LATERAL BUCKLING RISKS ASSOCIATED WITH CONTINUOUSLY WELDED RAILWAY TRACKS

Abstract

Contemporary railway tracks require continuity of rails as opposed to discontinuous joining of the rails via fishplates and bolts. Welded rails achieve their continuity thereby providing a smooth transition of the wheel from one rail to another. However, the continuity of the track results in the development of longitudinal compressive or tensile forces as the track contracts or elongates with changes in the ambient temperature. Correct estimate of the neutral temperature of the railway track, which is the conditional temperature at which the rails are welded, is essential to limit the development of longitudinal forces along the railway track. This interdisciplinary paper will introduce rail welding methodologies currently used to construct continuously welded railway tracks. Estimation of the neutral welding temperatures will be discussed along with methodologies to generate neutral temperature conditions within climates where the ambient temperature is different from the estimated neutral values.

Full text

4th International Conference on Welding Technologies and Exhibition (ICWET’16)
11-13 May 2016, Gaziantep-TURKEY
LATERAL BUCKLING RISKS ASSOCIATED WITH
CONTINUOUSLY WELDED RAILWAY TRACKS
Niyazi Özgür Bezgin1,a
1Istanbul University
aozgur.bezgin@istanbul.edu.tr
Abstract
Contemporary railway tracks require continuity of rails as opposed to discontinuous joining of the rails via fishplates
and bolts. Welded rails achieve their continuity thereby providing a smooth transition of the wheel from one rail to
another. However, the continuity of the track results in the development of longitudinal compressive or tensile
forces as the track contracts or elongates with changes in the ambient temperature. Correct estimate of the neutral
temperature of the railway track, which is the conditional temperature at which the rails are welded, is essential to
limit the development of longitudinal forces along the railway track. This interdisciplinary paper will introduce rail
welding methodologies currently used to construct continuously welded railway tracks. Estimation of the neutral
welding temperatures will be discussed along with methodologies to generate neutral temperature conditions within
climates where the ambient temperature is different from the estimated neutral values.
Key Words: Continuously welded track, aluminothermic welding, flash butt welding, neutral temperature, buckling
1. Introduction
Continuously welded rail requirement arises from the need to reduce the dynamic loads
transmitted from the wheels of the railway vehicle to the track supporting elements and to reduce
the necessary track maintenance. Butt-welding of the rails achieve continuity thereby providing a
smooth transition of the wheel from one rail to another. However, the continuity of the track
results in the development of longitudinal compressive or tensile forces as the track elongates or
contracts with changes in the ambient temperature. The compressive forces may cause buckling
in ballasted railway tracks. Risk of track buckling is enhanced if the lateral stiffness of the
ballasted track is insufficient. The tensile forces on the other hand, may cause rail fracture during
the passage of train especially in sub-zero climates
Occurrence of tensile or compressive forces relates to the rail temperature as the rails are welded.
This temperature, known as the neutral temperature relates to the climate of the region where
railway route exists. Correct estimate of the neutral temperature is essential to limit the
development of longitudinal forces along the railway track. As the ambient temperature rises
above the neutral temperature compressive longitudinal forces develop on the rail. As the
ambient temperature falls below the neutral temperature, tensile longitudinal forces develop on
the rail. The lateral buckling stability or axial tensile fracture strength depends on how much the
temperature varies with respect to the neutral temperature of the track at the particular location.
This paper, introduces the welding methods to achieve continuity along contemporary ballasted
railways followed by lateral buckling stability and tensile fracture conditions of contemporary

