The Precast Concrete Bridge Beam: The first 50 years

Paper: Taylor

Ordinary Meeting A paper to be presented and discussed at the Institution of Structural Engineers on Thursday 19 November 1998 at 6.00pm.

The precast concrete bridge beam: the first 50 vears

J

H. P. J. Taylor, BSc(Tech), PhD, FEng, FIStructE, FICE Tarmac Precast Concrete Ltd

e- Howard Taylor is Technical Director at Tarmac Precast Concrete. His career spans a wide range of experience in design, research and construction. AI the C&CA he became technical manager of a national research programme into ‘Concrete and the oceans’.

then a Director of Dow Mac Concrete Ltd, remaining with rhe company when ir was acquired by Costain

oublications. He was Presidenr of the Institution of

m He moved back into industry, as Chief Engineer and

f.. % Building Products. He has written more than 70

Introduction In the last 50 years, prestressed concrete, pretensioned bridge beams have developed from a few experimental applications to one of the key solutions to bridging problems in the short- to medium-span range, IO-40m. This paper is written to chart that progress and to put in context the current and likely future developments in precast concrete bridges.

This paper concentrates on UK development, although some attention is given to the progress of precast bridge construction in other countries. It is impossible to pin down the starting point of a 50-year interval that would allow a Golden Jubilee year to be celebrated with total precision, but, for reasons described in this paper, the years 1948-1998 have a good claim for 1998 to be considered the jubilee year.

The 50 years have been characterised by constant progress, with developments of standard beams, in analysis, in the development of standard bridges - a process which fell by the wayside - and, in the last decade, a reengineering round, leading tcr a new bridge beam family and integral bridges.

In the future we will see the impact of the European Codes, the use of new materials, and the building of intelligence into our bridges. The bridge beam story demonstrates the power of both incremental innovation and more radical change in what has been, overall, a success story.

It is hoped that this paper is more than a historical review in that it will give the engineer involved in bridge assessment a better understanding of the many precast bridge beam types, particularly those of the early years when non-current beam profiles were used.

Bridge beam development 1948-1973 This period saw the introduction of the first manufacturer-produced ranges of standard beams which later were overtaken by the national standard range. The year 1973 saw the introduction of the U-beam, putting in place the last standard design until the introduction of they-beam range in 1990.

Pretensioned, precast bridge beams were cast before 1948 but that year marked the introduction of the first manufacturers advertising and producing ranges of proprietary standard beams.

A number of beams were cast before World War 2 and stored by the Ministry of War Transport. Paul’ describes the construction of two bridges of box beam and I-beam construction, respectively, for the LMS and LNER railway companies. The time of construction was 1943. Thomas2, in a com- prehensive paper to an ICE conference in 1940, recorded that the beams were designed by Dr Mautner for the Prestressed Concrete Development Company. Brief details of these two early bridges are given in Fig 1, and further details may be found in a more accessible form in the recent ICE Proceedings volume dedicated to historic concrete’. It is of interest to note that these bridges were of integral construction.

The year 1948 can also be taken as the start of the jubilee years as, at this time, Francis Walley designed a range of pretensioned sections for the Costain Concrete Company. This range, the WR range, has a section similar to the modem Y-beam and covered a depth range from loin (250mm) to

2in (560mm) and spans from l l f t (3.35m) to 45ft (13.72m). More will be said about the similarity with the Y-beam later in this paper. The WR range is illustrated in Table 1.

At the same time Abeles had designed a beam with a shape not unlike the M-beam‘. These beams were made mainly by Dow Mac and used in various places in the LNER network. Dow Mac also introduced a standard range, the SBB (small bridge beam), at that time. The SBB range, again with a shape reminiscent of the Y-beam, was produced with three different mould sizes, designated SBB1,2 and 3, which gave bottom flange depths of 2X, 3% and 4Xin (63.5, 89, 114.3mm). The SBB2 design is still in production today by Tarmac Precast Concrete Ltd and is probably the oldest bridge beam design still being manufactured. This section is also illustrated in Table 1.

The author has copies of the original brochures and some calculations of these and other beam sections whi.ch provide a great deal of material that is of use to the assessment engineer. Unfortunately, the space available for this paper does not allow it to be reproduced, but enquiries are welcomed from any bridge beam manufacturer’s design office.

The influence of Government was very great in the early days, with encouragement of the use of prestressed concrete because of shortages of steel, and Government technical input was used to progress design, testing and the planning of prototypes.

