CEN/TR 17231:2018

SLOVENSKI STANDARD
01-oktober-2018
(YURNRG9SOLYLQDNRQVWUXNFLMH3URPHWQDREWHåEDPRVWRY0HGVHERMQLYSOLY
WUDþQLFHPRVW
Eurocode 1: Actions on Structures - Traffic Loads on Bridges - Track-Bridge Interaction
Eurocode 1: Einwirkungen auf Tragwerke - Verkehrslasten auf Brücken - Gleis-Brücken
Interaktion
Eurocode 1 : Actions sur les structures - Actions sur les ponts, dues au trafic - Interaction
voie-pont
Ta slovenski standard je istoveten z: CEN/TR 17231:2018
ICS:
45.080 7UDþQLFHLQåHOH]QLãNLGHOL Rails and railway
components
91.010.30 7HKQLþQLYLGLNL Technical aspects
93.040 Gradnja mostov Bridge construction
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

CEN/TR 17231
TECHNICAL REPORT
RAPPORT TECHNIQUE
August 2018
TECHNISCHER BERICHT
ICS 91.010.30; 93.040
English Version
Eurocode 1: Actions on Structures - Traffic Loads on
Bridges - Track-Bridge Interaction
Eurocode 1 : Actions sur les structures - Actions sur les Eurocode 1: Einwirkungen auf Tragwerke -
ponts, dues au trafic - Interaction voie-pont Verkehrslasten auf Brücken - Gleis-Brücken
Interaktion
This Technical Report was approved by CEN on 16 April 2018. It has been drawn up by the Technical Committee CEN/TC 250.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, Former Yugoslav Republic of Macedonia, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania,
Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland,
Turkey and United Kingdom.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2018 CEN All rights of exploitation in any form and by any means reserved Ref. No. CEN/TR 17231:2018 E
worldwide for CEN national Members.

Contents Page
European foreword . 5
Introduction . 6
1 Scope . 7
2 Normative references . 7
3 Terms and definitions . 7
4 Symbols and abbreviations . 9
5 Description of the Technical Issue . 10
5.1 General . 10
5.2 Axial effects . 11
5.2.1 Origin of axial forces and displacements . 11
5.2.2 Force transfer between track and deck ends . 11
5.2.3 Rail stresses . 11
5.2.4 Forces acting on the fixed point (e.g. Bearing forces) . 14
5.2.5 Interaction with sub-structure . 14
5.3 Vertical effects . 15
5.3.1 Effect of vertical forces and displacements . 15
5.3.2 Bridge deck end rotation . 15
5.4 Limits to the need for detailed calculations . 16
5.5 Calculation of multiple loading conditions . 17
5.6 Effect of bridge deformations . 17
5.6.1 Effect on track geometry . 17
5.6.2 Effect on stability of ballasted track . 18
5.6.3 Effect of ballast degradation over structural joints. . 18
5.7 Effects on track construction and maintenance activities . 18
6 History and background . 19
6.1 Existing codes and standards . 19
6.2 Differences between national rules. 21
7 Case studies . 21
7.1 Scheldt River Bridge (Belgium) . 21
7.2 Dedicated high speed lines in France and Spain . 21
7.3 Olifants River Bridge (South Africa). 21
7.4 Bridges on Denver RTD (USA) . 21
7.5 Historic bridges in central Europe . 22
7.6 Semi-integral bridges on German high speed lines . 22
8 Design considerations for track. . 23
8.1 Representation of axial behaviour of track. . 23
8.2 Understanding of ballast behaviour . 24
8.2.1 Ballast properties. 24
8.2.2 Importance of effective ballast retention . 24
8.3 Description/ limitations of available track devices for mitigation of effects . 24
8.3.1 Principles . 24
8.3.2 Practical solutions . 26
8.4 Description/ limitations of bridge design for mitigation of effects . 31
8.4.1 General . 31
8.4.2 "Steering bars” and virtual fixed points. . 31
8.4.3 Damper Systems . 32
8.5 Effects of track curvature and switches and crossings . 32
9 Design criteria . 33
9.1 General . 33
9.1.1 Rail stress . 33
9.1.2 Rail break containment . 33
9.2 Displacement limits . 33
9.3 Differentiation between ultimate- and service-loading . 35
9.4 Safety factors . 35
9.5 Differences between ballasted and ballastless tracks . 35
9.6 Calculations for configurations with rail expansion devices . 36
10 Calculation methods . 36
10.1 Methods in EN 1991-2:2003 . 36
10.1.1 General . 36
10.1.2 Software based on UIC 774-3R . 38
10.1.3 Linear analysis with manual intervention (LAMI) . 38
10.2 Load configurations . 40
10.3 Sensitivity analysis . 40
10.4 Numerical comparisons of calculation methods . 41
11 Information and process management . 46