4th International Conference on Welding Technologies and Exhibition (ICWET’16)
11-13 May 2016, Gaziantep-TURKEY
CWRs. The paper concludes by providing some statistics with respect to the performance of the
presented welding methods in terms of occurrence of weld fractures and estimations of neutral
temperatures with respect to differing ambient conditions.
2. Ballasted railway tracks and track buckling
Ballasted railway superstructures are composed of a superstructure and substructure. The
superstructure consists of the rails, rails attachment elements, sleepers onto which the rails are
attached, ballast layer supporting and surrounding the sleepers, subbalast supporting and
providing drainage and filtering for the supported track elements above. Substructure consists of
the filled layer and the supporting natural or enhanced soil or the natural base that is reached
after an excavation and the supporting natural or enhanced soil. Figure 1 shows a typical cross-
section of a single track ballasted railway cross-section.
Figure 1. Ballasted railway cross-section.
The rails are produced at different length depending on the method of track installation and site
access conditions. 30 m long rails are quite common. Historically, discrete rail were attached
together through bolting. Rails had drilled holes at their ends, where the neighbouring ends of
two consecutive rails were bolted together via steel plates known as fishplates. This
configuration not only connected the rails, but also allowed a gap between the rails that provided
the necessary space for the rails to elongate with increasing ambient temperatures. Figure 2
shows an example of bolted rails.
Figure 2. Bolted rail connection [1].
However, increased railway traffic and accumulated loads, increased impact loads over the
bolted connection as the train wheel rolled over the gap and the resulting damage to the rail-ends
and the supporting sleepers and the balast layer persuaded railway track designers to seek means
to provide rail continuity along their joints. To this end, rail-end welding methodologies

4th International Conference on Welding Technologies and Exhibition (ICWET’16)
11-13 May 2016, Gaziantep-TURKEY
developed to provide continuity among discrete rails. Figure 3 shows a welded railway
connection where the transition from one rail to the other is smooth.
Figure 3. Welded rail connection [2].
Although continuous transition became possible through welded joints, the freedom of rail
elongation and contraction was severely constrained. The resulting slender rails came under
compression at rail temperatures above the temperature of the rails when they were welded.
Ballasted tracks with insufficient lateral constraints buckled within the plane of track resulting in
track closures and costly repair procedures. Figure 4 shows an example of a buckled track. The
buckled track lengths can extend up to 150 m each direction from the buckled region.
Figure 4. A buckled ballasted railway track [3].

4th International Conference on Welding Technologies and Exhibition (ICWET’16)
11-13 May 2016, Gaziantep-TURKEY
3. Methodologies used to weld the rail-ends
The two common welding methods are aluminothermic welding also known as the thermite
welding and flash butt welding. Aluminothermic welding uses aluminum as the reducing agent.
The rail heads aligned with a gap of approximately 2 cm. A refractory mould is attached that
encompasses the ends to the two rails which are to be connected. The rail-ends are preheated to a
certain temperature depending on the cross section size of the rails and the targeted quality of the
weld. Special aluminum and iron oxide powder is released into the mould and ignited. The
resulting exothermic reaction, which generates temperatures at more than 2000oC produces iron
and aluminum oxide slag which is recovered through the side slag-bins of the mould. After the
weld has set, the refractory mould and the left over material are sheared-off the connected rails.
The connection is later rough grinded and then finished off to the required tolerance after cooling
[4, 5].
Flash butt-welding method, clamps the rail-ends with a compressive force and runs an electric
current through the circuit formed by the rail-ends to be welded. The generated heat of
approximately 1250oC and the clamping force generating approximately 90 MPa pressures
forges the rail-ends without the use of a filler material [6].
Quality of rail-end welding is strongly related to how well the welded elements are prepared.
Preheating temperatures, proper alignment and gap formation is extremely critical to achieving
quality welded rails in thermite welding. A typical sequence may take from 20 minutes per weld
up-to an hour per weld to completion depending on the site conditions, preparation requirements
and the achieved weld-quality. Thermite welding is heavily dependent on the skills of the
personnel. On the other hand, flash butt-welding is an automated process that requires little
human-interference and lasts approximately 5 minutes per weld to completion [7].
Flash butt welding provides more material and crystal structure homogeneity across the weld
compared to thermite welding as the latter requires a reducing metal that combines to form an
alloy with the rail. Changes in the material and crystalline character across the thermite welded
joint may have contributed to the statistical finding by SNCF in 2004 that 34% rail fractures
occurred on thermite welded joints as opposed to 3% of rail fractures that occurred on butt
welded joints [7, 8].
4. Formation of compressive and tensile forces on rails
Contemporary railways in Türkiye and elsewhere in Europe frequently use the UIC60 type rails.
These rails have a height of 17,2 cm, width of 18 cm, mass of 70 kg per meter and a cross-
section area of 76,7 cm2. Thermal expansion coefficient (c) of steel at 20oC is 11*10-6 K-1. The
elasticity modulus of steel (Es) is 210GPa. For a unit change in the temperature of steel, the
expected length of elongation/contraction for a rail that is 30 m long is:
If the elongation/contraction of this rail is constrained via continuous welding, the developed
compressive/tensile strain per degree of temperature increase is:

