Dynamic design guidelines for prestressed concrete sleepers

Faculty of Engineering - Papers
Source: OAI


Current design philosophy, outlined in AS 1085.14, is based on the analysis of permissible stresses resulting from quasi-static wheel loads and essentially the static response of concrete sleepers. In general, cracking can incur when the bottom fibre stress is larger than tensile strength of concrete. Premature cracking of prestressed concrete sleepers has been detected in railway tracks. The major cause of cracking is the infrequent but high-magnitude wheel loads produced by a small percentage of “out-of-round” wheels or railhead surface defects, which are crudely accounted for in AS 1085.14 by a single load factor. Based on the current design method, the cracked sleepers must be replaced by new ones, resulting in a costly maintenance budget each year. 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, in order to develop a new limit states design concept that is taking care of the realistic loading conditions and the true capacity of the sleepers. This paper presents a new limit states design concept for prestressed concrete sleepers. The paper also de-scribes the dynamic design guideline and unified design diagrams for railway concrete sleepers. The unified design diagrams have been developed for practical purpose in dynamic design and analysis of railway sleep-ers. The numerical investigations and case scenarios have been performed using a package for dynamic analy-sis of railway tracks, D-Track. The package was an achievement of the collaboration within the framework of the Australian CRC for Railway Engineering and Technologies. The dynamic design guideline covers the various effects on railway tracks due to a wide range of track occupancies, support conditions, vehicle types, rail gauges, and wheel/rail irregularities.

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    ABSTRACT: The prestressed concrete sleepers (or railroad ties), which are installed in railway track systems as the crosstie beam support, are designed to carry and transfer the wheel loads from the rails to the ground. It is well known that railway tracks are subject to impact loading conditions, which are attributable to the train operations with either wheel or rail abnormalities such as flat wheels, dipped rails, etc. These loads are of very high magnitude but short duration. In addition, there exists the potential of repeated load experience during the design life of prestressed concrete sleepers. Prestressed concrete has played a significant role in maintaining the high endurance of sleepers subjected to low to moderate repeated impact loads. In spite of the common use of prestressed concrete sleepers in railway tracks, their impact response and behaviour under repetitions of severe impact loads are not deeply appreciated nor taken into consideration in design. This experimental investigation was aimed at understanding the residual capacity of prestressed concrete sleepers in railway track structures under ultimate impact loading, in order to develop state of the art limit states design concepts for prestressed concrete sleepers. A high-capacity drop weight impact testing machine was constructed at the University of Wollongong to achieve this purpose. A series of severe impact tests on in-situ prestressed concrete sleepers was carried out, ranging from low to high impact magnitudes. The impact energy was evaluated in relation to the drop heights. The impact-damaged sleepers were re-tested under static conditions in order to evaluate the residual fracture toughness in accordance with the Australian Standard. It was found that a concrete sleeper damaged by an impact load could possess significant reserve capacity sufficient for resisting about 1.05 to 1.10 times the design axle loads. The applied impact energy and residual fracture toughness under different magnitudes of impacts are highlighted in this paper. The effects of track environment including soft and hard tracks are also presented together with a design guidance related to the serviceability and ultimate limit states design.
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    ABSTRACT: In the network of Iranian railways, pre-stressed concrete sleepers are more popular than the other types due to their high performance. One of the defects observed in these sleepers is longitudinal cracks caused by irregular and excessive pressure before and during the track operation. These cracks usually initiate from the rawlplug locations and propagate to the middle or even to the whole lengths of the sleeper. This excessive and additional pressure in rawlplug positions are due to the water freezing, existence of rock fines or fine aggregates in the fastening holes, deformation of the rawlplug or a combination this effects. This paper presents a test methodology named as Katrak test to exhibit failure planes in non-strengthened and strengthened (transversely reinforced) sleepers. In this study, the effects of the extra pressure in rawlplug positions are simulated by applying cylindrical pressure inside the rawlplug holes using a physical and numerical (FEM) models. The results show that the Katrak test and simulation performed could be an appropriate method for studying sleeper behaviour and formation of longitudinal cracks under excessive pressure.
    No preview · Article · Dec 2012 · Engineering Failure Analysis
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    ABSTRACT: The B70 mono-block pre-stressed concrete sleeper is one of the most widely used types of concrete sleepers for railways. One of the most important defects of this type of sleepers is longitudinal cracks both before and during railway track operation, which usually begins at the rawlplugs positions and continues up to the central and regional area of the sleeper. In this research, creation of longitudinal cracks before track operation has been studied. For this purpose, effects of exerted applied loads on the sleeper including pre-stressed loading and cylindrical pressure loading in the circular hole of rawlplugs positions (four different types) in creating longitudinal cracks were investigated using the ABAQUS Finite Element software. In addition, a sensitivity analysis of the tensile stress in the rawlplugs positions of the B70 pre-stressed concrete sleeper was carried out against changes in the pre-stressing force as one of the most important parameters in the structural design of this type of sleeper. According to the research results, the cylindrical pressure inside the rawlplug holes increases the tensile stresses around the holes and eventually leads to extension of longitudinal cracks. Furthermore, the implementation of loadings on the B70 mono-block pre-stressed concrete sleeper increases the intensity of the damage significantly. The numerical model also shows the linear behaviour of the changes in the pre-stressing force and, as a result, high values of pre-stressing force can be a main factor in creating longitudinal cracks.
    No preview · Article · Mar 2015 · Engineering Failure Analysis