12 GUIDANCE – Current best practice . 47
12.1 Bridge design principles . 47
12.2 Track design principles . 47
12.2.1 Ballasted track . 47
12.2.2 Ballastless track . 47
12.2.3 Special rail fastening systems . 48
12.2.4 Rail expansion devices . 48
12.2.5 Derivation of the behaviour . 48
13 Recommendations for future standards development . 49
14 Recommendations for future research and development . 49
14.1 General . 49
14.2 Improved input data for existing calculation methods. 49
14.3 Extension of existing models to include other track configurations . 50
14.4 Collecting data for better verification of analytical models . 50
14.5 Providing a basis for developing new, more rigorous, models . 50
Annex A (informative) Calculation of rail break gap . 51
A.1 Rail break gap for track with conventional fastenings (not on a bridge) . 51
A.2 Rail break gap for track on a bridge, with conventional fastenings . 52
A.3 Rail break gap for track with sliding (ZLR) fastenings . 54
A.4 Limiting values of rail break gap . 54
Annex B (informative) Algebraic studies of longitudinal track characteristics . 55
B.1 Algebraic representations of behaviour . 55
B.1.1 Sliding action . 55
B.1.2 The k-function . 56
B.1.3 Temperature change . 57
B.1.4 Temperature gradients . 67
B.1.5 Track springs . 67
B.1.6 Joint movements . 71
B.1.7 Track forces resulting from joint movements . 73
B.2 The Two Spreadsheet Method . 77
B.2.1 General . 77
B.2.2 The Temperature Stress Spreadsheet (TSS) . 77
B.2.3 The Additional Stress Spreadsheet (ASS) . 80
Annex C (informative) Examples of Track-Bridge Interaction calculations . 83
C.1 Introduction to calculation methods . 83
C.2 Example 1: Simply supported deck with no rail expansion device . 83
C.3 Example 2: Series of continuous decks with no rail expansion device . 85
C.4 Continuous deck with a rail expansion device . 88
Annex D (informative) Alternative method for determining the combined response of a
structure and track to variable actions . 91
Annex E (informative) Proposed revision of EN 1991-2:2003, 6.5.4 . 92
E.1 General . 92
E.2 Combined response of structure and track to variable actions . 92
E.2.1 General principles . 92
E.2.2 Parameters affecting the combined response of the structure and track . 92
E.2.3 Actions to be considered . 95
E.2.4 Modelling and calculation of the combined track/structure system . 95
E.2.5 Design criteria . 98
E.2.6 Calculation methods . 100
Bibliography . 104

European foreword
This document (CEN/TR 17231:2018) has been prepared by Technical Committee CEN/TC 250
“Structural Eurocodes”, the secretariat of which is held by BSI.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
Introduction
The subject of Track-Bridge Interaction has become particularly important with respect to longer span
bridges and viaducts supporting tracks, especially for those carrying high speed trains. However,
investigations which have been undertaken in order to address that specific issue have raised points
which are relevant to all types of railway bridge. Consequently, the content of this Technical Report is
intended to be applicable to all types of railway bridge, for both ballasted and ballastless track, and for all
types of railway (e.g. conventional railways, metro and light rail systems, and high speed railways).
It is also clear that the increased availability of computational methods of analysis, since the basis for
existing codes was laid down in the 1990s, has made it possible to consider approaches to calculation of
Track-Bridge Interaction effects which could not be expected to be used in routine procedures in the past.
There are three principal 'outputs' set out in the final sections of this Technical Report. They are as
follows:
1) Guidance for designers and maintainers of railway track and structures to assist them in adopting
current best practice in taking Track-Bridge Interaction effects into account (Clause 12 of this
report).
2) Recommendations for future development of standards, especially the revision of the relevant
section of the Eurocode EN 1991-2:2003 6.5.4 (Clause 13 and Annex E of this report).
3) Identification of areas in which new research and development is needed to make further
improvements in approaches to Track-Bridge Interaction (Clause 14 of this report).