4th International Conference on Welding Technologies and Exhibition (ICWET’16)
11-13 May 2016, Gaziantep-TURKEY
The resulting compressive/tensile stress:
For a UIC60 rail, the unit change of temperature would generate a compressive/tensile force
approximately 1,7 Ton-f on the rail:
If the change in temperature is greater, the resulting force P on the confined rail will be
proportionally greater.
Unless the evaluated route is within a cold region, occurrence of compressive loads and hence
tracks buckling is the greatest threat to confined tracks undergoing a variation in temperature.
4.1.Evaluations for neutral temperature
The following evaluation is for a railway line in Istanbul. Ambient temperature extremes and the
related rail temperature estimates are needed to determine the neutral temperature at which the
rails must be welded.
1. Minimum observed ambient temperature: Tmin = -15oC
2. Highest observed ambient temperature: Tmax = 40oC
3. Minimum expected rail temperature: Tmin,rail = -15oC
4. Highest observed rail temperature: Tmax,rail = +60oC
5. Safety margin: Tsafety = 5oC
(1)
(2)
(3)
The preferred application temperature is the higher limit, which in this case is 30,5oC.
However, all too often, the ambient temperature is below the required neutral temperature. Under
such circumstances, the condition of the rail at the neutral temperature is provided by heating the
rails until they elongate to the required length through equations (4) and (5):

4th International Conference on Welding Technologies and Exhibition (ICWET’16)
11-13 May 2016, Gaziantep-TURKEY
E. (4)
(5)
For example, if the ambient temperature is 10oC below the neutral temperature, in order to
commence with the construction of the railway line, a 30 m long rail must be elongated
according to equation (6) before it is welded.
(6)
Table 1, summarizes ambient temperature extremes for railways around the world [7]. Of course
for each country, a site specific analysis must be conducted for each project in order to determine
the sufficient neutral and application temperatures, but the table provides an insight into the
effect of varying climates on the required neutral temperature of the rails.
Table 1. Neutral and application temperatures for selected railways [7].
Rail temperature
Welding temperature
Country
Operator
Tmin (oC)
Tmax (oC)
Tneutral (oC)
Tapplication(oC)
Romania
CFR
-30
60
20
7 - 18
Czech Republic
CSD
-30
60
20
5 - 25
Germany
DB AG
-30
60
20
17 - 23
Hungary
MAV
-30
60
20
5 - 20
United Kingdom
NR
-20
50
20
21 - 27
Netherlands
NS
-30
60
20
25
Austria
ÖBB
-30
60
20
15 -20
Poland
PKP
-30
60
20
17 - 23
Switzerland
SBB
-30
55
17,5
22 - 28
Sweden
SJ
-40
60
15
15 - 20
France
SNCF
-15
60
27,5
20 - 32
Russia
SZD
-50
65
12,5
1 - 23
USA
Amtrak
-40
55
12,5
16 - 21
5. Development of track buckling
Figure 5 qualitatively presents the buckling phases of a railway track with an initial out-of-
straightness of d0. The allowed value depends on the particular railway operator and varies up to
4 mm. For instance the National Railway Society of France (SCNF) initiates track maintenance
to for lateral realignment when wb + d0 reaches 4 mm for a length of 10 m [8]. The condition at
wb is critical such that buckling may initiate, resulting in a lateral displacement wc that can be as

