The Railway Sleeper: 50 years of pretensioned, prestressed concrete

Paper: Taylor
Paper
The railwa sleeper: 50 years of pretensioned,
prestresse concrete
cy
H. I? J. Taylor, BScTech, PhD, FEng, FIStructE,FICE
Costain Building Products Ltd
Synopsis
This year,1993, marks 50 years fromthe introduction of the
pretensioned, prestressed railway sleeper. This paper describes
some history of the early development of prestressed sleepers and
of our Codes. The success
links the design development with that
of the prestressed sleeper is self-evident, butthe paper does
emphasise the successful way in which the prestressed sleeper
copes with a most hostile working environment and provides an
insight into the developmentof our design methodsfor
prestressed concrete.
Definitions
Monoblock sleeper: a one-piece sleeper which may be made
of timber,
concrete or steel, supporting two rails and tying them together (Fig1).
Rail block: a block of stone
or concrete which supports a single
rail (Fig l).
Flat bottom rail
Twin block sleeper:a sleeper consisting
of two concrete blocks tied together
with a metal bar (Fig 1).
Bullhead track:rail track consistingof bullhead rail held in cast-iron chairs
which are supportedby sleepers or blocks jointed in 60 ft lengths (Fig 2).
Flat bottom rail track: rail track consisting of flat bottom rail frequently
continuously welded into long lengths held by a fastening system directly
to sleepers (Fig 2). On occasion, baseplates are used between the rail and
the sleeper.
Fastening system: the system of connection between sleeper andrail. This
may be through a chair or with a tie or clip between the rail and sleeper.
Modem fastening systems are elastic, incorporating a spring clip, a pad
beneath the rail and insulators to provide electrical isolation betweenrails
for track circuiting purposes.
Bullhead rail track
Introduction
This paper discusses the introduction and use of pretensioned monoblock
sleepers over the last 50 years. However, concrete has been used in block
sleepers and, to a lesser extent, in reinforced concrete monoblock sleepers
for 100 years in slow speed track.
Monoblock sleeper
L
I
L/
d
1
r
1
l
Irl
0
-
,il
Y
Twinblock sleeper
m
Railblock
Fig I . Sleeper types
The Structural EngineedVolume 71/No 16/17 August 1993
Fig 2. Bullhead andflat bottom rail track
The Stockton to Darlington Railway used stone blocks to support the
rail
in 1825 (Fig 3). Difficulties were experienced with the absence of ties to
hold the rails together but, by the time that the Manchester and Liverpool
Railway - the world’s first passenger railway - was opened in 1830, tied
blocks were used. With the development
of reinforced concrete, experiments
were made in the first half of this century with RC sleepers. Some RC
sleepers were tried in World War 1. The need to find a replacement for
timber emerged at the start of World War 2 and, by then, early work on
prestressed concrete led tothis being an option intrials.
In 1941 two designs of reinforced concrete sleepers were produced by the
Chief Civil Engineer’s Department of the London Midland and Scottish
Railway, and some were made and put in a branch line near Derby. The
results were inconclusive, but experience was gained in strain measurement
and the difficulties of carrying out research on real track that was to prove
valuable in the future. In the next year
100 RC sleepers were put in the
mainline near to Watford and survived forjust 10 days! While this design
development was taking place, work was being carried out by a separate
group at Colwall in Worcestershire on prestressed concrete sleepers. This
work used an experimental long line prestressing bed built
by a Dowsett
company and was carried
out by engineers from the railway authorities, the
Board of Trade, and Dowsett. These sleepers were designed
by Dr Mautner
of The Prestressed Concrete Company, later to become ofpart
the Mouchel
organisation.
Fig 4 is a photograph of a painting by Dame Laura Knight exhibited at
the Royal Academy in 1945 which shows the workat Colwall and gives a
good impression of the excitement and pressureof those times.
