1

Testing criteria for non-ballasted track and embeddedtrack systems

Andr Van LeuvenDynamic Engineering

St Louis, MO

Tom Vanhonacker,

D2S InternationalHeverlee, Belgium

ABSTRACT

The EC co funded research project Urban Track aimsat reducing the total life cycle cost of urban rail systemsby 25%. This goal will be achieved through thedevelopment of low cost track designs that take intoaccount the maintenance aspects in order to reducemaintenance costs and increase track availability. Thenew track systems will have a low track modulus tominimize ground borne noise and vibrations andminimize rail wear. Acceptance of track systems dependson proving they are fit for service by amongst otherssuccessfully completing a repeated load test. However,experience has shown that under site specific conditionsand especially in small radius curves, very resilient railfasteners have failed in service despite the fact that thedesign passed the repeated load test in the lab accordingto the existing norms. In addition, there is no clearnormative frame for repeated load testing of embeddedtrack. Current practice is based on the standard for testingof direct fixation fasteners. This paper discusses thedevelopment of a method to define the repeated load testconditions for resilient rail fasteners and embeddedcontinuously supported fastenerless rail systems andsituations. For existing track systems, the proposedmethod is based on the use of experimental on site data;for new systems the proposed method is based on theresults of multi body simulations with for track andvehicle. The repeated load tests are executed by applyingan artificial force in one point of the railhead of a sampleassembly of a limited length. The artificial force isapplied in such a way that the same lateral and verticalrail displacements are obtained that were derived from themeasurements or the simulations. The method forexisting rail systems and situations starting from raildisplacement measurements is illustrated in detail as itwas applied to the testing of very resilient rail fasteners.

INTRODUCTION

Performance requirements and test methods for directfixation fasteners are described in following documents:

European norm EN 13481: Railwayapplications Track Performancerequirements for fastening systems (Part 1 to 7);

European norm EN 13146: Railwayapplications Track Test methods forfastening systems (part 1 to 8).

It has been found that these test methods andperformance requirements do not always correspond tothe needs and in service conditions experienced by veryresilient direct fixation fasteners. Due to their specificdesign and characteristics, their increased resiliency andheight (due to the presence of a more resilient base platepad), it is necessary to verify the test conditions describedin the above-mentioned documents. More specific, this isdone for the load conditions during repeated load testing.It is found that the loads applied during repeated loadtesting should be re-evaluated for very resilient railfasteners.

For embedded tram tracks, despite their increasingpopularity, there is at present neither a clear normativeframe nor a common set of functional requirements underwhich Embedded Rail Systems may be approved andcertified; no specific EN or ISO standards are available.

Up to now, Embedded Fastenerless Rail Systemshave been tested using test procedures derived from thespecifications for discrete and continuous supported railon slab track (EN-13481-5&6), the recommendations andguidelines compiled by CROW in the Netherlands(RAGERS 2001) and experience.

2/7

VERY RESILIENT DIRECT FIXATIONTRACK

For conventional direct fixation track, performancerequirements and test methods are described in the normsand documents cited earlier.

Although the norms make a distinction in testconditions for Main Line and Light Rail, it is also notedthat in order to perform a representative fatigue test, thereare many other contributing factors, which shoulddetermine the exact test conditions.

It is however difficult to list test conditions for eachpossible situation, especially for very resilient railfasteners as they are often only used in very particular andshort sections of the network.

One could argue that an analytical approach can beused to determine the forces acting on the fastener. Inreality, loads are distributed unevenly over the two railsand unevenly down to the rail fasteners. They are oftenvery difficult to quantify as they not only consist of staticor quasi-static loads but also of dynamic loads caused bytrack irregularities, discontinuities at welds and joints,switches, corrugations, vehicles defects such as wheelflats, hunting etc.

In general, the forces, which act on a railway track asa result of vehicle passage, can be split up in threecomponents:

- vertical;

- lateral;

- longitudinal.

The vertical and lateral loads generated by movingrailway equipment are applied by wheel treads andflanges to the rails, which in turn must be held in place byfastenings and sleepers and ballast or slab.