1 Scope
This document reviews current practice with regard to designing, constructing and maintaining the parts
of bridges and tracks where railway rails are installed across discontinuities in supporting structures.
Current Standards and Codes of Practice are examined and some particular case histories are reviewed.
The document gives guidance with respect to current best practice and makes recommendations for
future standards development and also identifies areas in which further research and development is
needed.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
• IEC Electropedia: available at http://www.electropedia.org/
• ISO Online browsing platform: available at http://www.iso.org/obp
3.1
track-bridge interaction
conditions under which forces and/or displacements in a railway track and its supporting bridge
structure are influenced by the fact that rails span discontinuities in a bridge structure e.g. structural
movement joints or bridge deck ends
3.2
additional load
load in an element of the track, (e.g. rail and rail fixing) on a bridge compared with what is expected in
that element if the same track system were to be installed with the same loading actions away from any
bridge
Note 1 to entry The word 'additional' is used in the same sense to describe additional stresses, additional forces
and additional deformations.
3.3
thermal fixed point
point in the structure of the bridge, without the track, which is assumed not to be displaced when there
is a change in temperature. (Otherwise known as the “centre of thermal displacement” or “thermal
centre”)
3.4
deck length
L
D
distance between structural movement joints in the bridge deck
3.5
span length
L
S
distance between vertical supports, e.g. piers and abutments
3.6
expansion length, L of a deck
T
distance between the thermal fixed point and the free end of the deck
Note 1 to entry: For bridge designs in which the thermal fixed point is neither at one end nor at the mid-point of
the deck, the distance from the thermal fixed point to the further free end is taken to be L . (See Figure1.)
T
3.7
effective expansion length, L at a joint
J
total of the distances from the joint to the thermal fixed point for the two bridge decks adjacent to the
joint
Note 1 to entry: See Figure 1.

Key
△ represents a 'fixed' support
○ represents a 'free' support
Figure 1 — Examples of expansion lengths L and L
J T
3.8
support stiffness
longitudinal stiffness of a single pier given by
F
K=
δδ++δ
phϕ
Note 1 to entry: Depending on the type of bearings used, the tolerance of the bearing and the shear stiffness may
have to be considered by calculating the longitudinal stiffness.
Note 2 to entry: For the case represented in Figure 2 as an example.
Key
(1) bending of the pier
(2) rotation of the foundation
(3) displacement of the foundation
(4) total displacement of the pier head
Figure 2 — Example of the determination of equivalent longitudinal stiffness at bearings
4 Symbols and abbreviations
For the purposes of this document, the following symbols and abbreviations apply.
E elastic (“Young's”) modulus. For rails, it is assumed that E = 210 GN/m.
F longitudinal force
K longitudinal stiffness at a single pier (see Clause 3 definition 7)
L deck length (see Clause 3 definition 4)
D
L effective expansion length at a joint (see Clause 3 definition 6)
J
L span length (see Clause 3 definition 5)
S
L expansion length (see Clause 3 definition 4)
T
SFT Stress Free Temperature. (Temperature at which the axial stress in the rail is zero for
unloaded track)
SLS Serviceabiity Limit State (see definition in EN 1990:2002 , 1.5.2.14)
ULS Ultimate Limit State (see definition in EN 1990:2002 , 1.5.2.13)
th −5 -1
α, 𝛼𝛼 coefficient of thermal expansion. For rails, it is assumed that α = 1,2 × 10 K
δ axial displacement of the bridge deck due to traction or braking forces
B
δ longitudinal displacement due to rigid body translation of the pier (see Figure 2)
h
δ longitudinal displacement due to bending of the pier (see Figure 2)
p
As impacted by EN 1990:2002/A1:2005 and EN 1990:2002/A1:2005/AC:2010.
As impacted by EN 1990:2002/A1:2005 and EN 1990:2002/A1:2005/AC:2010.
δ axial displacement of the bridge deck due to vertical loading
V
δ δ displacements due to rotation of the free end of the deck
θD, θR
δ longitudinal displacement due to rotation of the foundation of the pier (see Figure 2)
φ
λ ratio of span length (L ) to depth of bridge deck structure
S
θ angle of rotation of the free end of the bridge deck due to temperature difference
TD
5 Description of the Technical Issue
5.1 General
Interaction between the track structure and a bridge structure (i.e. the consequences of the behaviour of
one of those structures on the other) occurs because there is a physical connection between them,
whether the rails are directly fixed or there is a ballast bed in between the track and the bridge. The
interaction results in forces being applied to the track (rails, fastenings and ballast) and the bridge
substructure (foundations, piers, abutments, bearings). These forces are in addition to those which would
be expected if the track and bridge were analysed separately.