The next setof concrete sleeper tracktrials included both RC sleepers and
prestressed sleepers from Colwall. These
first prestressed sleepers were
281
Paper: Taylor
B
.r.
c
! I
-
A
c
.
t
I
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I
side a wide range of support reactions are found. These depend on the
nature of the ballast and formation below it aswell as the form of ballasting
and the quality of maintenance. The point of loading also definesthe points
of maximum moment and shear as coincidental. This
was brought out very
well inthe discussion of one of the earlyreview papers in the development
of sleepers', when a Mr Henzell remarked:
-
4575
I
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" 1 3 1
1
'A point to be noted about the sleepers described by the authors was that
they wereentirelywithoutstirrupsortransversereinforcement.The
engineers of the development work had to find out how to make a sleeper
which would stand up to railway traffic and remain prestressed in service
without stirrups or hooping, and that they had been able to do so was
remarkable. Even the French engineer, Freyssinet,the founder of modem
prestressing, rejected completely the idea of self-anchoring reinforcement
for wires of one-fifth of an inch diameter or more, and in his sleeper patent
before the outbreak of war he applied anchored wires, and a strong transverse hooping. The elimination of transverse reinforcement was still more
remarkable when it was realised that a sleeper was not like an ordinary
beam, with maximum bending and maximum shear occumng at different
points. The ordinary beam has maximumbending at midspan and maximum
shear at the supports,
the maximum of the one occumng
with the minimum
of the other, but in a sleeper, which had cantilevered ends, the maximum
bending and maximum shear were at almost the same point, and that point
was not in the middle of the span but near the end of the sleeper; and, as
there was no mechanical bonding of any kind, the wires had to be anchored
in a short distance. To do so effectively with plain wires without any mechanical means was remarkable'.
I
"B
Section
Section
A-A
B-B
Fig 3. Rail blocks
put inthe west coast mainline at Cheddington near Tring
to on 21 February
1943. That, and the building of the Dow Mac factory at Tallington in 194344, marks the start of the 50-yearperiod reviewed in this paper.
British Rail now has 35M prestressed, pretensioned monoblock sleepers
in track, and there are probably more than twice as many pretensioned
monoblock sleepers elsewhere in the world - notably Russia and China, also
in Canada and the USA and in small numbers in many other countries.
From these beginnings more than 50M sleepers have been made in the
last 50 years.
Stages in sleeper design
Railway sleepers are probably unique in the way in which they act as
structures. First, they are not subject to self-load stresses during their
working life as the staticself-weight of the rail is only in the orderof 0.1%
of the total design load. They only ever see significant loads asdynamic
loads. The impacts of a steel wheel on rail are absorbed to a certain extent
by rail pads between the rail and the sleeper,but even these areso stiff that
the demands on a sleeper are much more than a bridge deck loaded by pneumatic rolling wheels.
Sleepers rest on the ballast but are not tied down to it,thus the impact load
is able to make the sleeper oscillate. Sleepers are excited to their natural
freqencies. This is a significant load effect and has been recognised in the
past, but insufficient attention had been paid to ituntil relatively recently.
The points of application of load are defined totally,but on the support
Fig 4. Sleeper tests, 1943
I
282
The design process, with key steps and some significant variables, is
shown in Fig 5 . The vehicle applies its load to the track through its axles
and wheels. Axle loads depend on the design of the vehicle and on its maintenance. The axle loads the rail through its wheels. At this stage impact
effects have to
be'considered with significant variables: speed, vehicle type
and the levels of sprung and unsprung mass, the track structure; straight or
curving; and maintenance. From this stage to obtain sleeper rail reactions
and the consequent ballast pressures involves a consideration of rail stiffness, sleeper spacing,ballast packing and maintenance and the shapeof the
sleeper footprint. Design sleeper moments can be assessed from the loads
and ballast pressures except that,because of dynamic effects, the unloaded
resonant stresses have to be considered in developing a design moment
envelope. Finally, a sleeper may be designed with given material strengths
but with the remaining considerable freedom of varying the depth by
profiling the top face to giveoptimum stresses throughout.
With all these interactions and the need to consider effects that are so
difficult to determine,
quantify and maintain in the long term in practice, it
is hardly surprising that much design development relies on an empirical
approach taking into account what has gone before.
The next section of this paper discusses some of the pioneering work on
track from which the basic design rules on the loading side were derived and
follows this through to thedesign process in use today.
Assessment of load
The two societies, theAmerican Railway Engineering Association and the
American Society of Civil Engineers, set up committees in 1913 to instigate
a scientific study of deformations and stresses in railway track. This
committee, under the chairmanship of Professor A. N. Talbot, produced a
steady stream of work from 1918 to 1940. The seven Talbot reports2 are
important, not just for the data which were developed but for the way in
which laboratory tests and track trials werecarried out together. The work
of the Talbot Committee was verythorough. Using fixed reference locations
Talbot measured the displacement of the rail beneath single wheels, pairs
of wheels and complete locosgiving valuable data onspread - the distribution of wheel loads along the
rail to adjacent sleepers. The
deformation of
sleepers (always timber) was also measured, and this work was developed
to determine ballast pressure distributions on sleeper soffitsand at depth.
