www.railwaygazette.com

MAY 2022

Railway Gazette
INTERNATIONAL

Making tracks
to Münster

MAINTENANCE
Rethinking rail asset
management strategies
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Page 54TRACK MAINTENANCE Switzerland

Sittertobel
renovation
improves
WUDFN|IRUFHVSchweizerische Südostbahn worked with industry partners including Getzner:HUNVWRƪHWRUHQRYDWHLWVKLVWRULF6LWWHUWREHOYLDGXFWDQGLQVWDOODPRUHRSWLPLVHGWUDFNVWUXFWXUH7KHEHQHƬWVRIWKHFKDQJHVKDYHEHHQTXDQWLƬHG
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Dipl-Ing Martin Quirchmair
Corporate Development,*HW]QHU|:HUNVWRƪH

Dipl-Ing Stefan Werner
KPZ Fahrbahn AG

Dipl-Ing Beat Burgherr
62%,QIUDVWUXNWXUDr Harald Loy*HW]QHU:HUNVWRƪH,768QLYHUVLW\,QQVEUXFN

L

ocated on the approach to
St Gallen, the Sittertobel
viaduct on the Schweizerische
Südostbahn is one of the mostimposing railway structures in the
Alps. Completed in 1910, the 99 mhigh viaduct is the tallest railway bridgein Switzerland. The 365 m long
structure comprises two approach
viaducts and a 120 m long steel truss
girder main span across the Sitter

26

River; this so-called ‘ﬁsh belly’ is up to
12 m in height and weighsapproximately 920 tonnes.
Having passed its centenary, thestructure has been subject to close
attention. Routine inspections in recent
years revealed the natural eﬀects ofwear and tear on both the approach
viaducts and the main span. Because of
the strategic importance of the viaductto the SOB network, as well as its
cultural and heritage signiﬁcance,
the railway decided to undertake acomprehensive renewal programmewith the aim of equipping it to last afurther 50 years without majorintervention or replacement1.
Feasibility studies conﬁrmed that the
repairs were viable, both technicallyand commercially. Work started in2019 and was concluded in 2021. In
addition to the restoration of the
viaduct itself, the entire track

1910
Year theSittertobel
viaduct wascompletedsuperstructure was renewed, with aredesigned transition zone between the
open deck and the ballasted track
elements, and the relocation of the rail
expansion joint.
Prior to the reconstruction, theapproach viaducts were laid with aballast trough and wooden sleepers.
The southern (Herisau) end of thebridge has more approach spans than
the northern (St Gallen) end, and also
includes a 350 m radius curve.
Reﬂecting the load-bearing capacity ofthe steel truss girder, a considerably
lighter open deck was used for thecentre span. Specialist bridge sleepers
were mounted on rubber pads, which
provided additional elasticity to thetrack.
Expansion joints were ﬁtted to therails, with their centres approximately
4 m from the Herisau end of theﬁsh belly. However, pronounced void

May 2022 Railway Gazette International

Switzerland TRACK MAINTENANCE
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formation was apparent around thejoint, with signs of wear and ballast
degradation. This meant thatmaintenance intervals on this part ofthe viaduct were shorter thanelsewhere, with increased costs.
Addressing the varying track stiﬀness
around the joint was therefore a key
focus for the bridge renovation project.
On long bridges, the rail expansion
joints are typically located close to the
expansion joints for the structure itself.
They are thus able to absorb
longitudinal track movements as the
structure expands and contracts andprevent excessive rail stress resulting
from thermal length variations anddynamic loads.
In Switzerland, the requirements for
track over bridges are set out in SBB’s
technical standard I-22068. This also
describes the use of expansioncompensating devices according to the5m 
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9m

32m

length and type of bridge2. The
principle behind the rail expansion
joint is shown in Fig 2. In the centre of
the joint, the switch and stock rails arepressed against each other by braces.
The rail ends can slide against eachother, compensating for any movementin the bridge. Modern rail expansion
joints can typically expand by
1 200 mm or more.
However, the rail expansion joint
causes a local change in the structure’s
characteristics; this may result in load
peaks under dynamic loading
conditions. It was no surprise that
increased signs of wear, such as ballast
destruction and voids, were appearing
in the area around the expansion joint.