If these additional forces are too high this may lead to failure modes including tensile failure of the rail,
lateral buckling of the track, shear failure of bridge bearings, longitudinal failure of the bridge
substructure or uplift of track elements. These forces shall be taken into account in assessing both
serviceability limit state (SLS) and ultimate limit state (ULS) conditions of the structure, although only
SLS conditions should be taken into account for calculating stresses in the track (see 9.3 and 9.4),
As a general principle, track engineers prefer to have bridges which are designed to reduce the influence
of the bridge on the track to a minimum. Existing and proposed standards and codes set maximum
limiting values of stresses, forces and deformations. For specific projects the preferred practice at the
design stage may be to aim to achieve values well below those limits.
However, historically the problem of Track-Bridge Interaction has been solved by installing rail
expansion devices close to structure movement joints on longer bridges. Rail expansion devices are
expensive to install and to maintain, especially on high speed lines where impact forces arising from
imperfect joints in the rail cause deterioration of the track and the supporting structure. On many urban
railways there is a need to reduce the number of rail expansion devices to remove a source of noise, even
if the train speeds are lower.
The resolution of this apparent conflict between the interests of bridge designers and track designers
shall be based on an understanding of the economic implications of different solutions. At the simplest
level, this requires an understanding of the relative construction and maintenance costs of, for example,
installing rail expansion devices compared to modifying the bridge design (e.g. longitudinal stiffness of
sub-structures) and/or accepting higher operational stresses in the rails. However, even more significant
economic benefits may accrue from changes to the detailed design or even the overall configuration of
bridge structures.
In order to reduce maintenance costs, tracks are now designed to work with lower stresses in the rails,
and some other components, than they were some years ago. For example, in most European main line
tracks fifty years ago it was common to use rails of 50 kg/m to 56 kg/m with sleepers 650 mm to 700 mm
apart. With the same maximum axle loads, the same railways now use rails of 60 kg/m with higher
strength steel and with sleepers 600 mm to 650 mm apart. This means that there is a greater margin
between the operational rail stresses and the ultimate failure conditions.
The underlying principle in many of the cases described in this report is that in some critical locations it
may be desirable to use this margin to allow higher operational stresses in continuous rails (implying an
acceptance of a shorter rail life) in order to avoid the use of rail expansion devices.
5.2 Axial effects
5.2.1 Origin of axial forces and displacements
It is assumed throughout this Technical Report that track configurations which are to be considered can
also be used in applications where there is no bridge e.g. on earthworks, etc. and that in those situations
the track performs in an acceptable way. There is also an underlying understanding of the fact that
changes in temperature do not cause continuous welded rail (CWR) to expand or to contract. The length
of CWR remains the same as the temperature changes because the longitudinal and lateral restraint
provided by the ballast or slab, prevent movement of the track. Of course, the state of stress in the rail
does change with temperature.
The main purpose of this report is to consider additional forces and deformations which arise on bridges,
in particular the longitudinal forces which arise as a result of any actions which tend to open or close the
gap at structure movement joints. These may be due to:
— Changes in temperature (Mean temperature and temperature gradients).
— Application of traction and braking forces.
— Application of vertical forces.
— Creep and shrinkage of concrete elements.
— Movement of the thermal fixed point due to, e.g. Rotation of pier foundations due to settlement.
5.2.2 Force transfer between track and deck ends
Figure E.4, within the text for the proposed revision of EN 1991-2, shows plots of the k value, which is the
longitudinal shear resistance of the track (force/m), which is transferred from the track to the deck or
track formation, plotted against u, the relative longitudinal displacement between the track and the deck
or track formation. It is shown as an elastic-plastic function but it shall be recognized that, for calculation
purposes, this is greatly simplified from the measured plots of k versus the relative displacement u. Note
that in the recent past the parameter k is sometimes used for the generic force (as in UIC 774-3R,
Figure 5) and more often is used specifically for the plateau value of the force. The term ‘k-function’ has
therefore been adopted to represent the longitudinal shear resistance of the track over the elastic-plastic
range and k is used for the plateau value.