Fig 6 shows the results of a test of a sleeper inthe laboratory on a ballast
bed with pressure measurements measured by pressure capsules embedded
in the ballast. The work went on to consider stressesin rails and joints and
even to determinethe effect of flat spots onwheels and the consequential
stresses that they impose on the rails. Wheel flat spotsare still an important
factor in sleeper lifetoday, and will be discussed later inthis paper.
Talbot's work was available to the UK pioneers of RC and prestressed
sleepers, and some of the experimental techniques used by Talbot were also
used in the Cheddington trials.
A paper by Johansen3describes the work by the LM&S Railway Research
Department at Cheddington from 1942. In this work reinforced concrete
The Structural EngineerIVolume 71/No 16/17 August 1993
Paper: Taylor
Axle loads
Variables:
Vehicle type
Vehicle maintenance
Wheel load
Variables:
Axle load
Vehicle maintenance
Vehicle speed
Track structure
Track maintenance
Ballast pressure
Variables:
Rail stiff ness
Sleeper spacing
Ballast packing
Track maintenance
Sleeper plan shape
L
Sleeper moments
Variables:
Sleeper shape
Track maintenance
Dynamic effects
Sleeper design
Variables:
Materials chosen
Manufacturing process used
Fig 5. Design process
sleepers fitted with strain gauges were placed in track and monitored. The
sleepers, manyof which became severely cracked during the
trials, yielded
useful results, and many of Johansen’s conclusions are still very relevant
today. The work showed that the cracking
of the sleeper on the top surface
beneath therail was more important than
at ballast level, a point which was
finally solved only in the early 1980s. The importance
of damage being done
to sleepers immediately after they are laid during ballasting, before the
proper packing beneath them was completed, was recognised, as was the
influence of wheel flats and corrugatedrail. Both of these latter two effects
are of great significance today, and workis still going on to control them.
0
cu
10
I
.E 20
3
Thomas4, carried out further
tests at Cheddington where he used load cells
and otheringenious means of measuring chair reactionsandballast
pressures, developedby the Building Research Station where he worked. Dr
Thomas was, of course, a major figure in the development
of our concrete
Codes in that era. The work on rail reactions led to the measurement of a
frequency curve for loco and tender wheels and coach wheels. Figs7 and
8 reproduced from the paper show the results.
The distribution of pressure beneath the sleepers was measured with a
‘ball sandwich’ which consistedof two metal plates separatedby regularly
laid out ball bearings. The sandwich was fixed to the underside
of the
sleeper and sat on the ballast. The width
of the indentation of the balls into
the outer steel leaves ofthesandwichwasmeasuredandusedwith
calibration information to estimate the pressures. Thomas produced data
very like those of Talbot before him, and these were used, in a slightly
modified form, in BS9865.
BS986 gave design chair reactions and pressure distributions beneath
Table
shown
insleepers
as
Fig
1 and
9.
It is interesting that prestressed sleepers were to be designed to higher
loads than reinforced ones, and the explanation was given RC
thatsleepers
may crack and can be designed to mean working loads whereas prestressed
sleepers may not, as the bond may then be lost, and therefore they should
be designed to the highest loads ‘likely to be incurred’.
The 22 t load for mainline sleepers has been reduced since that time
although theP-P/4 bearing pressure designis still used. TheP-P/2 diagram
is not used nowadays, and mainline sleepers are used everywhere in the
network. Itis important to recognise, however, that BS986 gave no require-
\
30
40
50
Pressure below centreline of tie
Ballast depth 12 in
Fig 6. Distribution of ballast pressure
The Structural EngineedVolume 71/No 16/17
283 August 1993
Paper: Taylor
8
0 642
10
12
14
16
18
20
22
Chair reactions: Ton
Fig 7. Frequency curve, chair reaction dueto locoand tender wheels
ment for negative moment capacity underrail.
theBS986 also defineda load
test in whicha load was appliedat the rail seat position spread on
a 125mmwide rubber pad.Two support points, 125 mm wide and 150
mm apart, were
used, and a test load (which, for a class E sleeper, was 30 t) was defined
before which cracking was not allowed. A similar test is still used as an
acceptance criterion on randomly selected sleeperson eachproduction line
cast.
