Paper: Taylor

Paper

The railwa sleeper: 50 years of pretensioned, prestresse cy concrete H. I? J. Taylor, BScTech, PhD, FEng, FIStructE, FICE Costain Building Products Ltd

Synopsis This year, 1993, marks 50 years from the introduction of the pretensioned, prestressed railway sleeper. This paper describes some history of the early development of prestressed sleepers and links the design development with that of our Codes. The success of the prestressed sleeper is self-evident, but the paper does emphasise the successful way in which the prestressed sleeper copes with a most hostile working environment and provides an insight into the development of our design methods for 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 (Fig 1).

Rail block: a block of stone or concrete which supports a single rail (Fig l).

Twin block sleeper: a sleeper consisting of two concrete blocks tied together with a metal bar (Fig 1).

Bullhead track: rail track consisting of bullhead rail held in cast-iron chairs which are supported by 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 and rail. 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 between rails for track circuiting purposes.

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 r Irl

I d 1 - ,il 0 L/ 1 l Y

Twinblock sleeper

m Railblock

Fig I . Sleeper types

Flat bottom rail

Bullhead rail track 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 worlds 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 to this being an option in trials.

In 1941 two designs of reinforced concrete sleepers were produced by the Chief Civil Engineers 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 for just 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 part of 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 work at Colwall and gives a good impression of the excitement and pressure of those times.

The next set of concrete sleeper track trials included both RC sleepers and prestressed sleepers from Colwall. These first prestressed sleepers were

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B A - 4575 - .r. c.t

I c I I

I I I I 1

! I " 1 3 1

"B Section Section

A-A B-B

Fig 3. Rail blocks

put in the west coast mainline at Cheddington near to Tring on 21 February 1943. That, and the building of the Dow Mac factory at Tallington in 1943- 44, marks the start of the 50-year period 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 static self-weight of the rail is only in the order of 0.1% of the total design load. They only ever see significant loads as dynamic 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 are so stiff that the demands on a sleeper are much more than a bridge deck loaded by pneu- matic 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 it until relatively recently.

The points of application of load are defined totally, but on the support

Fig 4. Sleeper tests, 1943

282

I

side a wide range of support reactions are found. These depend on the nature of the ballast and formation below it as well as the form of ballasting and the quality of maintenance. The point of loading also defines the points of maximum moment and shear as coincidental. This was brought out very well in the discussion of one of the early review papers in the development of sleepers', when a Mr Henzell remarked:

'A point to be noted about the sleepers described by the authors was that they were entirely without stirrups or transverse reinforcement. 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 trans- verse 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 maximum bending 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 mech- anical means was remarkable'.

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 main- tenance. 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 stiff- ness, sleeper spacing, ballast packing and maintenance and the shape of 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 give optimum 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 the design process in use today.

Assessment of load The two societies, the American 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 were carried out together. The work of the Talbot Committee was very thorough. Using fixed reference locations Talbot measured the displacement of the rail beneath single wheels, pairs of wheels and complete locos giving valuable data on spread - the distrib- ution 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 soffits and at depth. Fig 6 shows the results of a test of a sleeper in the laboratory on a ballast bed with pressure measurements measured by pressure capsules embedded in the ballast. The work went on to consider stresses in rails and joints and even to determine the effect of flat spots on wheels and the consequential stresses that they impose on the rails. Wheel flat spots are still an important factor in sleeper life today, and will be discussed later in this 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 Johansen3 describes the work by the LM&S Railway Research Department at Cheddington from 1942. In this work reinforced concrete

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Axle loads Variables:

Vehicle type Vehicle maintenance

Ballast pressure Variables:

Rail stiff ness Sleeper spacing Ballast packing

Track maintenance Sleeper plan shape

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, many of which became severely cracked during the trials, yielded useful results, and many of Johansens conclusions are still very relevant today. The work showed that the cracking of the sleeper on the top surface beneath the rail 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 corrugated rail. Both of these latter two effects are of great significance today, and work is still going on to control them.

0 10

.E 20 3 30

40 50

cu \

Pressure below centreline of tie

Ballast depth 12 in

Fig 6. Distribution of ballast pressure

Wheel load Variables: Axle load

Vehicle maintenance Vehicle speed Track structure

Track maintenance

-L

Sleeper moments Variables:

Sleeper shape Track maintenance

Dynamic effects

Thomas4, carried out further tests at Cheddington where he used load cells and other ingenious means of measuring chair reactions and ballast pressures, developed by 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. Figs 7 and 8 reproduced from the paper show the results.

