3-a.k. snghal rajeev verma

7/28/2019 3-A.k. Snghal Rajeev Verma

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Design & Development of PSC Sleeper andPhilosophy of Design for High Speed

A. K. Singhal *Rajeev Verma **

Synopsis : The PSC sleepers were introduced on Indian Railways in the

late 60s and since then have been accepted as essential component ofmodern track, to meet the requirement of the increasing traffic, heavier axle

load and higher speed. Indian Railway is one of the largest user of concretesleepers amongst the world Railways. More than 100 million prestressconcrete sleepers equivalent to 60,000 track kilometers have been produced

in Indian Railway so far. Present rate of production is about 10 million perannum.

Design of Concrete sleeper differs from normal structural design in a waythat the loading and support conditions cannot be assessed to a high degree

of certainty. This paper summarises design requirements, design philosophy

and factors influencing the design of PSC for high speed track.

1.0 Introduction

The purpose of railway track is to transfer train loads to the formation.Railway track is a discrete system consisting of rail, sleeper andballast laid over formation. Load transfer works on the principle ofstress reduction, layer by layer. The maximum stress occurs betweenwheel and rail and reduces as the contact area increases. Sleeper

is a very important component of track as load from the rail istransferred through rail pad to the ballasted bed and ultimately tothe formation.

Sleepers are members generally laid transverse to the rails, on whichthe rails are supported and fixed through fasteners. The mostpreferred material for sleeper has been timber since the beginningof railway constructions but due to indiscriminate deforestation andrapid dwindling of wood all over the world, a need for developing astrong alternative to wooden sleeper was realized by RailwayEngineers, thus heralding the era of concrete sleepers.

* Executive Director/Track, RDSO, Lucknow** Deputy Chief Engineer/Con/Design, Western Railway, Churchgate


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Life of the sleeper normally depends upon the quality of material i.e.high strength concrete and prestressing wire used. Lateral andlongitudinal stability of sleepers is provided by the weight, shapeand length of the sleepers. Maintainability of track mostly dependsupon the formation conditions and the formation pressure causeddue to transmission of ballast pressure from the interface of sleeperbottom to formation through ballast.

Recently, RDSO has carried out a simulation study with CharmersUniversity of Sweden for factors influencing design of concretesleeper and manufacturing.

2.0 Basic design requirementThe design of concrete sleepers is determined by fundamentalrequirements arising from the role that the sleeper component playsin the entire track structure. The dimensional requirements for mono-block sleepers are as follows:

- The bottom plan area of sleeper must be such that the averageballast pressure should not exceed a certain value, normally 6

Kg/cm2

.- The end faces should preferably be of a size and shape to

provide maximum resistance to lateral movement.

- As the prestressing wires (tendons) are straight, the crosssection can be varied so that the geometry of a particular sectionmay be used to provide prestress eccentricity to best match thebending moments, positive or negative. Consequently the shape

of the sleeper body is tapered from both ends to the centre inheight and sometimes width with gradual change of sectionalprofile to avoid stress concentrations.

The role of the sleeper is not only to distribute the vertical load fromthe train, but also to take care of the lateral movement induced bythe train and in the track itself due to temperature variations. Themain factor involved in lateral stability is friction between the sleeperbase and the supporting ballast. There is also a significant

contribution from compacted ballast shoulders at the sleeper endsand compacted ballast in the cribs.


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A sleeper without any crack is desirable because cracks in theconcrete caused by tensile bending moments lead to large increasein the stress range of the prestressing steel, which could cause afatigue failure of the tendons. The most critical bending momentalong the sleeper body often occurs at the rail seat, causing tensilestresses at the bottom of the sleeper. The design and the degree ofprestressing are thus most focused on the conditions at the rail seat.The aim is to develop full prestressing at the position of the rail seatin order to prevent bending cracks in the serviceability state.Consequently, the requirements of the anchorage capacity of thetendons are tough and create a complex stress situation at the endsof the sleeper. The centre of the rail seat of sleepers is generally

placed at a distance of 500 mm or less from the sleeper end, asshown in figure due to other considerations. It is thereby obvious

that a short transmission length is essential for pre-tensionedsleepers.