Values of k and u are set out in Table E.1. Although the values for ballasted and ballastless track are
included in the same table, this obscures the very different behaviour between the two cases. With
(unloaded) ballasted track u marks the relative movement when the sleepers start to move through the
ballast. The ballast shears at the level of the underside of the sleeper, and the ballast between the sleepers
is moved with the sleepers. With ballastless track, the relative movement occurs in the fastenings and
with larger movements the rail slides through the fastenings. Sliding in the fastenings can occur for loaded
ballasted track where the shear resistance of the ballast is greater than the frictional resistance of the
fastening.
5.2.3 Rail stresses
Figure 3 — Typical configuration of track across a structural joint
Figure 3 shows schematically a typical situation in which a continuous rail is fastened across a
discontinuity in the structure. High local rail stresses may be generated, principally by two mechanisms:
• Opening and closing of the joint by axial displacement of the bridge deck ends, or
• movement of the joint due to rotation of the bridge deck ends.
Thermal expansion or contraction of the structure leads, principally, to longitudinal displacements of the
free ends of the structural elements, opening or closing the gap at the joint.
The relationship between the movement of the joint and the stress in the rail depends on the elastic or
non-elastic (e.g. slip) shear stiffness of the components between the rail and the structure. In qualitative
terms, this behaviour is quite well understood by track engineers as it is fundamental to the design of
track with continuous welded rail when considering lateral track stability (i.e. resistance to buckling). It
is less familiar to bridge engineers.
If there is enough provision for relative movement between the bridge and the rail, the bridge will expand
and contract as the temperature changes, and the rail will simply remain in position with no displacement
and no change in stress. Where there is not “enough” provision for relative movement between the bridge
and the rail, the rail will be dragged along by the moving bridge deck. In these circumstances the relative
displacement between the rail and bridge deck is reduced but the absolute displacement of the rail is
increased. Additional strain due to the movement of the joint is imposed on the rail (equal to the slope of
the curve of absolute rail displacement v. position along the rail) and that results in an additional axial
stress in the rail. This mechanism is illustrated in Figure 4.
Key
1 displacement / stress
rail displacement rail stress
deck displacement
Figure 4 — Effect of longitudinal joint movement on rail stress
In order to create any kind of a model for calculation, it is necessary to characterize this behaviour
mathematically. Most models represent the behaviour with a simple bi-linear (elastic-plastic) force-
displacement curve as described in 8.1 i.e. it is assumed that the longitudinal displacement of the rail
increases in a linear, elastic response to the applied force until it reaches a critical relative displacement,
u , beyond which the rail slips through the fastening system with a constant restraining force, k. The
implications for structural design calculations are considered in more detail in 10.2 of this report.
The bi-linear representation is a useful approximation but the true behaviour of the track components is
much more complex. The components which can affect the behaviour include rail fastening systems,
under-sleeper or under-ballast mats and the ballast itself.
For a real bridge structure the rail stress distribution may be calculated from first principles, treating the
track, the bridge and the connection between them as a single structure for numerical analysis. Figure 5
shows a typical form of axial rail stress distribution for a simple, single span bridge. A point to note is that
although the deck displacements are limited to the length of the bridge, the rail displacements and
stresses are affected for some distance beyond the ends of the bridge.
Key
rail stress bridge displacement
rail displacement
Figure 5 — Rail stress due to bridge temperature variation across a simple, single span bridge
5.2.4 Forces acting on the fixed point (e.g. Bearing forces)
The longitudinal forces transmitted to the bridge deck through the bearings depend on the detail design
of the bearing, the shear stiffness of the bearings and the supporting substructure. In general terms the
shear stiffness of bearings which permit longitudinal movement is taken to be zero, but for more accurate
calculations the elastic stiffness of some types of moving bearings (e.g. elastomeric bearings) should be
taken into account.
The stiffness of the support, K, including its foundation, with respect to displacement along the
longitudinal axis of the bridge caused by a horizontal force, F, is given by
F
K=
∑δ
i
In which ∑δ represents the sum of the horizontal displacements due to elastic shear of the bearing,
i
displacement and bending of the pier (see 3.7).
