Under Sleeper Pads in Turnouts
Modern railway tracks distribute the loads from rail vehicles via the rails, rail
seats and sleepers as evenly as possible. There are limitations when it comesto turnouts. Points show an irregularity in regard of the vertical elasticity. Mostly
they are too stiff. But their elasticity can be adjusted by under sleeper pads.
1 Challenge
Modern railway tracks need to be able tobear the loads from rail vehicles via therails, rail seats and sleepers as evenly as
possible and distribute such to the track
superstructure and to the subgrade. By distributing these loads sufficiently, stresses
can be kept as low as possible, helping tominimize maintenance expenses and thusincrease the operating life of the track system.
Although one can draw on tried and trusted
calculation methods, as per Zimmermann[1] for beams on elastic foundation in respect of the load distribution effect, there
are limitations when it comes to turnouts.
Due to their geometry (Fig. 1), turnouts
show an irregularity which can result in

Variations in stiffness along the tongue andfrog area, as well as the constantly changing load-bearing surface area of the sleepers in the ballast superstructure result in
discontinuity of the track system.
The differences in the load-distributing over
the turnout length is a 3-dimensional problem which can be grasped and analyzed
using Finite Elements Methods (FEM). By
installing additional elastic elements withdefined stiffness it is possible to increase
the load-distributing effect of the rails. At
the same time the load-bearing characteristics of the track frame can be optimized.
One cost-effective way of approaching thistarget is to use under sleeper pads (USP)
with varying degrees of stiffness.
In the following a short presentation of thefundamental impact of the bedding modulus, the possible increase in superstructure
elasticity via USP and a 3-dimensional FEM
turnout model for optimizing the USP arrangement is provided.

2 Bedding Modulus

Fig. 1: Turnout with rigid crossing frog andcheck rails

various degrees of rail deflection within different areas, even if other boundary conditions remain unchanged.

Harald Loy

System Development
Address: Getzner Werkstoffe GmbH
Herrenau 5, A-6706 Buers
harald.loy@getzner.com

The static and dynamic loads on the track
superstructure stemming from rail traffic mainly depend on the behaviour of the
track bed, as well as the geometry andstiffness characteristics of the track frame.
For traditional ballasted track, the elasticity
derives primarily from the flexibility of theballast bed and the subgrade. It is generally expressed via the bedding modulus C
and represents the relationship between
the surface pressure and the related rail
deflection.
p[N/mm3] withyp = pressure between sleeper and ballast[N/mm2]y = rail deflection [mm]C=

In simplified terms, the bedding modulus
indicates how much pressure [N/mm²] results in a deflection of 1 mm. As surfacepressure is already included, the bedding
modulus is used to describe stiffness incases where the elasticity primarily resultsfrom a surface mounting, for example withtraditional ballasted track, or for characterizing flat elastic elements such as ballast
mats and USP.
The bedding modulus of different kindsof ballasted track ranges from roughly
0.05 N/mm3 (very soft) to more than
0.40 N/mm3 (very hard) on average, depending on the installation conditions [2].
For new rail lines, the higher stiffness mainly
results from the construction of compacted
substructure and anti-frost layers. Moreover, the use of the Dynamic Track Stabilizer(DGS) in ballasted track also results in consolidation. While measures of this kind increase the load-bearing ability of the track
bed, at the same time the rail’s function asa load-distributing element is reduced, withnegative ramifications for dynamic effects
in wheel/rail contact, and this can lead toincreased stresses on the ballast.
Higher degrees of bedding stiffness due
to consolidated ballast and subgrade withgreater load-bearing capacity can be adjusted by installing elastic elements with lower
levels of bedding modulus.
Regarding the beam on elastic foundationaccording to Zimmermann, a reductionof the bedding modulus from C to C* decreases the ballast pressure by the factor.
4
C * /CIf the effect of a reduced ballast pressure
is quantified with regard to considerationof track stability using the 2nd power law,a reduction in the ballast pressure of 15%
results for example in a lengthening of theintervals for track maintenance by a factorof 1.4. With regard to consideration of trackstability using the 4th power law, the same
reduction in the ballast pressure results inlengthening the interval by a factor of 1.9,
i. e. the duration until the next track maintenance is almost doubled!
The applicability of this assessment of
track stability is backed up adequately by
the derived results of the AASHO Road
Tests [3] and the experience gathered by
Deutsche Bahn following introduction ofheavy superstructures using UIC 60 rails[4]. In this regard, reference is made to a
direct ‘hard mounting’ of the sleepers on
the ballast.

