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The track on bridges is affected by cyclic and dynamic action arising from the train passage and the reaction of the track supporting structure. Concerning the track, the current bridge design procedures include two major engineering tasks: On the one hand the stability of the track supported on the bridge deck has to be satisfied. In order to reduce the deterioration of the track the elasticity of the track has to be adapted for the stiff bridge deck. Furthermore, lateral stability has also to be satisfied. The vibration of the bridge structure has to be taken into account because it can affect the stability of the ballasted track. On the other hand the transition zone between bridge structure, abutment and the track on the subgrade is a matter of permanent concern. The changes in stiffness, in the dynamic behavior arising from the rolling stock and the relative deflection between the bridge structure and the abutment induce a nearly unavoidable problem for the long term behaviour on the track.
BAM has conducted intensive experimental work to study the behaviour of the ballasted track on dynamically excited bridges. The state of the art is a design value for the deck acceleration of 0.35 g as established in the ENV 1991-1. In our experimental investigations the design value was verified. The influence of a frequency content higher than 30 Hz and of single vibration amplitudes on the track deterioration was evaluated. Both vertical and lateral stability were investigated. The influence of vibrations on the lateral stability can be more severe since the lateral stability could involve a sudden failure of the track. Furthermore, a simulation tool was developed to calculate the lateral stability on a vibrating bridge deck. For this simulation tool a parameter study was undertaken [1] where combinations of the curvature of the bridge-track system, the design temperature and the bridge deck vibrations were investigated. Parts of this study are incorporated in this paper.
In a current research project the investigations are extended to the interaction of the train-track-dynamics and the long term behaviour of the transition zone. The main emphasis is on the integration of the possible deterioration of the structural components in the numerical modelling of the relevant structural elements of the vehicle, the bridge and bridge abutment structure. An outlook is given to a complete model for the assessment of the track behaviour at railway bridges.
The bridge design for railway bridges is far more dependent on the interaction with the traffic and the carriageway than for road bridges. This is especially true for the specific demands of the track in highspeed railways. Both maintenance and safety of the track have to be considered. The most relevant sections for the design criteria can be located at the bridge transition zones. Based on experimental investigations it is shown that bridge joint displacements, changes in stiffness and uplifting of the sleeper are causes for an increased degradation and loss in strength of the ballasted track. With respect to high speed vibrations of the bridge deck can have an even more decisive impact. Bridge deck vibrations can lead to destabilization of the bailast. In a numerical study the behavior of the track at the bridge is illustrated.
Offshore wind turbines enter unknown territory, especially where the foundations are concerned. This is because offshore wind power can only make use of the experience from the common offshore constructions used by the oil and gas industry to a limited extent. The offshore wind industry has tried to reduce foundation dimensions, especially the pile lengths, as much as possible compared with those of the oil and gas industry. This is because with the large number of wind turbines involved it can provide considerable economic advantages. On the other hand, the stability of the foundations is additionally at risk because due to the much larger number of cyclic loads they are subjected to it is very difficult to predict how they will behave. Since offshore wind farms are manufactured in series, every systematic fault in the foundation acts as a series fault for a large number of turbines. This calls for monitoring – and the right dimensions of pile foundation, the most common type of foundations used for wind turbines
The stability and geometric nonlinearities of slender structures are a major topic in structural design. While this topic is most relevant in the field of Structural Engineering, e.g. for steel or concrete structures, only few applications take the role of soil-structure-interaction explicitly into account. The focus of this paper is placed on the impact of soil support and its modelling for the buckling analysis based on examples both for pile foundations and for railway track stability. The general interaction between steel design and the geotechnical input will be addressed. The paper discusses and summarizes a range of subtopics based on experience and current research at the author’s institute.
Offshore piles have to withstand predominantly cyclic axial loads when they are installed in multi-pile configurations, as in jacket foundations. The dimensions of the pile are governed by both the internal capacity and the fatigue behaviour of the steel cross-section as well as by its external capacity in the pile-soil interaction. Owing to the large numbers of piled foundations required for current and future offshore wind farms, there is an urgent need to optimize the dimensions and related costs of single piles. With regard to the pile capacity, two major topics of research are the determination of possible capacity gains due to pile ageing effects and proper consideration of cyclic degradation. In order to investigate both effects, a large-scale testing facility has been constructed at the BAM TTS site in Horstwalde near Berlin. This open-air facility allows large tubular driven piles to be loaded cyclically in both tension and compression while studying the ageing effects by introducing delays between the testing campaigns. First results already show a moderate increase in pile capacity over time. Concerning the anticipated capacity degradation of cyclically loaded piles, preliminary results show an unexpected behaviour. Additional tests are currently being conducted for further clarification.
Monopiles are currently the predominant foundation type for offshore wind turbines in Europe. Due to the increasing dimensions of the turbines, pile diameters beyond 10m become necessary. A design-relevant failure mode of monopiles is the local buckling of the pile wall in the embedded sections. Relevant buckling guidelines do not consider the soil-structure interaction specifically, although the embedment may allow for a reduction of wall thickness. However, Eurocode-based design concepts require a validation with comparative buckling cases for validation, either in terms of buckling curve parameters for both the algebraic stress-based and semi-numerical LBA/MNA design concept or as a calibration factor kGMNIA for fully numerical GMNIA calculations. These parameters are not yet available for embedded shells. To close this gap, we have conducted experiments on piles embedded in sand to investigate local buckling under soil-structure-interaction. The results will be used to calibrate numerical models. This research was carried out as part of the VERBATIM research project, funded by PTJ/BMWK and supported by the Carbon Trust's Offshore Wind Accelerator consortium.
The main degradation process at bridge transition zones due to traffic loads is the appearance of differential settlements. Abrupt stiffness changes, repeating traffic loads and relative displacements of the superstructure ends on bridges often aggravate this problem. In this contribution, a 3D finite element (FE) model extended with a boundary formulation in the frame of the scaled-boundary finite element method (SBFEM) for a transient analysis of train-track-bridge interaction is presented. This numerical model permits an assessment of bridge transition zone with respect to permanent deformations of the track. The main focus lies on the modeling strategies for the vehicle and their impact on suitable assessment criteria for bridge transition zones. For this purpose, two different modeling strategies for the vehicle, a moving load model and a multibody model, have been compared and discussed on the basis of the assessment criteria. The results indicate that the model of the vehicle has a minor effect for an assessment on the embankment, but that the assessment on the bridge may show significant differences depending on whether the inertial components of the vehicle (multibody model) are considered.