3.0 Design philosophy

Railway sleepers are probably unique in the way in which they actas structures. First, they are not subjected to self-load stresses duringtheir working life as the static self-weight of the rail is only of the

order of 0.1% of the total design load. They are subjected mostly todynamic loads. Sleepers rest on the ballast but are not tied down toit, thus the impact load is able to make the sleepers oscillate. Thepoints of application of load are defined, but on the support side, awide range of support reactions are found. These depend on thenature of the ballast, state of compaction and formation below it aswell as the form of ballasting and the quality of maintenance. Thepoint of loading also defines the points of maximum moment and

shear as coincidental.

The vehicle (wagon/coach) applies its load to the track through axleand wheels. Axle load depends on the design of the vehicle and its

Fig. 1 : A Concrete Sleeper


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maintenance. Ascertaining the load coming on an individual sleeperand consequent ballast pressure involves the consideration of trackstiffness, sleeper spacing, ballast packing, maintenance conditionof the track and the bottom profile of sleeper. The load coming onthe sleeper must also account for dynamic augment due to speed,rail wheel irregularity and track defects etc. The design sleepermoments are assessed from the sleeper loads and ballast pressure.Finally, a sleeper is designed with given material strengths. Manyfactors are difficult to be determined, quantified and maintained inthe long term in practice, therefore, much design development relieson a probabilistic approach taking into account what has gone before.

4.0 DesignThere are a number of steps in the design i.e. assessment of loadon the sleeper, ballast pressure distribution, selection of dimensionsof sleeper followed by bending moment calculations at rail seat &centre of sleeper and finally checking the stresses in sleeper andcalculating factor of safety and load factor.

4.1 Loading

It has been an internationally accepted fact that the combination of

a number of elastic media between wheel and formation coupledwith the dynamics of track and rolling stock irregularities introducemany indeterminates that preclude determination of exact rail seatdesign load.

The first step towards an economical design is therefore to find outthe rail seat design load correctly to the possible extent. Rail seatload depends upon number of factors such as wheel load shared by

sleepers adjacent to the particular wheel, dynamic loading due towheel, properties/condition of rubber pad and maintenance practicesof the track.

4.1.1 The wheel load coming on a rail is shared by many sleepers.The maximum load on a particular sleeper is derived by loaddistribution factor multiplied by wheel load. Load distributionfactor of 0.50-0.55 for PSC sleeper (sleeper spacing 60 cm)design has been adopted by Indian Railways.

4.1.2 Dynamic augment is a factor which is multiplied to static loadto calculate the design load. Dynamics of the rail wheelinteraction is very complex phenomenon. Due to moving load,


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vibrations of very low frequencies of the order of 1 Hz to highfrequencies of the order of 2000 Hz are generated. Thesevarying frequencies vibrations are to be absorbed by differentcomponent of the bogie and track. Concrete sleeper aresusceptible to impact load especially in the frequency rangingfrom 25-300 Hz. Considering all the factors, dynamic augmentfor speed and rail wheel irregularities has been adopted as2.5 by Indian Railways.

4.1.3 The effect of softer pads of 5 mm/6.5 mm thickness vis--visharder EVA pads on loading has also been studied. This studyshows that properties/quality of rubber pad do have effect ondynamic loading condition of sleeper.

4.1.4 Ballast pressure plays an important role in the design ofconcrete sleeper. Presently, a length of 1040mm from sleeperend is being considered as a zone of ballast reaction on IR.This is in near ideal track condition. With passage of time,track deteriorates and central portion of the sleeper alsoimparts partial reaction, which is considered as 40% of thatimparted by end zones.