The distribution of pressure beneath the sleepers was measured with a ball sandwich which consisted of two metal plates separated by 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 of the sandwich was measured and used with 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 sleepers as shown in Table 1 and Fig 9. I

It is interesting that prestressed sleepers were to be designed to higher loads than reinforced ones, and the explanation was given that RC sleepers 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 the P-P/4 bearing pressure design is still used. The P-P/2 diagram is not used nowadays, and mainline sleepers are used everywhere in the network. It is important to recognise, however, that BS986 gave no require-

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0 2 4 6 8 10 12 14 16 18 20 22

Chair reactions: Ton Fig 7. Frequency curve, chair reaction due to loco and tender wheels

ment for negative moment capacity under the rail. BS986 also defined a load test in which a load was applied at the rail seat position spread on a 125mm- wide 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 sleepers on each production line cast. 0 1 2 3 4 5 6 7 8 9 1 0 1 1 1 2 1 3

Chair reactions: Ton

Fig 8. Frequency curve, chair reaction due to coaches Prestressed concrete design 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 to tests carried out by Freysinnet in 1936. His other reference to design theory was to a paper in The Structural Engineer in July 19406. In the appendix to this paper Dr Mautner presented as clear an exposition of prestressed theory as can be found today. In two pages he covered elastic theory, treatment for 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 and from then, through the Institution of Structural Engineers first report of 195 1 and CPl lY, we are led to BS8 l 10 in use today. The development of the 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 most exactly suited the strengths of concrete used in sleepers.

The Chairman of the CPl15 committee was Dr Thomas, and his Vice- Chairman 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 pre- stressed railway sleeper was central to the development of the design process, or whether it was just one of the many uses of the technique in the 1940s and O OS, can be answered only by a historian. Without doubt, the survival of prestressed railway sleepers in their arduous environment must have given great confidence to all early practitioners in prestressing.

1498 -.I +

I I I l I

I I l =

Distribution of pressure under sleepers

For class A, B or C sleepers pc = P/2 For class D sleepers pc = P/3 For class E sleepers pc = P/4 except for that for sleepers to be used with tracks with ash ballast, pc = P/3

Fig 9. Distribution of pressure under sleepers (BS986, 1945)

Development of the manufacturing system The early method of manufacture used for pretensioned sleepers was highly automated. The Tallington factory was designed on foundry systems with

TABLE I - Chair reactions, BS986

Chair reaction R in ton for:

sleepers Class

of sleeper

Type of track For design For design

at section under rail

Joint sleepers

Intermediate sleepers at centre of sleeper

5.5 5.5

9 10

~ 1 1

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

10 12.5 15 14 22

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TABLE 2 - Prestressed concrete design data

Reference

Criteria

Working load Comp. stress&, (N/fMl2) Tensile stress&, (N/mm2) Principal lo&, (N/mm2) Transfer strength

Ultimate load Global safety factor

Losses Elastic strain*/ 1 N/mm2 of stress @ transfer 6 " t = 35) Creep strain/ 1 N/mm2 offcc Shrinkage

BS986 1945

36 x 10-6

44 x 10-6

300 x 10-6

First report

[StructE 1951

2.5

30 X 10-6

58 X 10-6

300 x 10-6

CP115 BS8110 1959 I 1985

0.33LU

2.07 0.05 f C u

1.8 0.045 fCu 0.5.L

1.5 x DL +2.5 x LL

0.33.L.

0 . 4 5 K

0 . 2 4 K

0 . 5 ~

1.4 x DL +1.6 x LL

30 x lo6

48 X 10-6

300 X

36 X 10-6

55 x 10-6

100 x 10%

fa concrete characteristic strength (cube) * this is basically a consequence of the recommended or allowable E 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 had the advant- age 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 in place 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 about 1985 the sleeper system was under a constant process of development. Through this time the A to D classes were dropped, the E range was sometimes replaced by a F range, an intermediate EF range and even a G range. Each change was in response to experience in 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 IO. Production system, 1943