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BEFORERENOVATIONAFTERRENOVATION
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than timber. In the area around thetransitions between the bridge track
and the ballasted track, it was decided
to deploy heavier concrete turnout
sleepers.
The problem areas were physicallyseparated to alleviate the problems that
had been caused by the combination of
the transition zones and the rail
expansion joint. The centre of the rail
expansion joint was moved outwards to
14 m from the end of the ﬁsh belly.
Elastic rail seats from Delkor were
installed on the main span in order toadapt the stiﬀness of the open deck
bridge track to that of the ballasted
sections (Fig 1).

Track elasticity
Redesigned superstructure
Redesign of the superstructure wasmanaged by Kompetenzzentrum
Fahrbahn AG. Because of diﬃculty inreplacing the sleepers on the ﬁsh belly,
KPZ decided early on to dispense withwooden sleepers and use weatherproof
ﬁbre-reinforced foamed urethane
sleepers. Elsewhere, the standard
ballasted track has been relaid using
conventional concrete sleepers rather

The adjustment in stiﬀness at thetransitions between the ﬁsh belly andthe approach viaducts was carried out
with the help of FEM modelling. The
structural design concept had beendeveloped by KPZ, and from there
the elasticity of the track could be
optimised.
Based on calculations carried out
using various product combinations,the conﬁguration shown in Fig 1 was)LJ7KHEDVLFDUUDQJHPHQWRIWKH
UDLOH[SDQVLRQMRLQW
where the switch
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each other by rail
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DEVRUEORQJLWXGLQDOPRYHPHQWVRailway Gazette International May 2022

27

TRACK MAINTENANCE Switzerland
(

determined to be the most appropriate.
A stiﬀness value of 20 kN/mm was set
for the rail seats on the open deck.
A D1519 under-ballast mat with abedding stiﬀness of 0·15 N/mm3 waslaid under the ballasted track on theapproach spans in the vicinity of therail expansion joint, which was also
supported on FFU sleepers. The
turnout sleepers which support theconnection to the standard track
superstructure were equipped withelastoplastic SLB 2210 G under-sleeper
pads. These are designed to preserve
the track alignment and protect theballast. Fig 3 shows the restored
superstructure of the approach viaductfrom the Herisau end.
Guard rails have also been installed
next to the 54E2 running rails; these
extend across the entire length of the
viaduct and function as a safetymeasure, preventing a derailed train
from reaching the edge of the bridge.
From a technical perspective, they arealso used to adjust stiﬀness in thetransition zones. However, thisapproach can sometimes lead toproblems in the longer term, as theguard rails can aﬀect the mechanicalloading of the superstructurecomponents.
This question therefore had to beaddressed when dealing with thetransition zone. The stiﬀening eﬀect of
the guard rails on the ballasted track
sections generates higher forces,
particularly at the connection between
the ﬁsh belly and the approach spans.
This happens because guard rails on the
open deck section have almost no
impact on track stiﬀness, which isdominated by the soft rail seats that siton the rigidly bedded FFU sleepers andalmost completely decouple therunning rails from the guard rails.
In particular, the ﬁrst sleeper on theballasted track is held in position by theﬁxing of the guard rails on the open
deck. This results in less deﬂection andproduces an imperfection in the track.
The higher forces at the transition canbe seen in the calculated deﬂection
curve (Fig 4). A simple way to mitigate
this eﬀect was to remove theFig 3. A view of theapproach viaductfollowing restoration.

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entire length of thestructure.



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Below: Fig 4. The
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curve at thetransition between
WKHƬVKEHOO\DQG
approach spans. The
guard rail is fastened
to or loosened from
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the ballasted track.
Right: Fig 5.
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connection of the guard rails from theﬁrst FFU sleeper on the ballasted track
section. Modelling showed that thiswould result in a signiﬁcantly morehomogeneous deﬂection in thetransition zone, which in turn had adirect impact on the dynamic forces.
Fig 4 shows the computational
simulation of this change, which could
then be incorporated into the planningof the restoration work3.

Measuring the results
To verify the improvements brought
about by the structural restoration,
Getzner Werkstoﬀe performed‘before and after’ track geometry
measurements in May 2019 and
September 2020.
In order to analyse track
performance in the transition zone, theGetzner team focused on measuring
rail deﬂection. Four measuring sections
marked M1 to M4 were set out asshown in Fig 5.