Fig. 2 : Different Support Conditions for Sleepers

4.2 Bending Moments

4.2.1 The bending moments (BMs) at rail seat and centre arefunctions of the bottom profile (plan) of the sleeper. For a givenlength of sleeper, the BMs at the two sections are influenced

by bottom width at various sections.4.2.2 BM at rail seat is influenced by the overhang of the sleeper

and hence, by its length and track gauge. Relief in BM at rail


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seat is offered by the uniformly distributed rail seat load up tothe longitudinal neutral axis of sleeper.

4.2.3 BM at centre section is controlled by the distance betweenrail seats (gauge), coefficient of centre binding and the width

at centre bottom and increases with all the three parameters.

4.2.4 The Indian Railway sleeper suffers in comparison to itsEuropean/ American/Australian counterpart in the matter ofincreased centre-moments/centre top tension because of the17% wider track gauge. Even with a coefficient of centrebinding of 0.5, the magnitude of hogging BM at centre remainscomparatively smaller for the standard gauge sleepers thanfor BG. Centre of gravity of the set of HTS wire will dependupon two moments i.e. at rail seat & at centre and, therefore,it is very important to decide CG of set of HTS wire accordingly.

4.2.5 Longer length of sleeper reduces formation pressure butincreases BM at rail seat with more demand of steel thoughcentre section BM is reduced. Reduction in length has areverse effect on the BMs at the two critical sections. In additionthe margin of transmission length reduces in case of fully

bonded pre-tensioned designs. However, the length of thesleeper is to be decided based on the aim to establish fullprestressing at the position of the rail seat in order to preventbending cracks in the serviceability state.

4.3 Section of Sleeper4.3.1 Bottom width

Larger width at the rail seat reduces ballast pressure. The

area in the tamping zone should be large enough to restrictballast pressure at the sleeper ballast interface to 5-6 kg/cm2,the width being constrained by considerations of sleeperspacing and mechanised tamping. Very large widths causesproblem of tamping and may result in broken edges, unevenballast pressure and torsional stresses. The width at centresection should be as small as possible but the transition fromrail seat to centre should be gradual to avoid stress

concentration.4.3.2 Top width

Nature of fittings like dowels, inserts, slide chairs, bearingplates/rubber pads etc. dictate the choice to top width at rail


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seat. Top width at centre section, being the smallest, shouldbe adequate to accommodate HTS wire in top tier and shouldhave adequate cover for durability.

4.3.3 Depth

Increased depth means greater relief in bending stresses atrail seat, higher section modulus at rail seat bottom and hencelarger factor of safety against the imposed BM and larger loadfactor against failure. On the other hand smaller depth resultin reduced section modulus and very high bending tensilestress that cannot be overcome by prestressing forces,already weakened due to a much smaller number of HTS wireshaving to be placed in the reduced sectional area.

The depth at centre section has to be decided consideringthat the section has adequate modulus to restrict bendingtensile stresses at top which can be effectively brought withinthe permissible compressive and tensile stresses in concreteby the superimposed prestressing forces. The transition fromrail seat section to centre section should be gradual to avoidstress concentration and cracking under impact of moving

loads.4.4 Permissible stresses4.4.1 Concrete

Final stresses in concrete (i.e. algebraic sum of bending stressand stress due to prestress) at critical sections i.e. at rail seatand centre must remain well within its permissible limits. 0.4Fc and 0.04 Fc (where Fc is the specified concrete strength)are the present permissible limits in bending compression andtension respectively.

4.4.2 HTS Wires

0.2% proof stress for HTS wires has a generally stipulatedvalue of not less than 85% of UTS. The initial prestressingforce varies, according to the available information in differentdesigns of the world from 70% to 75% of UTS. But it is not theinitial prestress that really matters. It is the residual prestress

after losses that influence not only the crack resisting capacityof the section but also its durability under pulsating loads.


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Very high permanent prestressing force is likely to causelongitudinal splitting of concrete also.

4.4.3 Loss of Prestress of HTS Wire

- Losses in prestress occur due to relaxation of steel, creep,

shrinkage and elastic shortening of concrete. Presently normalrelaxation steel is being used. Low relaxation steel will help inreducing losses.