0
1
2
3
4
5
6
7 8
910111213
Chair reactions: Ton
Prestressed concrete design
Fig 8. Frequency curve, chair reaction due to coaches
In the discussion to ref. 4, Dr Mautner gave the basis of the design of the
‘Dow Mac’ sleepers used in the trials. He covered permissible stresses in
some detail and referred back tests
to carried out by Freysinnet in 1936. His
other reference to design theory was toa paper in The Structural Engineer
1498
+
in July 19406. In the appendixto this paper Dr Mautner presented as clear
-.I
an exposition of prestressed theory as can be found today. In two pages he
covered elastic theory, treatmentfor losses, and principal stresses for shear.
The design Codes, in terms of materials data, have developed more
slowly. The data given in ref. 5 were quickly followed by those in BS986
I
I
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I
and from then, through the Institution of Structural Engineers’ first report
of 1951 and C P l l Y , we are led to BS8l 10 in use today. The development
ofthe criteria is shown in Table 2. In the table, because of different
approaches, it is difficult to make direct comparisons but, in developing the
table, data relevant to the highest strengths of concrete were used as these
I
most exactly suited the strengthsof concrete used in sleepers.
The Chairman of the CPl15 committee was Dr Thomas, and his ViceChairman was Dr F. Walley who is Chairman of the committee that looks
after BS8110 today. The continuity of our Codes and their progression is
as much a human issue as a technical one! Whether the success of the preDistribution of pressure under sleepers
stressed railway sleeper was central to the development of the design
process, or whetherit was just one of the many usesof the technique in the
For class A, B or C sleepers pc = P/2
1940s and O OS, can be answered only by a historian. Without doubt, the
survival of prestressed railway sleepers in their arduous environment must
For class D sleepers pc = P/3
have given great confidence toall early practitioners in prestressing.
For class E sleepers pc = P/4 except for that
l =
Development of the manufacturing system
The early methodof manufacture used for pretensioned sleepers was highly
automated. The Tallington factory was designed on foundry systems with
l
I
for sleepers to be used with tracks with
ash ballast, pc = P/3
Fig 9. Distribution of pressure under sleepers(BS986, 1945)
TABLE I - Chair reactions, BS986
Chair reaction R in ton for:
Class
of
sleeper
5.5
For design
at centre
of sleeper
5.5
9
10
11
10
12.5
15
Intermediate
sleepers
Lightly worked sidings
Heavily worked sidings and refuge
sidings, goods, loops, and the like, over
which the speed is limited to 30 mile/h
Tertiary
Secondary
Primary
284
sleepers
Type of track
~
Joint
sleepers
14
For design
at section
under rail
22
The StructuralEngineer/Volume 71/No 16/17 August 1993
Paper: Taylor
TABLE 2 - Prestressed concrete design data
Reference
Criteria
BS986
1945
First
report
[StructE 1951
Working load
Comp. stress&,
(N/fMl2)
Tensile stress&,
(N/mm2)
Principal lo&,
(N/mm2)
Transfer strength
0.33LU
2.07
I
BS8110
1985
0.33.L.
0.45K
0.05f C u
1.8
0.045 fCu
0.5.L
Ultimate load
Global safety
factor
Losses
Elastic strain*/
1 N/mm2of
stress @ transfer
6 " t = 35)
Creep strain/
1 N/mm2offcc
Shrinkage
CP115
1959
2.5
0.24K
0 . 5 ~
1.5 x DL
+2.5 x LL
1.4 x DL
+1.6 x LL
36 x 10-6
30 X 10-6
30 x lo6
36 X 10-6
44 x 10-6
58 X 10-6
48 X 10-6
55 x 10-6
300 x 10-6
300 x 10-6
300 X
100 x 10%
fa concrete characteristic strength (cube)
* this is basically a consequence
of the recommendedor allowableE values of concrete
# outdoor exposure
the moulds being brought to the casting station on rollers and then, after
filling with concrete, they were winched on
rollers along the already stressed
tendons. The method is described fully in ref. 1.The process hadthe advantage of having fixed work stations with a higher productivity than would be
achieved if the men moved from mould to mould to demould, clean and
refill them.These systems remained inplace until the end of the 1970s when
fixed gang moulds with machines that passed over them were developed.
The current automated gang mould systems are very efficient and the
considerable investment in the factory and machines results in individual
sleepers being made with very low labour content and with much more
consistent quality than under the previous method. Figs 10 and 11 contrast
the early and current manufacturing systems.