RTR 2/200935

 Under Sleeper Pads in Turnouts
This is also valid for upgrading of tracks, as
the costs involved for such work are also
comparatively low.
With ballasted track, elastic USP not only
allow for a longer rail bending line, which reduces loads on the ballast, they also help toprevent contact abrasion as the top layer ofballast can become embedded in the pads.
Hard contact points between the bottom of
the sleeper and the ballast are alleviatedand the track mounting is more homogeneous. The pads also help to prevent sudden
settling of sleepers due to cavitations [7].

Fig. 2: Concrete sleepers with Sylomer®
under sleeper pads (USP)

In regard to the flexibility of the ballasted
track, the resulting bedding modulus of thestandard gauge track should be set in a waythat sufficient rail deflection is ensured,
which should not be less than 1.0 – 1.2 mm[6], taking into consideration the load-distributing effect in the track and the turnouts.
The limiting factor is generally the maximum
permissible amount of rail foot tension.

Due to stabilization of the top ballast layer,
migration of ballast rocks due to dynamic
forces is shifted to lower layers, which canhave a benign effect on the long-term quality of the superstructure.

3 Increasing Superstructure
Elasticity with Under Sleeper
Pads (UPS)

Even under the assumption of declining effectiveness over the effective lifetime of
the rail, USP still cannot result in any detrimental impact on the track superstructure.
In this regard, they can be seen as fail-safe
elements. The track superstructure will always exhibit more favourable characteristics than structure without any USP.

USP are a cost-effective way for subsequently increasing the elasticity of the superstructure and reducing wear and tearon the ballast (Fig. 2). USP are elastic elements located between the sleeper and theballast and are today available in a continuous range from approximately 0.02 N/mm3
to harder than 0.30 N/mm3 for standard
products, Determination of bedding modulus is given by BN 918 145 – 01 on a prescribed load plate with ballast profile [5].

There are numerous ways to ensure adequate adhesion of the pads to the sleepers. One possibility is to glue the pad to thecured concrete, but the general trend in the
future shows that the pads are integrated
directly into the sleepers as a part of the
manufacturing process. A tight connection
can be achieved using a plastic mesh, halfof which is integrated into the sleeper pad
and the other half of which can be vibrated
into the wet concrete of the sleeper.

The investment cost is far lower than thosefor the installation of ballast mats. Ballast
mats are mainly used where direct mounting to a hard subgrade, e. g. on a bridge,
should to be avoided.

Tests in Germany have shown that the useof sleeper pads allows for significantly improved track behaviour and dynamic vibration behaviour compared to traditional ballasted track [8]. In Austria, turnouts with
USP were already installed in 2002 andmeasurements have shown a reduction ofvibrations in the 40 Hz – 50 Hz frequency
range. Moreover, substantially less subsidence was found in turnouts with rigid crossing frogs, even compared to constructions
with moveable crossing frogs [9]. The posi-

USP does not necessitate the installationof elastic railpads. The track frame consisting of the rails and concrete sleepers,
which is rigidly connected into a load-bearing structure via the rail fasteners, can thus
remained unchanged in its traditional role.

switch rails
switch toes

switchingmachineCompared to straight track, for which calculations are easy to conduct due to the relatively homogenous geometry with constantrail profiles and sleeper mounting surfaces,
calculating elastic elements in turnouts is
far more complex. The main reasons forhigher effort include the varying profiles of
the rails, the additional construction elements and the generally strong variations inthe sleeper conditions. These parameters
result in varying degrees of vertical load
deflection.
Looking at a standard right-hand turnout,
there is an initial jump in stiffness at thetransition point from the stock rail sleepers, which are not supposed to be tampedin the middle, to the heavier turnout sleepers which are often located in front of theswitch panel (Fig. 3).
In the switch panel itself, switching machines which are integrated into hollow
steel sleepers can result in additional discontinuities. The switch rails themselves
have an increasing moment of inertia fromthe toes to the centre of the turnout. During
passage of the vehicle, while the stock rail
is loaded with one wheel load, the softer
switch rail results in a lower distribution ofload when subject to the load from the opposite wheel load. In the area of the closure rail, the continuously growing sleeper
areas are also noticeable. The massive frog
together with the wing rails results in thegreatest stiffness. Due to the check rails
and the long sleepers, the distribution of
the loads is greatest in this area. Immediately after the last long sleeper there areoften eccentrically placed short sleepers
with shortened sleeper heads on the interior side. This results in a strong load on
one side, with corresponding torsion of the
track, depending on the track design.
The geometrically determined discontinuity