- In general, the better the compressive strength of concrete,the lower the losses. A high level of quality control on materials,concrete manufacturing and curing is essential to attainconsistent high strength leading to lower losses.

- Practices in assumption of losses in prestress depend onproduction techniques. Assumption of losses is one thing butto ensure that no more losses take place than assumed is ameasure of technology and quality control in the plant.

4.5 Factor of safety (FOS) & load factor (LF)

4.5.1 The designed MR at rail seat bottom is equal to BM x FOS.The FOS indicates that crack may occur under working loadof the order of rail seat design load x FOS. A high factor ofsafety ensures greater immunity from cracks except undervery high loads with a very low probability of incidence. A crackfree section imparts longevity because it precludes ingress ofmoisture or chemicals that cause stress corrosion in HTS.With a smaller factor of safety the margin between thepermissible tensile stress in concrete and its modulus ofrupture reduces and cracks may appear at lower and morefrequent loads.

4.5.2 Load factor is the ratio of failure moment (MF) capacity andimposed BM. MF is a function of the quantity of HTS in tensilezone, concrete strength and effective depth of section. A loadfactor of about 3 is considered adequate. Load factor indicatesthe capacity of the section to resist failure under rarelyoccurring very high impact loads as in case of derailment.

5.0 Recent Simulation Study5.1 Recently, RDSO has done a project in consultation with

Chalmers University of Sweden on 'Optimisation ofPrestressed Concrete Sleeper' under UIC Asia Regional


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Cooperation Programme. In the project, various designparameters of the sleeper have been studied by computersimulation and recommendations have been made forimprovement in design philosophy. As per study, loaddistribution factor and dynamic augmentation factor dependon track irregularities, wheel flat, stiffness of rubber pad, ballastpressure distribution etc. and Dynamic augmentation factoris largely affected by speed, wheel flat and rubber pad stiffness.Thus, detailed analysis and design of sleeper throughsimulation studies becomes very important for heavy haul andhigh speed traffic.

5.2 DIFF Model has been developed by CHARMEC using FEM

analysis for a length of 70 sleepers' bays. Rail has beenmodeled with 16 beams element in each sleeper bay. Onlyhalf sleeper / track model is developed owing to symmetry oftrack structure. Four wheel loads (2 from each adjoiningbogies) have been considered in the model. Uniform ballastbed stiffness distribution has been taken with value of ballastbed stiffness equal to 225 MN/m/m2 and ballast bed viscousdamping factor of 1.0.

5.3 In the modeling study, the DIFF model has been analysed toassess influence on Dynamic Augment factor due to rail wheelirregularity, rail pad stiffness, rail type, ballast bed stiffnessand wheel flat etc.

5.4 Based on studies, following important conclusions have beendrawn -

5.4.1 The value of load distribution factor is different for differenttrack conditions such as rail corrugation, rail joint, wheel flats,rail pad stiffness etc. Load distribution factor (i.e. ratio betweenrail seat load to wheel load) is 0.40-0.43 under static conditionwhich is presently being taken as 0.55 in RDSO design.

5.4.2 The value of dynamic magnification is different for differenttrack conditions such as rail corrugation, rail joint, wheel flats,rail pad stiffness etc. Maximum dynamic magnification factor

for rail seat bending moment is 1.73 for BOXNHL and 2.24 forLHB coach for the case of wheel flat.


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5.4.3 Influence of Dynamic rail pad stiffness and vehicle speedon sleeper Bending Moments

a) There is moderate increase in rail seat & centre bendingmoment and rail seat load due to increase in rail pad stiffness.

b) Rail pads with higher stiffness lead to a significant increase inthe maximum contact forces, rail seat loads and sleeperbending moments.