Development of sleeper systems
From 1950 to about1985 the sleeper systemwas under a constant process
of development. Through this time theA to D classes were dropped, theE
range was sometimes replaced by a F range, an intermediate EF range and
even a G range. Each change was inresponse to experiencein track but, by
the 1980s, the exact definition of strength grades was lost.The current F40
sleeper was developed in the mid-l980s, and some of the thought which
went into this will be described later.
Fig 11. Production system, 1993
In parallel with the changes in strength of sleepers, great changes took
place in the rest of the track system. Bullhead jointed track gave way to
continuously welded flat bottom rail and chairs gave way to baseplates
which themselves gave way to elastic fasteningsystems with pads beneath
the rail. Manyfastening systems were tried, often with unsatisfactoryresults.
The difficulty of providing a fixing to the concrete
that will hold the spring
fastening was nearly always underestimated. Early systems often failed by
fatigue in the fasteningitself or by the fixing to the concrete
pulling out or
fracturing. The major reason for replacement of most early sleepers is
because of a fracture of the interface between fastening and concrete. In
1960 the Pandrol clip was first introduced into the BR network, and this has
proved to be, both in theUK and worldwide, an excellent fasteningsystem.
The clip, illustrated in its latest form in Fig 12, is familiaralltowho travel
by rail and has hold down
force-displacement properties which are excellent
in holding the rail to a vibrating sleeper and at thesame time restraining the
pad in place. (Fig 13.)
The Pandrol system is also one which, unlike previous
systems, requires minimum maintenance and, as it does
not rely on a screw
fastening, can be quickly checked visually.
Design process
The simple steps of the design process have been described. This section
covers them in more detail and gives an idea as to how much of current
design is science, art, and experience.
Sleepers always see dynamic loads. With the normal sleeper spacing the
time taken from a wheel to pass over three successive sleepers at 200 km/h
is about 0.025 S. On heavy haul lines, sleepers are
more closely spaced and
speeds are lower so that, typically, the time would be 0.095 S. Within this
time the peak value would be applied for some0.01 or 0.04 S, respectively.
Wheel flats, typically the size of a 50p piece, may occur if a wheel skids
under braking. Over time these can develop to 75mm or longer, imposing
an additional hammer blow as theyhit therail. Wheel flats are common, as
any regular train traveller can easily recognise. Out of round wheels and
frozen suspensions, the latter being not uncommon in trucks with certain
suspension types sometimes carrying cement or coal, can also cause loads
F irg 12. Pandrol railfastening
Fig IO. Production system, 1943
The Structural EngineerIVolume 71/No 16/17August 1993
285
Paper: Taylor
1000
800
600
400
Vibration mode found in practice
200
0
6
04
2
8 161014 12
Clip toe deflection (mm)
1
Fig 13. Pandrol rail fasteningpeqormance
to be impulsive. Finally, as shown by Thomas4, the wheel passing over a
joint or aweld in welded track causesa further impact effect.
Fig 14 gives a summary of 12 different formulae for deriving the impact
factor for the wheel onto the rail. The factor is speed related but is also
greatly influenced by maintenance of the whole railway. Clearly, we must
be led by experience in deciding on this factor!
The distribution of load between sleepers or spread has been defined
mathematically by Zimmermdn9. Zimmerman’sformula is:
(
R S L = W 1--
W
Bending moment from sideforces
Fig 15. Moments.from dynamic effects andside loads
3r2:2)
where
W is the enhanced wheel load(for impact)
i = 6 EI/L3C
E is the rail modulus
I is the rail inertia
L is the sleeper spacing
C is the support spring rating
2.0 -
1.5 -
10
150
50
100
Speed km/h
1 Steam loco, Peterson formula
2 Clark formula highend of range
3 Diesel loco, Peterson formula
4 Indian formulafor light track 1
5 AAR formula for diesel locos 1
6 Indian formulafor light track 2
Fig 14. Impact factor formulae
286
7 German formula
8 Schram
9 Clark formula
10 AAR formula for steam locos 2
11 Ore 071 formula 1
12 Ore 071 formula 2
Once again the value of C to be used is a matter of conjecture and, before
a distribution factor can be assumed, some allowance must be made for
combinations of loads from adjacent wheels on the vehicles.