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 fastening systems with pads beneath the rail. Many fastening systems were tried, often with unsatisfactory results. 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 fastening itself 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 the UK and worldwide, an excellent fastening system. The clip, illustrated in its latest form in Fig 12, is familiar to all who travel by rail and has hold down force-displacement properties which are excellent in holding the rail to a vibrating sleeper and at the same 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 some 0.01 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 75 mm or longer, imposing an additional hammer blow as they hit the rail. 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 rail fastening

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1000

800

600

400

200

0 0 2 4 6 8 10 12 14 16

Clip toe deflection (mm)

Fig 13. Pandrol rail fastening peqormance

to be impulsive. Finally, as shown by Thomas4, the wheel passing over a joint or a weld in welded track causes a 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. Zimmermans formula is:

RSL=W 1-- ( 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 50 100 150 Speed km/h

1 Steam loco, Peterson formula 7 German formula 2 Clark formula high end of range 8 Schram 3 Diesel loco, Peterson formula 9 Clark formula 4 Indian formula for light track 1 10 AAR formula for steam locos 2 5 AAR formula for diesel locos 1 11 Ore 071 formula 1 6 Indian formula for light track 2 12 Ore 071 formula 2 Fig 14. Impact factor formulae

Vibration mode found in practice

1

W

Bending moment from side forces

Fig 15. Moments.from dynamic effects and side loads

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, depend- ing on the type of railway, axle loads, maintenance, etc. The temptation to derive these combination effects backwards from the performance of sleepers of known strength with known vehicle types and speeds is 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 may be 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 (Fig l S ) , particularly towards the inside fastening location.

One of the major causes of vibration damage to sleepers is rail corru- gation. In the late 1970s an increase of incidence of corrugated rail caused some loosening of the iron shoulders that hold the fastening into the sleeper and caused top cracking of 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 consists of the development and propagation of a regular wave on the rail top. Passengers can easily recognise a corrugated section, as it sings when a train passes over it. Corrugation is still not fully under- stood; 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. Research in this area continues. Rail grinding with special machines which reprofile the rail head is used 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 (Fig 16). This sleeper is somewhat shorter than its predecessors for operational reasons, to enable it to be laid with machinery which itself is within a safe working gauge for relaying to

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1 C Rail ' ~ 1210+ 1210 -

I I I 15

E 0 5

K

v) a,

.-

15 5 >

30 t 1210 605 0 605 1210

Dimensions in (mm)

Fig 16. Moment strength envelope, F40 sleeper

proceed on a weekday with the adjacent line being used by normal traffic. The importance of a dialogue between the railway users of sleepers in many areas, the installer, the designer and the manufacturer cannot be over- emphasised if a satisfactory design conclusion is 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 standard 5 mm to 6 mm pads. It was also decided, in modern well- maintained 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 gives a good idea of the level of loading actually experienced by sleepers. In the late 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 sleepers at 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 give a larger prestress under the rail seat and effectively hold any cracks together.

l 1 I I I 1

I Point (6) I I I I 80

40

0

-40

L I I I I 1

0 0.01 0.02 0.03 0.04 0.05 0 0.01 0.02 0.03 0.04 0.05

Time in S - 0 start arbitrary Time in S - 0 start arbitrary

Cycles recorded from a loco type 87 travelling at speeds of 100 mile/h measured at two points

0 0.01 0.02 0.03 0.04 0.05

Time in S - 0 start arbitrary

-40

0

40

80

h

v Y

a t

0 0.01 0.02 0.03 0.04 0.05 Time in S - 0 start arbitrary

Cycles recorded from a carriage at the centre of the same train travelling at 100 mile/h measured at two points

Fig 17. Measured strain readings, West Coast Main Line, 1984

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The criticality of this element of the sleeper design is interesting. Fig 17 shows the result of tests on instrumented sleepers in the WCML where strains were measured using resistance strain gauges and a high speed datalogger. The class 87 loco, which already had achieved some notoriety in causing more rapid track deterioration than previous locos, imposed strain changes of 100 x compression and 10 x tension in the sleeper under the rail. The passenger coaches of the same train give 50 x compression and 50 x tension.

The pattern of oscillation shows 6 to 7 cycles of near identical amplitude 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 then sits back onto the ballast where it is well damped. The frequency is approximately 500 Hz and is different from that caused by rail corrugations which, typically, is 900 Hz at 160 km/h.