Fig. 3 : Influence of dynamic rail pad stiffness and vehicle speed on

(left) maximum sleeper bending moment at rail seat and (right)minimum sleeper bending moment at centre. Reference track model

and reference freight vehicle model BOXNHL, see Section 3, and 60mm

wheel flat (depth 0. 40 mm)

c) As per studies, Maximum rail seat load for axle load of 22.9 Twith wheel flat & withSoft rail pad (stiffness 100 kN/mm) = 0.6 x 229/2 = 68.7 kNMedium rail pad (stiffness 200 kN/mm) = 0.8 x 229/2 = 91.6kNHard rail pad (stiffness 400 kN/mm) = 1.1 x 229/2 = 126 kNWhereas, as per RDSO existing practice, rail seat load = 0.55x 2.5 x 229/2 = 157 kN

d) Thus, it can be concluded that load distribution factor anddynamic augment factor are not constant but vary with thetrack conditions viz. rail corrugation, wheel flat, rail joint, ballaststiffness distribution, rail pad stiffness and speed etc.


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5.4.4 Increased ballast bed stiffness leads to increased rail seatload and rail seat bending moment but decreased centrebending moment.

5.4.5 Different types of ballast stiffness distribution (based on

distribution factor ) have been considered depending uponpacking condition, as shown below:

By considering the above distributions in analysis, it can besaid that all possible conditions occurring in actual fieldconditions have been covered in design.

5.4.6 Rail seat bending moment is maximum for the distributioncorresponding to ballast shoulder at ends and centre bending

moment is maximum for distribution ballast shoulder at centre.

Fig. 4 : Different load distributions

5.4.7 Influence of ballast bed modulus distribution and vehiclespeed

a) Effect of geometrical properties of sleeper on bendingmoments has been studied. Uniform cross section leads toreduced rail seat bending moment. Sleeper with narrow cross-

section at centre leads to reduced sleeper centre bendingmoment.

b) Sleeper response for dynamic loading is different from staticloading. During dynamic loading, maximum sleeper


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displacement and maximum sleeper curvature / bendingmoment are not synchronized.

c) As per European standards, the characteristic bendingmoment is representing the required capacity at the end ofthe service life of the sleeper, i.e. 50 years. The test bendingmoment is calculated by adding the bending moment capacityreduction given by the expected loss of prestress andreduction of tensile capacity of the concrete from the time oftesting to the end of the service life of the sleeper. The testbending moment is normally 1.25 - 1.4 times the characteristicbending moment depending on the geometry of the sleeperand the prestress level. The fundamental idea is that thesleeper should have a capacity that is higher than thecharacteristic bending moment requirements during its entireservice life. The bending capacity of the sleeper is believed tovary during the service life due to prestress losses andchanges in the tensile capacity of the concrete.

Fig. 5 : Influence of ballast bed modulus distribution and vehicle speedon (left) maximum sleeper bending moment at rail seat and (right)minimum sleeper bending moment at centre. Reference track model

and reference freight vehicle model BOXNHL, see Section 3, and 60mm

wheel flat (depth 0. 40 mm)


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5.5 Study for Manufacturing Process

As per study, following improvements in sleeper production processcan be carried out for an effective sleeper production system thatproduces high quality sleepers suitable for high speed.

a) PrestressingThere should be Parallel control of both the elongation andthe applied hydraulic pressure. There should be long beds inorder to avoid large influence of wire length and to providemeans for an effective quality control. Simultaneousprestressing of all wires in the bed is desirable.

b) Casting

The majority of the compaction of the concrete should beaccomplished by use of internal poker vibrators in order toensure proper compaction of the concrete along the entirelength of each sleeper in the bed. Compacting the concreteonly by vibration of the entire mould should be avoided inorder to ensure that the geometry of the moulds remainsmaintained and that the compaction of the concrete does notvary due to vibration nodes of the mould.

c) Curing

The Cement fineness (blaine) of at least 440 m/kg should beused. The moulds should be covered by tarpaulins directlyafter casting in order to utilise the heat created by the hydrationprocess. The maximum allowed temperature in the concreteduring the curing should be limited to 60 C.

d) Anchorage of prestress

Both single wire strands and three-wire strands can be used.An indented strand surface should be used in order to ensurethe bond capacity of the strands.

e) Cutting of sleepers

The cutting of the sleepers should be performed by mean of adiamond disc cutter that simultaneously goes through boththe concrete and the wires. No corrosion protection should

be applied at the sleeper ends in order to enable futureinspection of possible wire slippage at the ends of the sleeper.