crossing nose
flange way

through rails

switch heels
check rails

switch panel

closure panel

Fig. 3: Components of a standard right-hand turnout

RTR 2/2009

4 Geometrical Discontinuities at
Turnouts

wing rails
closure rails

stock rails

36

tive experiences with USP in turnouts have
led to the development of a standardized
design for the Austrian Federal Railways.

crossing panel

Fig. 4: FEM turnout model for the optimizationof under sleepers pads

Under Sleeper Pads in Turnouts in the turnout area causes strong localized
variations in the loads on the superstructure. With a lower bedding modulus, thebending lines can be lengthened which reduces the amount of pressures on the ballast. At the same time, by optimizing the
distribution of the bedding modulus withvarious USP it is possible to smooth out
the differences in stiffness stemming fromthe geometry. Thus, careful design and installation of various types of sleeper pads
can achieve a double positive effect in theturnout area: Loads on the superstructure
can be reduced and the turnout can be‘smoothed out'.

5 FEM Turnout Model
It is necessary to understand the entire
system in order to be able to transpose thepositive experiences gained with padded
sleepers to applications involving turnouts.
And furthermore to balance out the geometrically determined differences in vertical load distribution. The Finite Elements
Method (FEM) can help us to grasp thissystem (Fig. 4).
Using FEM and high-performance computers, it is possible to generate a complete
turnout with padded sleepers and to perform analyses in respect of its load-bearing
functions. In contrast to the manual calculation method according to Zimmermann,
there is no need for idealization by transformation of the entire structure (transverse
sleepers in longitudinal sleeper superstructure, resultant stiffness for several elastic levels). But in order to cut calculation
times, the individual components are reduced by base elements to their relevant
functionality and the necessary degrees of
freedom. Another advantage of the model isthat variable parameters can be assigned
for all geometries and stiffness in the rail
elements (tongue, crossing frog, checkrails, etc.). The same is valid for sleeper
parameters and the non-linear behaviour ofFig. 5: Deflection pattern of the rail through the entire turnout by reduced bedding modulus stepby step

the rail fasteners (rail plate/baseplate function under pressure loads - fastening clips
function under tension loads). Moreover,the behaviour of the bedding (sleeper pads
and ballast) can be modelled using any desired non-linear deflection curves.
As the individual elastic levels are not ‘superposed’ by the formation of resultant
spring stiffness, it is possible to simulate adivision of the flexibility in levels above andbelow the sleepers. As a result, it is also
possible to individually observe deformation variables.
Parameterization of the model allows forany standard turnout to be generated withthe defined characteristics and for calculations to be carried out with a moving
load collective, corresponding to the load
impact of the bogies. As a result, one obtains all of the vertical deformation in theentire structure (subsidence and rises);
the strains result from the kinematic conditions and the parameters of the construction components. A holistic model ofthis nature can be helpful for grasping thesystem comprising the elastically mounted
turnout in the ballasted track and making
targeted adjustment for the further development of such.

6 Analysis of Load-Bearing
Behaviour
In order to understand the bedding impact
on the vertical rail deflection, the stiffnessof the ballast and the subgrade throughout
the entire turnout area is uniformly andstepwise reduced within the framework ofa simulated calculation. The maximum rail
deflection can therefore be calculated foreach load position, based on the load col-"VOJRVFJOTJHIUUPUIFXPSMESBJMXBZNBSLFU
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es compared to a turnout without USP. Even
with the relatively stiff pads used in thiscase (> 0.2 N/mm3) the forces transmitted
into the superstructure can be reduced on
the order of 10 % to 30 %, depending on thepart of the turnout.

7 Summary
The use of under sleeper pads (USP) can
increase the elasticity of track superstructure with relatively low investment costs. At
the same time the ballast, which is a latentsource of track instability, is stabilized, as
individual ballast rocks can become embedded in the surface layer of the USP. Loads
on the superstructure are reduced by amore homogenous mounting of the sleepers and track stability is improved.