5.2.5 Interaction with sub-structure
Forces on the sub-structure are influenced by the presence or absence of rail expansion devices, the
stiffness of the thermal fixed point and a number of other parameters. In order to comply with the
requirements for longitudinal force capacity the sub-structure has to be very stiff longitudinally. For the
abutments, much of the stiffness is in the backfill (particularly where the longitudinal movement is in the
direction of the backfill) and this shall be quantified, taking into account the difference between tension
and compression. For intermediate deck spans away from the abutments, the anchor pier and its
foundations shall be designed around the need for stiffness and this may require the use of A-frames or
longitudinal shear walls.
The stiffness of the soil may also depend on the loading rate and may be higher in dynamic loading cases
e.g. train braking cases. For larger bridges, this factor should be determined by geotechnical engineers
and taken into account in any analysis.
5.3 Vertical effects
5.3.1 Effect of vertical forces and displacements
The axial forces and stresses in the track are affected by the vertical loads applied to the track, and the
resulting forces from the track onto the deck. The effects caused by loads applied to the track are the same
as those away from the bridge, so they do not contribute to the additional stress as defined in 3.2.
However, bridge design for vertical loads is generally based on specific load models defined in the
Eurocode (e.g. “Load Model 71” as a model representing European main line traffic conditions) whereas
track strength away from bridges may be determined using other assumptions about applied vertical
loads (e.g. “Rail Fastening Categories” in the EN 13481 series of standards). The two approaches give
similar, but not necessarily identical, results.
The effects of the forces from the track onto the deck shall be considered. In particular the end-rotations
of the decks need to be calculated to evaluate δ and δ . These effects may be observed anywhere on the
θD θR
bridge. The effects are most significant at the free ends of the deck.
5.3.2 Bridge deck end rotation
At the free ends of the deck, bending of the bridge under vertical train loading and under vertical
temperature gradient results in relative rotational movement of the structural elements. The rotational
movement of the bridge deck end leads to vertical and horizontal movement. (See Figure 6). In this case,
if the rail is continuous across the structural joint the stress in the rail and other track components
depends on the resistance of the track to uplift and the bending stiffness of the rails. The additional
bending stresses in the rail are neglected, but on ballastless track the effects of uplift forces on the rail
fastening system shall be taken into account. It is important to note that the bending stress ahead of the
first wheel due to this effect is in the opposite direction to the bending due to the weight of the train. If
the track is ballasted, the resistance to uplift is simply the force required to lift the track (rails, sleepers,
etc.) out of the ballast. This action may lead to degradation of the ballast. For ballastless track, it depends
on the uplift characteristics of the rail fastening system. The uplift mechanism is illustrated in Figure 6.
In principle, bridge deck end rotation effects should be calculated using the vertical load case applied in
the general bridge design calculations, positioned in the worst possible location. In practice, sufficiently
accurate analysis can usually be carried out using a uniformly distributed load e.g. a UDL of 80 kN/mm in
place of Load Model 71.
Key
1 abutment
2 bridge deck
3 last rail fastening position on deck
4 axis of bridge bearing
δδ+⋅ϕ ü
L
Figure 6 — Effect of bridge deck end rotation
5.4 Limits to the need for detailed calculations
It is the differential movement between the track and the structure which creates additional stresses. For
short expansion lengths on stiff bridges, movement at structure movement joints is very small and
therefore it is not necessary to provide any mitigation measures in the track to reduce rail stresses. The
definitions of a “short” and “stiff” bridge are not universally agreed but on the basis of many years of
experience most railway administrations do not require the installation of rail expansion devices on
bridges with expansion lengths less than a specified value – typically around 30 m. It is suggested,
therefore, that detailed calculations should not be required for ballasted deck bridges with effective
expansion lengths, L , less than 30 m. For these cases the bearing forces can be calculated with sufficient
J
accuracy by using the “simplified method” in 6.5.4.6.1 of EN 1991-2:2003.
Recent experience suggests that even if the expansion length is more than 30 m, continuous rail can be
used safely without installing rail expansion devices but that as expansion lengths increase, more care
shall be taken at the design stage. For ballasted deck bridges with continuous rail on a single deck section
and with an effective expansion length, LJ, greater than 30 m but still less than 60 m for steel bridges or
90 m for a concrete bridge, it is still not necessary to make detailed calculations for rail stresses but more
detailed structural analysis is required to determine the bearing forces and stiffness requirements
For longer bridge decks it is necessary to make detailed calculations for rail stresses and bearing forces
and to consider the use of detail features that can be utilized in design to mitigate the effects of Track-
Bridge Interaction.
For very long expansion lengths there is no alternative to the use of rail expansion devi
...