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f) Cleaning and oiling

A controlled cleaning and oiling of the moulds must be donein order to obtain good surfaces of the sleeper and avoidingbuild up of concrete in parts of the mould that will lead to the

need of a higher demoulding force.

6.0 High speed - Issues involved

High Speed is a relative term. There is no defined speed at which atrain can be called a high speed train. We can say anything at orover 100mph (160 kmph) is fast and should be thought of as highspeed. Viewing the advantage that high speed train offers over roadand air travel in intercity journeys, particularly between metros, Indian

Railways have decided to conduct pre-feasibility studies for runninghigh speed trains at 300-350 kms per hour in all the four regions ofthe country. This will mean heavy investment in construction of newdedicated tracks for high speed trains, development and acquisitionof high speed rolling stock and development & installation ofSignalling and OHE. High speed trains hope to capitalise on rapidgrowth of Indian economy, rising industrialisation, urbanisation andunprecedented growth in intercity travel.

Load distribution factor, dynamic augmentation factor depend ontrack irregularities, wheel flat, stiffness of rubber pad, ballast pressuredistribution etc. and dynamic augmentation factors is largely affectedby speed, wheel flat and rubber pad stiffness. Thus, detailed analysisand design of sleeper through simulation studies becomes veryimportant for heavy haul and high speed traffic.

The dynamic coefficient partially consists of a dynamic factor due to

wheel defects. Typically, this dynamic factor is taken as 150% forspeed below 200 kmph and 175% for speed above 200 kmph.AREMA code specifies an impact factor of 200% and a separatespeed factor between 0.7 and 1.2 providing a dynamic factor from140% to 240% respectively. AS1085.14 specifies 150% for a vehiclespeed of 80 kmph and 200% for a vehicle speed of 115 kmph.

As per AREMA, Rail seat characteristic bending moment

Mdr = B x V x T

B - unfactorised bending moment

V - Speed factor

T - Tonnage factor


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As per UIC, speed factor for design of sleeper on soft pad & hardpad are 1.39 & 1.50 for 25T axle load (upto speed 120 kmph), 1.59& 1.75 for high speed standard ( 22.5 T axle load & speed 200 kmph)& TGV (18 T axle load & speed 300 kmph).

7.0 Conclusion

z The design of concrete sleeper differs from normal structuraldesign as loading and support condition cannot be assessed toa high degree of certainty. PSC Sleeper is designed for NOcrack condition as crack reduces the fatigue life of sleeper apartfrom decrease in load carrying capacity.

z On the basis of past experience, Indian Railways have specifieddesign criteria considering the certain degree of uncertainty.

Due to variations in track and rolling stocks, correct assessmentof design loading is not possible and analysis of the dynamicbehavior of sleeper is still a matter of research.

z Imposed bending moment largely depends upon base area &length of sleeper and resisting moment is a property of sectionat different locations. Therefore, profile of the sleeper plays amajor role in the strength of sleeper against cracking and failure.Revision of design criteria is an ongoing process and IndianRailways is continuously engaged in rationalising the variousdesign parameters through research as well as experiencegained.

Tonnage per annum - Million Gross Tons (MGT)

Fig. 6 : Function of the tonnage and the speed factors

Speed (Kmph)


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z Recent study shows, load distribution factor, dynamicaugmentation factor depend on track irregularities, wheel flat,stiffness of rubber pad, ballast pressure distribution etc. Thesimulation study and determination of these factors is must fordesign of sleeper for high speed. Rail wheel irregularity to theminimum possible extent, joint less track, good quality of weldsand track maintenance of high standard are essentialprerequisite for sleeper for high speed track.

z Improvement in manufacturing process of concrete sleeper isimperative to achieve high quality in sleeper production suitablefor high speed as elaborated in para 5.5.

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3-a.k. snghal rajeev verma

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