On balance the combination of the increasing impact factor and the
reducing spread factor comes to somewhere between 1 .S and 2.5, depending on the typeof railway, axle loads, maintenance,etc. The temptation to
derive thesecombination effects backwardsfromtheperformance
of
sleepers of known strength with known vehicle types and speedsis almost
irresistible.
The rail seat reactions derived in
this way are applied to the pressure
diagram in Fig 9. The centre pressure may be zero immediately after the
sleeper has been tamped beneath the
rail, it maybe P/4 during service
running and tend towards P/2 as the track becomes in need of retamping.
The effect of this will be to reduce the positive (sagging) moment beneath
the rail and increase the negative, hogging moment at the centre.
The effects of curved track and of vibration in one mode both lead to
tensile stresses at the top of the sleeper under the rail (Figl S ) , particularly
towards the inside fastening location.
One of the major causes of vibration damage to sleepers is rail corrugation. In the late 1970s an increase of incidence of corrugated rail caused
some looseningof the iron shoulders that hold the fastening into the sleeper
and caused top crackingof sleepers. The trend to raising the position
of the
prestress tendons came from this time, as did a redesign of the embedded
stem of the shoulder.
Rail corrugation consistsof the development and propagation aofregular
wave on therail top. Passengers can easily recognisea corrugated section,
as it ‘sings’ when a train passes overit. Corrugation is still not fully understood; it is a feature of all road systems supporting wheels and
is even
found in highways, but with
a very different pitch and amplitude. Rail
corrugation appears to depend on rail metallurgy, wheel and vehicle type
brake system (rim or disk) and line speed. Researchin this area continues.
Rail grinding with special machines which reprofile the rail headused
is as
a maintenance measure, but even then the corrugation can recur over
a
period of months or years.
The current BR mainline standard sleeper shows the effect
of these
demands in its bending moment diagram (Fig16).This sleeper is somewhat
shorter than its predecessors for operational reasons, to enable it tobe laid
with machinery which itself is within a safe working gauge for relaying to
The Structural EngineerNolume 71/No 16/17 August 1993
Paper: Taylor
'
proceed on a weekday with the adjacent line being usedby normal traffic.
The importanceof a dialogue between the railway users
of sleepers in many
areas, the installer, the designer and the manufacturer cannot be overemphasised if a satisfactory design conclusionis to result.
The under rail sagging capacity in the F40 is less than that of previous
sleepers, as it is shorter and picks up less load from the cantilever and
because it has thicker and much more resilient 10 mm rail pads than the
previous standard5 mm to6 mm pads. It was also decided, in modern wellmaintained high speed lines, that it was important to raise the prestressing
tendons in the section to improve the reverse hogging moment capacity, as
this negative moment demand was speed related.
The F40 sleeper was developed in the early
1980s in a programme of test
and trial application which givesa good ideaof the levelof loading actually
experienced by sleepers. In thelate 1970s problems were experienced with
the then standard F27 sleeper in the most arduous parts of the London to
Glasgow West Coast Main Line (WCML). The problems were on isolated
sections and showed up as cracks in the sleepersat the top, at each side of
the cast in rail fastening housings. This resulted in a new design of the
housing with better pullout resistance and torsional grip and also stimulated
a new look at the sleeper design.
After a study of various design options and measurement
of sleeper
movements under traffic, it was decided to raise the prestressing strands in
the section to givea larger prestress under therail seat and effectively hold
any cracks together.
1
C Rail
1210+
~
1210
I
I
I
15
E
0 5
.-K
v)
15
5a,
>
t
30
1210
0
605
605
1210
Dimensions in (mm)
Fig 16. Moment strength envelope, F40sleeper
I Point (6)
I
I
I
I
80
40
0
-40
l
0
1
I
I
0.01
0.02
0.03
I
0.04
1
L
0.05
0
I
0.01
Time in S - 0 start
arbitrary
Time
in
I
I
0.02
S
0.03
I
0.04
1
0.05
- 0 start
arbitrary
Cycles recorded from a loco type
87 travelling at speedsof 100 mile/h measured at two points
-40
h
0
40
Y
v
t
a
80
0
0.01
0.02
0.03
0.04
0.05
0
Time inS - 0 start arbitrary
0.01
0.02
0.03 0.05
0.04
Time in S - 0 start arbitrary
Cycles recorded from a carriage at the centre
of the same train travelling at100 mile/h measured
at two points
Fig 17. Measured strain readings, West Coast Main Line,1984
The Structural EngineedVolume 71/No 16/17 August 1993
207
Paper: Taylor
The criticality of this element of the sleeper designis interesting. Fig 17
TABLE 3 - Hypothetical distribution of axle loads, West CoastMain Line
shows the result of tests on instrumented sleepers in the WCML where
strains were measured using resistance strain gauges and a high speed
% of total
Axle load (t)
datalogger. The class 87loco, which already had achieved some notoriety
5
12.5
in causing more rapid track deterioration than previous locos, imposed
80
17.5
strainchangesof 100 x
compressionand 10 x
tensioninthesleeper
10
under the rail. The passenger coaches of the same train give 50 x
22.5
compressionand 50 x
tension.
