Sakdirat Kaewunruen

Dynamic Crack Propagations in Prestressed Concrete Sleepers in Railway Track Systems Subjected to Severe Impact Loads

Prestressed concrete sleepers (or railroad ties) are the crosstie beam support in railway track systems. They are designed and constructed under flexural constraints in order to carry and transfer the dynamic wheel loads from the rails to the ground. Under perfect wheel and rail conditions, the dynamic loading on railway tracks could be treated as a quasi-static load using a dynamic impact factor. The current design method for the prestressed concrete sleepers taking into account the quasi-static effect is based on allowable stress where crack initiation is not permitted. In reality, the impact events are often detected due to the uncertainties of wheel or rail abnormalities such as flat wheels, dipped rails, etc. These loads are of very high magnitude but short duration. Over the design life span of the prestressed concrete sleepers, there exists the feasibility of extreme and repeated impact loading events. These have led to two proposed limit states for the consideration of structural engineers: ultimate limit states and fatigue limit states. Prestressing techniques have been long used to maintain the high endurance of the sleepers under repeated impact cycles. In spite of the most common use of the prestressed concrete sleepers, their impact behavior and capacity under the repetitions of severe impact loads are unclear. This paper presents the experimental investigation aimed at understanding the dynamic crack propagations in prestressed concrete sleepers in railway track structures under repeated impact loading. The impact forces have been correlated against the probabilistic track force distribution obtained from an Australian heavy haul rail network. The effects of track environment including soft and hard tracks are highlighted in this paper.

Investigation of free vibrations of voided concrete sleepers in railway track system

Abstract Concrete railway sleepers in ballasted track are laid on ballast and subgrade supporting systems. Full contact between sleepers and ballast is typically assumed for analysis and design purposes. Often, voids and pockets in the sleeper/ballast contact interface form between sleepers and the ballast underneath that could cause problems to both the sleepers and the track system as a whole. The current paper investigates the effects of ballast voids and pockets on free vibration response characteristics of in situ railway concrete sleepers. Finite-element modelling was employed to develop a dynamic model of the railway track incorporating concrete sleepers. This model includes the dynamic interaction of sleepers and ballast as part of the free vibration analyses of the in situ railway concrete sleepers. Several patterns of voids and pockets underneath railway sleepers were studied. The emphasis was placed on partial and full interaction between sleepers and ballast. The information on the vertical vibration modes provides an important insight into the dynamic response of concrete railway sleepers in different void-and-pocket configurations.

EFFECT OF IMPROPER BALLAST PACKING/TAMPING ON DYNAMIC BEHAVIORS OF ON-TRACK RAILWAY CONCRETE SLEEPER

Ballasted railway tracks are impaired due to either normal or abnormal operations. One of the problems is the differential settlements along the track. Clearly, there is the need to maintain periodically the track substructures by means of ballast packing/tamping. Inappropriate conducts result in the nonlinear distributions of support stiffness. This study firstly demonstrates the effects of improper ballast packing/tamping on the free vibration behaviors of in situ railway concrete sleepers. The two-dimensional finite element modeling of an in situ concrete sleeper was employed in the parametric studies. This model takes into account the coupled flexural-and-shear deformations of concrete sleepers, elastic displacements of fastening system, and nonlinear dynamic interaction between the sleeper and ballast support. Dynamic interaction between sleepers and ballast was investigated based on the nonlinear distribution of ballast support stiffness underneath the sleeper. Effects of both symmetrical and asymmetrical stiffness distributions on dynamic behaviors of the local in situ concrete sleeper were also highlighted.

Strategic framework to achieve carbon-efficient construction and maintenance of railway infrastructure systems

Sakdirat Kaewunruen*, Joseph M. Sussman and Herbert H. Einstein 1 Birmingham Centre for Railway Research and Education, School of Civil Engineering, University of Birmingham, Birmingham, UK 2 High Speed Rail Research Group, Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA 3 Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA *Correspondence: sakdirat@hotmail.com; s.kaewunruen@bham.ac.uk

Reliability-based conversion of a structural design code for railway prestressed concrete sleepers

Ballasted railway track is very suitable for heavy-rail networks because of its many superior advantages in design, construction, short- and long-term maintenance, sustainability, and life cycle cost. An important part of the railway track system, which distributes the wheel load to the formation, is the railway sleeper. Improved knowledge has raised concerns about design techniques for prestressed concrete (PC) sleepers. Most current design codes for these rely on allowable stresses and material strength reductions. However, premature cracking of PC sleepers has been found in railway tracks. The major cause of cracking is the infrequent but high-magnitude wheel loads produced by the small percentage of irregular wheels or rail-head surface defects; both these are crudely accounted for in the allowable stress design method by a single load factor. The current design philosophy, outlined in Australian Standard AS1085.14, is based on the assessment of permissible stresses resulting from quasi-static wheel loads and essentially the static response of PC sleepers. To shift the conventional methodology to a more rational design method that involves a more realistic dynamic response of PC sleepers and performance-based design methodology, comprehensive studies of the loading conditions, the dynamic response, and the dynamic resistance of PC sleepers have been conducted. This collaborative research between several Australian universities has addressed such important issues as the spectrum and the amplitudes of dynamic forces applied to the railway track, evaluation of the reserve capacity of typical PC sleepers designed to AS 1085.14, and the development of a new limit states design concept. This article presents the results of the extensive analytical and experimental investigations aimed at predicting wheel impact loads at different return periods (based on field data from impact detectors), together with an experimental investigation of the ultimate impact resistance of PC sleepers required by the limit states design approach. It highlights the reliability approach and rationales associated with the development of limit states and presents guidelines pertaining to conversion of AS 1085.14 to a limit states design format. The reliability concept provides design flexibility and broadens the design principle, so that any operational condition could be catered for optimally in the design.