M1 was located on the main spanand captured the deﬂections on the
open deck. Deﬂection at the rail
expansion joint was measured at M2.
Because the centre of the rail expansion
joint was moved during the restoration,
the two locations are marked in Fig 5 as‘2019’ and ‘2020’. Sections M3 and M4
were used to determine the deﬂectionof the FFU sleepers and the padded
turnout sleepers. These two sections
were located at the start of the 350 m
radius curve on the Herisau approach.
Vibrations in the structure were
measured at M5. In this context, thevibrations were seen purely as ameasure of the dynamic load of apassing train. If these could be reduced,
the load exerted on the track
superstructure and hence on the
structure itself, would also be reduced.
Rail deﬂection at M1, M3 and M4
was measured using the approachshown in Fig 6. On the assumption thatthe tilting of the rail φ is low and anycollection points in2019 and 2020 toenable a ‘before andDƱHUpFRPSDULVRQRI
the behaviour of thesuperstructure.

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May 2022 Railway Gazette International

Switzerland TRACK MAINTENANCE

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our assessment of the improved track

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   alignment used the average resultsfrom all the train passes before andafter the restoration. The following
discussion looks at an example of a
Flirt EMU travelling towards Herisau4.
Fig 8 shows the deﬂection curves forboth 2019 and 2020. In both cases, M1
on the ﬁsh belly measured a uniform
deﬂection with no tilting of the rail.
Before restoration, the amount ofdeﬂection was primarily dictated by therubber pads under the bridge sleepers.
After reconstruction, the required levelof elasticity was provided by the elastic
rail seats.
The rail expansion joint wasmeasured at M2. Signiﬁcant diﬀerenceswere observed after the reconstruction,and the new position of the joint can beseen in the time signal. In 2019, a large
deﬂection was observed during thepassage of a train, which is particularly
noticeable in the graph of the stock rail
(marked ‘outer’). This showedsigniﬁcantly more deﬂection than theswitch rail (‘inner’). The deﬂection of
more than 4 mm was also signiﬁcantly
greater than the rest of the track. After 

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In regular service, three types ofpassenger trains normally pass over
the Sittertobel viaduct: SOB’s Stadler
Flirt EMUs, Thurbo’s GTW EMUs, andSOB’s Voralpenexpress inter-regional
services; these were initially operated
by locomotive-hauled push-pullformations, but were replaced frommid-2019 by new Stadler Traverso
EMUs.
Because of the restoration workon the track superstructure and theviaduct itself, a speed restriction was inplace at the time of the measurements;
trains were limited to 50 km/h, rather
than the regular line speed of 80 km/h.
In measuring sections M1, M3 and M4,

Measurement results)LJ7KHGHƮHFWLRQ
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Evaluating the outcome

the deﬂection in the middle of the railfoot was calculated. For the rail
expansion joint (M2), the deﬂection in
the middle of the rail foot could not bemeasured or calculated in any
meaningful way due to the split in therails. It was decided to use the outer
stock rail for the evaluation, as it would
be bearing the greater part of the loadat the measurement location.
The maximum rail deﬂection values
under the most heavily loaded axles asmeasured at M1 to M4 were used to
calculate an average value for each train
and measuring point. Four passes ofeach type of train were used for theevaluation, ensuring a representative
average. This method enabled adeﬂection value to be derived from thebefore and after measurements. The
vibrations at M5 were evaluated for thesame four passes of each type of train.

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movement in the y-direction negligible,
a geometric relationship enables thedeﬂection Δz at the middle of the railbase to be derived, and from that thedeﬂection at the two rails (RFinner andRFouter).
A maximum error of less than
3% was estimated for the expected
deﬂection ﬁgures, and this was deemed
to be suﬃciently accurate. In the centreof the rail expansion joint (M2), the railis split between the switch and stockrails. The measurements were taken
directly on the rail base, so that theobserved deﬂections could be used in
the evaluation without having to betransformed.
The ﬁeld setup is shown in Fig 7.
Vibrations were measured at M5; theacceleration sensor was bonded to the
granite structure precisely at theconnection between the ﬁsh belly andthe approach span.
As the measurements could only betaken with people on site at the
beginning, it was decided to trigger
them automatically, using an
acceleration sensor placed on a sleeper
on the ﬁsh belly. This allowed around
15 h of normal operation to berecorded, for both the before and after
campaigns.