An estimate of the fatigue life of the concrete gives interesting results. Assuming that the WCML carries approximately 10 M gross t p.a. and that the axle loading falls between 12.5 t and 32.5 t, in the pattern of traffic proposed in Table 3, it is possible to come to some simple conclusions. The loco gives the compression worst case and the coach the tension worst case.

Looking at the tension case, 430 000 x 7 cycles are experienced p.a., 3.2 x 106 cycles. Assuming that this causes the majority of the fatigue damage, it is possible to predict the fatigue life of a sleeper under the rail, using data produced more recently by Cornelissen & ReinhardtO (Fig 18). This, for the F27, suggests a life of 6.2 x lo6 cycles or approximately 2 years. This life before cracking was consistent with experience at the most arduous locations on the WCML, although it should be realised that a cracked sleeper survives for some years beyond the time of cracking before it needs to be replaced.

The revised F40 has sufficient prestress not to go into reverse stress under this level of cyclic loading and has therefore given excellent durability in track to date.

The compression side also has a reassuringly high fatigue life. The average prestress levels, with the cyclic loading imposed on them, still give compressive fatigue lives of the order of 1 020 cycles.

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 become one.

50 years on The replacement of bullhead chaired rail on timber sleepers by continuously welded flat bottom rail on concrete sleepers is practically complete on the mainlines of the BR network. Table 4 gives an idea of the extent of the current network, with the track categorised in terms of speed, A-D, and gross annual tonnes carried. The only area of the network where there is still timber sleepered track in any significant amount is in categories C 1 and D 1, with lesser amounts in C2 and D2. These are rural lines where it is harder to commercially justify the levels of investment needed to relay.

Further developments of complex sleepers or bearers for switch and crossing work has taken place, after a thorough research programme. Currently, prestressed concrete bearers up to 6 m long are used for switches and crossings where they offer greater stability than timber and overcome the problem of sourcing the high quality hardwood previously used in these locations.

This paper demonstrates that the design of track is not just an ongoing

1 .o

0.8

5 0.6 E c \

X

2 0.4 b

0.2

0.0

\\

compression - tens> I

l bry specimens 6 Hz

0 1 2 3 4 5 6 7 8 9 Log N

Fig 18. Fatigue of plain concrete with stress reversals

288

TABLE 3 - Hypothetical distribution of axle loads, West Coast Main Line

Axle load (t)

1 32.5 4 27.5

10 22.5 80 17.5 5 12.5

% of total

TABLE 4 - B R system track miles (simplified table)

Tonnage x lo6 1 2 4 speed k m h 1

Fig 8. End of project

Acknowledgements The author expresses his appreciation to all his colleagues who worked on the stack project. He is particularly grateful for the support of George Maddison (M) (Allott & Lomax).

References 1.

2.

3.

4.

5 .

6.

7.

8.

9.

BS CP3: Chapter V: Part 2: Wind loads , London, British Standards Institution, 1972 Bolton, A: Design against wind-excited vibration, The Structural Engineer, 61A, No. 8, August 1983 ESDU 82026 Strong winds in the atmospheric boundary layer: Part 1: mean hourly wind speeds, London, Engineering Sciences Data Unit, August 1984 BS 4076 Specification for steel chimneys, London, British Standards Institution, 1978 ESDU 85038 Circular-cylindrical structures: dynamic response to vortex shedding: Part I : Calculation procedures and derivation, London, Engineering Sciences Data Unit, May 1986 ESDU 86035 Calculation methods for along-wind loading: Part I : Response of line-like structures to atmospheric turbulence, London, Engineering Sciences Data Unit, December 1987 Hirsch, G: Control of wind-induced vibrations of civil engineering structures, Aachen, 4th Colloquium on Industrial Aerodynamics, June 1980 Roshko, A: Experiments on the flow past a circular cylinder at very high Reynolds number, Journal of Fluid Mechanics, May 1961 CICIND Model Code for steel chimneys, International Committee on Industrial Chimneys, May 1988

10. Design and construction of steel chimney liners, American Society of - . -

Civil Engineers, 1975

Paper: Bloomer Paper: Taylor

Paper: Taylor continued from page 288

Acknowledgement The author is pleased to acknowledge the assistance of 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 1. 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, Rail- way 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 construc- tion with resulting economy in the use of steel - Technical Appendix by K. W. Mautner, The Structural Engineer, July 1940, pp626-642

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8. CP115 The structural use of prestressed concrete in buildings, 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

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