Year

2007

Degree Name

Doctor of Philosophy

Department

School of Civil, Mining & Environmental Engineering

Abstract

Rail operators are consistently demanding higher axle loads to compete effectively with other modes of transport, particularly for heavy haul freight of minerals. However, high axle loads can only be achieved by ensuring that the existing rail infrastructure can cope with the greater static and dynamic loads associated with the wheel-rail interactions. Premature cracking of prestressed concrete sleepers is often the result of high-intensity dynamic loading caused by wheel or rail irregularities. The high-magnitude wheel loads produced by a small percentage of “bad” wheels or rail head surface defects are crudely accounted for in the Australian Standard AS 1085.14 by a single load amplification factor. In addition, there is a widespread perception within the railway engineering community that the carrying capacity of the existing track infrastructure is not fully utilised, and concrete sleepers possess significant amounts of untapped reserve strength. This can be attributed to the current design philosophy for concrete sleepers, outlined in AS 1085.14. This is based on the assessment of permissible stresses resulting from quasi-static wheel loads and essentially the static response of concrete sleepers, making it unduly conservative and very costly for the railway organisations. This thesis addresses the identified deficiencies of the current design method through an in-depth analysis of the dynamic response of concrete sleepers under realistic loading conditions and proposes a more rational design procedure.

In order to shift the conventional methodology to a more rational design method that involves more realistic dynamic response of concrete sleepers and performance-based design methodology, a significant research effort within the framework of the Cooperative Research Centre (CRC) for Railway Engineering and Technologies has been carried out to perform comprehensive studies of the loading conditions, the dynamic response, and the dynamic resistance of prestressed concrete sleepers. The collaborative research between the University of Wollongong (UoW) and Queensland University of Technology (QUT) has addressed such important issues as the spectrum and amplitudes of dynamic forces applied to the railway track, evaluation of the reserve capacity of typical prestressed concrete sleepers designed to the current code AS 1085.14, and the development of a new limit states design concept.

The comprehensive literature review highlighted the extremely limited research work that previously had been done in this field of research. In order to enhance an understanding of the dynamic performance of railway tracks, the first part of this thesis investigates the dynamic characteristics of the global railway track and its individual components with particular reference to rail pads and prestressed concrete (PC) sleepers. The experimental techniques for extracting dynamic properties of track components, developed in the laboratory, have been successfully applied in field trials. Moreover, this thesis provides an intensive review aimed at predicting wheel impact loads due to the wheel/rail irregularities at different return periods (based on the field data from wheel impact detectors).

The experimental and numerical investigations into the dynamic behaviour of prestressed concrete sleepers subjected to severe impact loading are then presented. The impact tests were carried out using the prestressed concrete sleepers manufactured in Australia. A track test bed was simulated in the laboratory and calibrated against the frequency response functions obtained for real tracks. A series of incremental impact loading tests for the prestressed concrete sleepers was performed, ranging from a typical design load to a severe wheel load. The cumulative impact damage and crack propagation in concrete sleepers were identified. The effects of track environment together with the relationship between the bending moment of prestressed concrete sleepers and the applied impact force are also presented.

The later part of this thesis identifies the responses of prestressed concrete sleepers in railway track structures under both single and repeated impact loads associated with different probabilities of occurrence. The residual capacities of the damaged prestressed concrete sleepers are studied in order to clarify the notion about the reserve strength of the concrete sleepers. The numerical investigations of the static and impact behaviours of railway prestressed concrete sleepers under static and dynamic loads were also undertaken to supplement the experimental findings in this thesis.

A proposal for the reliability-based design concepts and rationales associated with the development of limit states design procedures for the conversion of AS 1085.14 to a limit states design format is one of the key outcomes of this thesis. The new limit states design concepts and procedures for railway prestressed concrete sleepers are presented as the design guidelines for the railway engineers. The new methodology is aimed not only to save the material resources to achieve financial gains, but also to reduce the amount of cement production which would otherwise emit carbon dioxide as a contributing factor towards global warming.

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