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29

TRACK MAINTENANCE Switzerland

Homogenous behaviour

Heading Herisload from the switch
rail to the stock railau

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measurement data.

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OCK RAST

of all the passing trains evaluated. At
ﬁrst glance, it seems surprising that
there was no visible improvement in the
31·5 Hz to 63 Hz range. The mostprobable reason for this is the choice ofelastic components, which weredimensioned with a focus on optimising
the transition. If one considers thenatural frequencies of the ﬁsh belly withelastic rail fastening systems and thoseof the ballasted track with a rigid
under-ballast mat, the result lies in the
30 Hz to 40 Hz range.
Using a simple mass-spring model,
an improvement starting at
approximately 60 Hz would be possible,
although, to make a fair comparison,
the natural frequencies prior to therestoration would also have to beconsidered. However, in this situationa rough estimate is suﬃcient, as thechange from a lattice girder structure
to a stone arch bridge makes theconditions in the transition zone
increasingly undeﬁned, giving us lessconﬁdence in the analytical result.
Nevertheless, the measurement as

Velocity Level (dB)Deflection (mm)After averaging out all the evaluatedtrain passes, it is clear that the newsuperstructure design exhibits a much
more homogeneous deﬂection curve(Fig 10). There has been a signiﬁcant
improvement to the transition zone,
including the rail expansion joint.
The vibration measurements at M5
were used to assess the homogeneityof the transition with regard to thetransfer of kinetic energy to thestructure as a train passes. This is ameasure of the uniformity of the track
bed where the superstructure changes,
and the force peaks arising as a result.
Here the priority was to assess themeasured spectrum as a whole, as
impacts caused by faults in the
superstructure generally lead towide-band excitation.
The before/after comparison in Fig 11
shows the measured third-octave
spectrum. The scatter band is deﬁned
by the minimum and maximum values

Fig 9. Following
restoration, thetransfer of the wheel 

reconstruction, the amount ofdeﬂection for the switch and stock rails
was more or less identical. The time
oﬀset of the two measuring signals
shows how the wheel load transfersfrom switch rail to stock rail (Fig 9).
The rail expansion joint now has adeﬂection of about 2 mm and is very
well adapted to the standard track.
Slightly less rail deﬂection wasevident at M3 and M4. What wasapparent was that the rail tilted
noticeably, particularly at M4. This was
located well into the curve, where theouter rail was superelevated. As thetrains were restricted to 50 km/h at thetime of the measurement, we believethat the train slipped towards thecentre as it took the curve, pressing on
the head of the inner rail. An increased
rotation of the rail was therefore to be
expected, with movement in the y
direction. As such, it is possible thatslightly diﬀerent deﬂections would havebeen observed had the trains beenrunning at normal speeds.

0.0
-1.0
-2.0

H R A IL  




5
dB

Reductionin vibration
OHYHOVDƱHU
the renovation
work%HORZOHƱ)LJ|Average railGHƮHFWLRQEHIRUH
 DQGDƱHU
 UHVWRUDWLRQ
Below:)LJ|Vibrationmeasurement results
at the transition
EHWZHHQWKHƬVK
belly and theapproach viaduct.

a whole shows a reduction in thevibrations transmitted into the viaductof around 5 dB, with the levels of theestimated frequency bands deemed
critical in terms of resonance staying
the same. The vibration measurements
support the results showing a morehomogeneous transition as observed in
the deﬂection measurements.

References
1. Wagner, R., Instandsetzung desSitterviadukts; 19. Symposium
Brückenbau in Leipzig; Ausgabe 1/2
2019.
2. SBB; standard I-22068: Anforderungender Fahrbahn an Brücken undoberbautechnische Massnahmen im(LQƮXVVEHUHLFKGHU%UÕFNHQ, 2015.
3. SOB Plan 1392.3302, 2018.
4. *HW]QHU:HUNVWRƪH5HSRUW
REV3: Gleismessungen am Sitterviadukt
im Gleisnetz der Schweizerischen
Südostbahn AG; Internal Report, Bürs,
2020.

120

110

100

90

-3.0

RAIL
EXPANSIONJOINT 2019

-4.0

RAIL
EXPANSIONJOINT 2020

80

-5.0
-20

-10

0

10

20

30

40

50

Distance (m)Average Value 201930

Average Value 2020

Frequency (Hz)Average Value 2019

Average Value 2020

May 2022 Railway Gazette International