Fig. 6: Comparison of rail deflection patterns with optimized track using different USP along theturnout

Moreover, in turnouts, the geometricallydetermined differences in stiffness can besmoothed out. To achieve this target, USP
with various degrees of stiffness can beused, positioned in a way that the entire
construction features improved load-bearing conditions. This allows the track to besmoother. Track irregularities resulting fromturnouts can thus be mitigated and vibrations can be reduced.
Using a turnout model based on the Finite
Elements Method, it is possible to analyze
the load-bearing behaviour of the construction. As data can be varied for different parameters, it is possible to analyze a very
wide range of geometric boundary conditions and stiffness conditions. The challenge involved is to take into consideration
the non-linear bedding properties.
Using the FEM turnout model, an optimized
arrangement of Sylomer® USP can be calculated for any situation.

Fig. 7: Reduction of rail seat forces along the turnout with optimized solution
References

lective for a typical bogie (axle load 220 kN,
3 m spacing). The deflection patterns presented in Fig. 5 represent the envelope forthe maximum vertical deformations of therelevant rail.
One can see that a lowering of the bedding
modulus results in an increase in rail deflection as desired. But at the same timethe various differences in the stiffness canalso be seen along the path of the turnout.
The differences in the deformation curves
are magnified. The lower the bedding modulus is, the more pronounced the differences are. In this regard, the amount of deflection is lowest in the crossing frog due to thehigher level of rail stiffness and the large
surface area of the sleepers, whereas immediately after the last long sleeper.
By optimizing the arrangement of USP withvarying degrees of stiffness, these differences can be smoothed out. Fig. 6 shows a
38

RTR 2/2009

comparison with an improved arrangementof stiffness using various types of USP.
The first curve shows as an example thedeflection curve without any additional USP.
The rail deflection of 0.8 mm in the regulartrack results exclusively from the defined
bedding modulus of the ballast and the subgrade of 0.2 N/mm3. If the vertical deformation is increased by the uniform installationof additional under sleeper pads, the resulting bedding modulus of roughly 0.10 N/mm3
leads to an increase in deflection to
1.3 mm before and after the turnout. The
differences in vertical rail deflection can besmoothed out by the installation of various
USP, in the event of identical initial levels.
This allows for a more homogenous patternof deflection to be achieved (using different
USP types). The turnout is smoothed in its
function as a load-bearing element.
Fig. 7 shows the reduction of rail seat forc-[1] Eisenmann, J.: Die Schiene als Träger und Fahrbahn. Die Eisenbahnschiene (Herausgeber Fastenrath). Verlag Wilhelm Ernst & Sohn 1977, S.9…78.
[2] Eisenmann, J. und Rump, R.: Ein Schotteroberbau
für hohe Geschwindigkeiten. ETR – Eisenbahntechnische Rundschau 46, (1997), Heft 3, S.99…108.
[3] AASHO-Road-Test Special Report 61G. DeutscheÜbersetzung in Betonstraßenjahrbuch 1962/63,
Beton-Verlag, Düsseldorf.
[4] ORE-Frage D.161, Bericht Nr.4 vom Sept. 1987: Dynamische Auswirkungen der Anhebung der Radsatzlast von 20t auf 22,5t und der geschätzte Anstieg
der Oberbauelastizität.
[5] DB Systemtechnik: BN 918145-01: Technische
Lieferbedingungen – Spannbetonschwellen mit elastischer Sohle – Elastische Schwellensohlen, (Jan.
2004)[6] Eisenmann, J., Leykauf, G. und Mattner, L.: Vorschläge zur Erhöhung der Oberbauelastizität. ETR-Eisenbahntechnische Rundschau 43 (1994). H. 7/8,
S.491…494[7] Müller-Boruttau, F., Kleinert, U.: Betonschwellen mit
elastischer Sohle, ETR 50 (2001). H3 S. 90-98.
[8] Leykauf, G., Stahl, W.: Untersuchungen und Erfahrungen mit besohlten Schwellen. EI Der Eisenbahningenieur (55) 6/2004, S. 8-16.
[9] Schilder, R.: USP Under Sleeper Pads –Schwellenbesohlungen – Ein Bericht über bisherige
Erfahrungen, ÖVG Salzburg Congress, Band 65,
Sept. 2004