Lateral, vertical and torsional stiffnesses of the raildistribute the lateral loads to the fasteners andconsequently to the sleepers and ballast or slab.

The amplitude of the loads, which must be restrained,depends on not only the dimensions, configuration,weight, speed, and tracking characteristics of the rollingstock but also upon the geometric characteristics of thetrack structure. The latter includes not only the trackalignment but also the local track geometry such as trackirregularities and deviations from the original design.

One could assume that lateral load distributionmimics the vertical load distribution to the discretesupports. However, the AREMA manual (AmericanRailway Engineering and Maintenance of- WayAssociation) warns this assumption may be non-

conservative since field observations suggest that such adistribution underestimates the actual load environment.

The in-service situation is considerably complex dueto the presence of several coupled wheel sets, variouspositions the vehicle can assume in a curve and theadhesion forces between the wheel and the rail. The totalforces (especially lateral forces) cannot be predicted withgreat accuracy.

Longitudinal loads are caused by accelerating andbraking, temperature forces, track creep and shrinkagestresses caused by rail welding.

Resiliency related issues

Besides the difficulties to quantify, the exact forcesacting on rail fasteners, very resilient rail fasteners arecharacterized by a specific design that influences the in-service loads to a greater extend than what is the case forconventional direct fixation fasteners.

Most very resilient fixation fasteners have two elasticlayers: a rail pad and a base plate pad. The base plate padis responsible for a significant increase in height of thefastener. Due to this increase, more pronounced rotationmoments are exercised on the fastener, causing additionaldisplacements.

The high vertical resiliency will also decrease thevertical reaction forces on the fastener whilst this is notnecessarily the case for the lateral reaction forces.

For example, the fastener loads can be calculatedusing the Zimmermann approach considering the rail asan elastically supported beam. When varying the verticaldynamic stiffness of the support points from 120 kN/mmto 5 kN/mm and considering a 24 ton axle load (120 kNwheel load), a UIC 54 rail, a fastener spacing of 600 mm,the wheels on the same bogie spaced at 2400 mm and thewheels of the nearest bogie of the adjacent car at4800 mm and 7200 mm, than the maximum load on asingle fastener decreases according to the figure below.

Fastener load vs. fastener stiffness

0

10

20

30

40

50

60

70

0 20 40 60 80 100 120 140

Fastener Stiffness

Fa

ste

ner

Lo

ad

Figure 1

3/7

It can be seen that for dynamic fastener stiffnessabove 70 kN/mm the wheel load of 120 kN results in afastener load of 56 kN and higher. However, for thefastener stiffness of 20 and 30 kN/mm, the fastener load isreduced to respectively 35 and 40 kN. While the fastenerloads are decreased, vertical deflections have increasedfrom below 0.8 mm to respectively about 2 and 1.5 mm.

A similar calculation is done using Finite Elements,showing the load distribution of a bogie (20 ton axle load)over the adjacent fasteners for four different verticalstiffnesses. It can be seen that the resiliency not onlyinfluences the maximum reaction forces but also thedistribution of the loads over the different fasteners. Thedifference in load distribution and resiliency will result indifferent rail displacement patterns.

Figure 2

The current approach of fastener load estimationseems valid and safe for stiff track. However, for veryresilient rail fasteners, the load distribution pattern isdifferent. The current approach does not account for theeffect of the highly increased resiliency. In addition, theapproach does not to cover variations in sleeper spacingand wheel spacing. Considering the complexity of theforces acting on the fasteners and the increaseddisplacements caused by an increased resiliency andspecific fastener design, it can be concluded that fatiguetesting imposing a certain test load, trying to represent themost adverse in-service load, is too safe or even not safewhen it concerns very resilient direct fixation track. Forvery resilient rail fasteners, an approach simulating themost adverse rail displacements is more accurate, as thesedisplacements are directly responsible for fatigue failure.The prediction and/or measurement of rail displacementsare practically more feasible.

Current testing loads and angle

Th