the following the numerical results, considering the load arrangement shown in Figure 5, are described. Figure 9 shows the predicted maximum principal stress of the side element with values up to 8.5 N/mm² (area of fillet, position 1). As not even the mean value but also the design value of the tensile strength (Table 1) of the concrete used is exceeded by the numerical results, cracking and thus failure of the side element is expected due to the respective design load. Areas of stress concentration with tensile stresses larger than 8.5 N/mm² up to 64 N/mm², are the consequence of the assumed linear elastic material behaviour in combination with the contact relationship.Figure 9: Maximum principal stress [N/mm²].The numerically determined regions of failure of the side element agree quite well with the observed one, confirming the applicability of the FE-model. Both, the numerical and the experimental investigations on the structural behaviour reveal, that the first proposal of the side element is insufficient for fulfilling the code requirements regarding the ultimate limit state.Figure 10 shows a revised proposal for the geometry of the side element as result of an iterative optimization process consisting of further Finite Element simulations and experimental investigations.Figure 10: Revised proposal for the geometry of the side element. References[1] E. Hohnecker, “Befahrbarkeit von eingleisigen Eisenbahntunneln mitStraßenfahrzeugen”, Abschlussbericht, Karlsruhe, März, 2008.[2]Eurocode 2 EN 1992-1-1: Design of concrete structures – Part 1-1: Generalrules and rules for buildings, 2009.。