4
27.5
The pattern of oscillation shows 6 to 7 cycles
of near identical amplitude
1
32.5
followed by a very rapid decline. This was put down to the
fact that the
sleeper comes away from the ballast and vibrates freely momentarily after
a wheel passes and thensits back onto the ballast whereit is well damped.
TABLE 4 - B R system track miles(simplified table)
The frequency is approximately 500 Hz and is different from that caused
by
rail corrugations which, typically, is 900 Hz at 160 km/h.
41
2
Tonnage x lo6
Category
An estimate of the fatigue life of the concrete gives interesting results.
<2
2<t<5
5<1:2
12<t
speed k m h
Assuming that the WCML carries approximately10 M gross t p.a. and that
the axle loading falls between 12.5 t and 32.5 t, in the pattern of traffic
1248
A
15
957
> l 60
0
proposed inTable 3, it is possible to come to some simple conclusions.
The
loco gives the compression worst
case and the coach the tension worst case.
2949
B
120< <l60
578
2249
563
Looking at thetension case, 430000 x 7 cycles are experiencedp.a., 3.2
x 106 cycles. Assuming that this causes the majority
of the fatigue damage,
80< < l 20
218 1213
C
3599 3529
it ispossible to predict the fatigue life of a sleeper under therail, using data
produced more recentlyby Cornelissen & Reinhardt’O (Fig 18). This, for the
D
4 0
329 957 21
2694
F27, suggests a life of 6.2 x lo6 cycles or approximately 2 years. This life
beforecrackingwasconsistent
with experienceatthemostarduous
locations on the WCML, although it should be realised that a cracked
technical problem, as it involves nearly all aspects of the running of the ra
network. One interesting development in the last few years demonstrates this
sleeper survives for some years beyond the time
of cracking before it needs
quite clearly. In 1986
an ‘intelligent rail’ was put into the west coast mainline
to be replaced.
at Braidwood, southof Glasgow. The installation consistedanofinstrumented
The revised F40 has sufficient prestress not to go into reverse stress
under this level of cyclic loading and has therefore given excellent durabilitylength of rail with a datalogger, minicomputer and telemetry module at the
track side. The systemwas capable of measuring and recording all axle loads
in track to date.
as they passed by. The system was set up with a trigger level of a 30
t wheel
The compression side also has a reassuringly high fatigue life. The
load which rang an alarm in the Glasgow Control Office. Glasgow was able
average prestress levels, with the cyclic loading imposed on them,
still give
to question the computer system, to identify the guilty train, vehicle and axle.
compressive fatigue lives of the order of 1020cycles.
Statistical data were collected weekly by the British Rail Centre at Derby
As we have seen from 50 years of satisfactory use, compressive fatigue
of concrete in prestressed sleepers is not a problem and is never likely to
using a modem link to download the data.
Initially, up to 10 such excursions
a week were detected and, after directed maintenance, this level is now less th
become one.
one/week. The vehicles at fault were a combination of some locos, many
freight vehicles (some of which were privately owned) with a set of faults
50 years on
which would not have been always readily visible during normal maintenanc
The replacementof bullhead chairedrail on timber sleepersby continuously
Such was the success of the Braidwood experiment that now BR has three
welded flat bottom rail on concrete sleepers is practically complete on the
such monitoring stations in operation and is installing a fourth.
mainlines of the BR network. Table 4 gives an idea of the extent of the
The final most interesting area to be considered
is how long the sleepers
current network, with the track categorised in terms
of speed, A-D, and gross
last and howdo they fail? The second partof this question can be answered.
annual tonnes carried. The only area of the network where there is still
The vast majorityof sleepers that are replacedfail because of the difficulty
timber sleepered track in any significant amount
is in categories C1 and D 1,
of controlling the arduous environmentinwhichtheyoperate.Track
with lesser amounts in C2 and D2. These are rural lines where it is harder
maintenance, rail corrugation, joints, wheel and vehicle defects, and even
to commercially justify the levels of investment needed to relay.
derailment, all cause damage leading in some cases to replacement. A very
Further developments of complex ‘sleepers’ or ‘bearers’ for switch and
small number of sleepers of less than adequate strength find their
way into
crossing work has taken place, after
a thorough research programme.