4th International Conference on Welding Technologies and Exhibition (ICWET’16)
11-13 May 2016, Gaziantep-TURKEY
high as 15 cm – 75 cm, influencing a total railway length of up to 300 m [3]. Initial
misalignment is unavoidable but it is influential on the reserve resistance of the track against
buckling.
Figure 5. Development of buckling [3].
Track curvature and track loading; among other factors, are highly influential on the buckling
mode and its occurrence along the track. Figure 6 shows the three common modes of observed
cases of buckling [3].
Figure 6. Common modes of buckling [3].
Figure 7 relates the lateral shift of a high quality track with temperature increase above the
neutral value. Based on extensive tests [3,8] it is estimated that a high quality track free of
vertical train loads is expected to buckle as the rail temperature increases 50oC above the neutral
value. However for a loaded track, the stored compressive energy within the track due to strained
rails can be triggered with the passing of a train at a much lower rail temperature increase of
35oC above the neutral temperature.

4th International Conference on Welding Technologies and Exhibition (ICWET’16)
11-13 May 2016, Gaziantep-TURKEY
Figure 7. Lateral track displacement as a function of increased temperature above neutral [7].
The variation of lateral track displacement with increased temperature above the neutral
temperature value is strongly influenced by the lateral track stiffness as shown in Figure 8. With
the increased lateral track stiffness, buckling becomes more abrupt as the track resists buckling
up to the critical temperature when the lateral confinement is overcome by the stored
compressive strain energy. However, as the lateral confining stiffness is reduced, buckling and
the lateral displacement becomes more gradual.
Figure 8. Buckling behavior as a function of lateral track resistance [9].

4th International Conference on Welding Technologies and Exhibition (ICWET’16)
11-13 May 2016, Gaziantep-TURKEY
6. Conclusion
Railway superstructures are not only exposed to loadings due to wheel loads but also load due to
temperature variations. Contemporary railways require continuous welding that expose the track
superstructure to compressive strains, which may lead to buckling. Buckling strength of the track
superstructure is controlled primarily by track lateral strength. On the other hand, mitigation of
track buckling requires the control of the accumulated compressive strain within the track. To
this end, the neutral temperature of a track must be carefully evaluated based on the climatic
conditions along its route. Temperature extremes must be evaluated and a weighted average
temperature must be determined that represents the most likely temperature within the evaluated
region.
Flash butt welding requires less human interference and does not require an extra welding
material between the rails. Based on statistical data, rail fractures are noted to occur more on
thermite welded rails compared to flash butt welded rails.
7. References
[1] https://maxfaqs.wordpress.com/tag/track-circuit/
[2] https://en.wikipedia.org/wiki/Track_(rail_transport)
[3] U.S Department of Transportation, Federal Railroad Administration, Track Buckling
Prevention: Theory, Safety Concepts, and Applications. DOT/FRA/ORD-13/16. Final Report
March 2013.
[4] Gantrex, Rail Welding – Aluminothermic Welding, Rev-1, 10/2009
[5] Australian Rail Road Track Corporation, Aluminothermic Welding Manual RTS 3602, Issue
1, Revision 0, 2013.
[6] Zaayman L., Continuous welded rail using the mobile flash butt welding, SAICE, Civil
Engineering, May 2007
[7] Lichtberger B., Track Compendium, Eurail Press, 2nd Edition, 2011
[8] Girardi, Louis, Bourdon, Yves. Rail Grades and Grinding Optimization. VTM 2005, Track
and Maintenance . Paris. 2005
[9] Kish, Samawedam, Track Buckling Preventation: Theory, safety, concepts and applications,
2011
Dr. Niyazi Özgür Bezgin, İstanbul University, Avcılar Campus, Civil Engineering
Department, 34320, Avcılar, İstanbul, 212.473.7070 – 17947, ozgur.bezgin@istanbul.edu.tr
Niyazi Özgür Bezgin – Assistant. Prof. Dr. Bezgin, received his Doctorate of Philosophy degree
from Rutgers, the State University of New Jersey in 2005 with the work he conducted on soil-
structure interaction. Upon graduation, he worked in the industry within the fields of
geotechnical engineering, structural engineering and transportation engineering. He was in the
design team of the Ankara-Konya High Speed Railway Project where he designed the
prefabricated high strength concrete sleepers. He joined the academic staff of Istanbul University
Civil Engineering Department in the field of transportation engineering structures in 2012.