NONLINEAR TRANSIENT ANALYSIS OF A RAILWAY CONCRETE SLEEPER IN A TRACK SYSTEM

Railway sleepers in a track system are usually subjected to a wide range of loading conditions. A critical type of loading condition that causes cracking in the railway concrete sleepers is the dynamic transient wheel force. The transient wheel forces are often due to wheel or rail abnormalities. This paper presents a dynamic finite element model of a railway concrete sleeper in a track system, aimed at raising the consideration of dynamic effects in sleeper design. The railway concrete sleeper is modeled using the beam-on-elastic-foundation theory. Since in the actual tracks the ballast underneath does not provide any tensile resistance, the finite beam elements employed in this investigation take into account the bending and shear deformations, together with the tensionless nature of the elastic support. This paper places emphasis on the effect of the transient periods on the flexural responses of railway sleepers in track systems. Using the robust finite element software STRAND7, the finite element model of the railway concrete sleeper was previously established and validated against experimental data by the authors. The numerical analyses present the ratio between the dynamic and the static bending moment resultants, the dynamic magnification factor, of the railway concrete sleeper under different sinusoidal pulse durations.

Composite slabs for railway construction and maintenance: a mechanistic review

Australian railway networks alone require timber components in excess of 280,000 m3 per year for railway construction and maintenance. The relatively high turnover of timber sleepers (crossties in a plain track), bearers (skeleton ties in a turnout) and transoms (bridge cross beams) is responsible for producing greenhouse gas emissions six times greater than equivalent reinforced concrete counterparts. This study reviews the railway maintenance problems and solutions for the replacement of these high maintenance members. Alternative composite materials to replace timber components in railway infrastructure construction and maintenance have been developed extensively in recent years. This critical review outlined an innovative solution incorporating composite slab theory and combining with the capabilities of being precast and modular, in order to reduce the depth, weight and required installation time relative to conventional track slab systems. Systemic risks, compatibilities and compliances are highlighted to warrant real-world applicability of composite structures.

Structural Safety of Railway Prestressed Concrete Sleepers

Abstract This paper carries out the assessment of reliability indices or structural safety of railway prestressed concrete sleepers designed in accordance with Australian Standard AS1085.19. The current design approach of the prestressed concrete sleeper relies on the permissible stresses over cross-sectional area. Loading condition acting on railway sleepers is considered from axle burden and dynamic amplification factor. On the basis of Australian design of railway prestressed concrete sleepers, only service limit states are considered; however, the design challenge is to provide adequate resistance of certain cross sections to both positive and negative bending moments. In this paper, the service limit states functions are formulated taking into account the permissible compressive and tensile stresses at both initial and final stages, and applied positive and negative bending moments at railseat and middle sections. Random variables in the reliability analysis include railway track design parameters, axle load, material and geometrical properties, prestressing force and its losses, and model uncertainties regarded to the structural resistance and load effects. Statistical properties of related parameters are adopted from previous studies. Two analysis methods are used: firstorder moment reliability method (FORM) and second-order moment reliability method (SORM). Sensitivity analyses of the reliability indices for flexural capacity according to the requirements of the limit states functions are also investigated in order to evaluate the major influences of dynamic load factors, strengths of materials, track parameters and model uncertainties.

Acoustic and Dynamic Characteristics of a Complex Urban Turnout Using Fibre-Reinforced Foamed Urethane (FFU) Bearers

A special track system used to divert a train to other directions or other tracks is generally called a ‘railway turnout’. A traditional turnout system includes rails, switches, crossings (special track components), steel plates, fasteners, screw spikes, timber bearers, ballast and formation. The wheel/rail contact over the crossing transfer zone has a dip-like shape and can often cause detrimental impact loads on the railway track and its components. The large impact also emits disturbing noises (either impact or ground-borne noise) to railway neighbors.

Current state of practice in railway track vibration isolation: an Australian overview

Abstract Inevitably, train and track interaction generates a travelling source of noise and vibration along the railway corridor. Railway noise is generated in various forms and spectra. The undesirable railway sound and vibration include rolling noise, impact noise, curve noise, mechanical noise, airborne noise, wheel/rail noise, structure- and ground-borne noises. The noise that is carried through the vehicle body mainly affects ride quality, customer experience and structural integrity of the rolling stocks, whereas the vibration that is transmitted from the rails to the supporting structure of the track plays a main role in rapid track degradation and potentially affects the surrounding structures. This study highlights the practical guidelines for track vibration isolation resulting from the operation of railways. Its emphases are placed on the contemporary vibration mitigation methods used in existing and ageing railway infrastructures (the so-called ‘brown field project’). Its aim was to provide the overview and lessons learnt for the future development of new vibration isolation strategies in practice.