Currently, prestressed concrete bearersup to 6 m long are used for switches track, as could be expected from any product accepted for use following a
and crossings where they offer greater stability than timber and overcome
statistical procedure based on random test. The experience of BR is that
the problem of sourcing the high quality hardwood previously used in theseprestressed concrete sleepers last for 30 years in the mainline, followed
by
replacement and cascading down to less heavily trafficked lines where a
locations.
further 10-20 years’ life may be possible.
This paper demonstrates that the design of track is not just an ongoing
1
1
1
1
Conclusion
1.o
0.8
E 0.6
5
c
\
X
2
b
\
\
0.4
compression - tens>
0.2
l
6 Hz
0.0
0
1
2
3
4
5
6
Log N
Fig 18. Fatigue of plain concrete with stress reversals
288
I
bry specimens
7
8
9
The prestressed, pretensioned railway sleeper has been a success-story both
for prestressed concrete and for British Rail. The sleeper has been central
to the development of the BR network from a railway in needof modernisation in 1945 to the network that we use now. BR’s achievement is that
its modernisation went from steam on bullhead jointed track to high-speed
mixed trafficked routes with continuously welded rail. The robustness of
prestressed sleepers allows passenger trainsof 250 k m h to share the same
track with slower goodstraffic.
In Japan and France the high speed network has been developed on new
dedicated lines, with the advantage that
the track design has only one
loading type. Britain’s challenge tofit high-speed railways into an existing
network is much harder, and our success
is insufficiently emphasised. Even
now, train speeds are limited more
by the need to ensure passenger comfort
on the original curving alignments thanby inadequacies of hardware.
The challenge for the future is in financing the secondary lines and in
maintaining the high standards of track in the primary network. In both of
these areas, prestressed concrete will surelystill have a decisive role.
Continued ot1 page 295
The Structural EngineedVolume 71/No 16/17 August 1 9 9 3
Paper: Bloomer Paper: Taylor
Paper: Taylor
continuedfrom page288
Acknowledgement
The author is pleased to acknowledge the assistanceof his colleague John
Surtees for his gift of historical and technical knowledge essential to the
paper and also his now retired colleague John Snasdell who taught him
much about sleepers. The helpful coments of Dr D. Cope, of British Rail,
are also gratefully acknowledged.
References
Barker, R. S. V., Lester, D. R.: ‘The development and manufacture of
prestressed concrete units’, Society of Engineers, May 1946, pp41-75
discussion contribution, Henzell, J. S.
2. Stresses in railroad tracks - the Talbot Reports 1918-1940, reprinted
by American Railway Engineering Association, Washington, DC,
1980
3. Johansen, F. C.: ‘Experiments on reinforced concrete sleepers’, Proc.
ICE, Railway Division, May 1944, pp3-20
4. Thomas, F. G.: ‘Experiments on concrete sleepers’, Proc. ICE, Railway Division, 1944, pp21-66
5 . BS986 Concrete
railway
sleepers, London,
British
Standards
Institution, first pub. 1941,2nd rev. 1945
6. Everitte, T. J.: ‘Recent developments of prestressed concrete construction with resulting economy in the use of steel - Technical Appendix
by K. W. Mautner’, The Structural Engineer, July 1940, pp626-642
7. Firstreport on prestressedconcrete, London, IStructE, September
1951, pp31
8. CP115 The structuraluse of prestressed concreteinbuildings,
London, British Standards Institution,1959
9. Zimmerman, H.: Die Merechnung des Eisenbabnoberbaues (original
pub. c. 1890) W. Ernst & Sons, 1941,3rd ed.
10. Cornelissen and Reinhardt: ‘Fatigue of plain concrete with stress
reversals’, CEB Bulletin No. 188, Lausanne, CEB, 1984
1.
Fig 8. End of project
Acknowledgements
The author expresses hisappreciation to allhis colleagues who worked on
the stack project. He is particularly grateful for the support of George
Maddison (M) (Allott & Lomax).
References
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295