The public and private sectors combined in the Prestressed Concrete

Bridge over LMS Railway 1943 Span 42ft Length 44ft

Bridge over LNER 1943 Span 50ft Length 52ft

2R 3in

Fig 1. Bridge beam sections, 1940

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Paper: Taylor

TABLE l -Bridge beam sections

Beam Notes Width Depth Sections

17%n 1948 developed by Francis Walley for Costain Concrete Ltd for infill & voided decks

SBB I x I 225mm-600mm Circa 1948 developed by Dow Mac Concrete Ltd. Still in production. Original 600mm design had lower flange depths of 2Xin, 3Xin & 4Xin

380mm - 8 15mm

T 495mm 1951 PCDG section still in production

Eagle I I to 30in 1965-1970 developed by Dow Mac Concrete Ltd for infill decks 19Xin

I I 1 I 710mm- 1980mm constructed within span diaphragms 410mm- Early 1960s PCDG design to long span beam & slab bridges, usually

Box I 0 1 510mm- 1510mm Early 1960s PCDG design. Many variants, 1200 wide box by Dow Mac. Shear 970mm connector beam by Maunsell etc

Cheshire Late 1960s use on a variety of bridge jobs in the county 850mm 762mm-13711~11

Lancashire Late 1960s. Used on M62 600mm 750mm - 1300mm

M I 1 1 720mm - 1360mm Late 1960s. Accepted as a standard bridge beam. Originally designed to accept an 970mm in situ long flange

U I I 800mm - 1600mm 970mm 1 1973. Standard bridge beam. Voided construction

Development Group (PCDG) which was founded also in 1948 and was subsumed into the Concrete Society when it was founded in 1966. The first Chairman of the PCDG was Dr A. R. Collins and the Secretary P. Gooding, both of the Cement & Concrete Association. By 1958 the PCDG had 650 members and had as the next target lOOO!

The PCDG drove the research and dissemination phase of prestressing technology. In recognition of the difficulty of getting a take up of prestressed bridge beams in a commercial environment that required specified goods to be obtainable from a number of suppliers by competitive tender- ing, the concept of national standard bridge beams was born. The first PCDG beam, the inverted T-beam, is still in production after its introduction in 19615. The full PCDG range was completed with the introduction of the box and I-beams by 1964"','".

It is worthwhile to cover developments in analysis at this stage, as these were an integral part of the success of the bridge beam. Rowel', in his book in 1962, defines the analysis problem of a bridge plate or grillage as having been approached in three ways:

(1) By dividing the bridge deck into individual longitudinal and transverse members and developing slope deflection simultaneous equations that were solved by hand or by relaxation method. (2) By separating longitudinal and transverse members and representing loading and deflection by sine series. These methods were developed by Hendry & Jaeger as harmonic analysis to provide for general cases. (3) The deck is considered to obey isotropic or orthotropic plate theory and from this a series of load distribution factors can be derived. Guyon and Massonnet were early pioneers, but Rowe, Morice, Little and others developed the process to the stage of being a working tool.

The many references from this work are not reproduced in this paper, as it is not part of the central theme. Rowe's book gives an excellent reference list and starting point for further reading.

This was clearly a time of development of analysis - indeed, the author remembers in 1961 his final-year degree project, making and testing a grillage and attempting to solve the equations by relaxation and by using the computer at Manchester. The relaxation analysis was tedious and did not come to a stable solution!

408

The load distribution method was validated by the Cement & Concrete Association with microconcrete models of in situ and prestressed beam bridges in the laboratory and in tests on real bridges. Rowel' described tests on a bridge at St Martins, Stamford, in 1952 (Fig 2). This bridge, which takes the old A1 over the railway, was renovated after reassessment in the early 1990s when a few of the original 32 beams were replaced.

By 1960, therefore, bridge deck analysis methods were in general use, Morice & Little having first published in The Structural Engineer in 1954. The Cement & Concrete Association in the 1960s acted as the dissemina- tor, with its training courses and many backup publications. At the time of writing of Rowe's book, Lightfoot & Sawko had just published the results of computer analysis of grillage equations". The method is easily compre- hended, and once the power of computers increased, the use of load distribution theory reduced. West in 1993" published the results of a further validation of analysis with respect to practical tests and gives simple

Fig 2. St Martins Bridge, Stamford, I952

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Paper: Taylor

I Fig 3. Westway, London, 1965

recommendations for the use of grillage analysis. Finally, Hambly’s book, first published in 1976”and now in a second edition in 1991, provides an excellent up-to-date, physical interpretation of the bridge deck analysis problem.

The advances in analysis were mirrored with those in Codes, except that, such was the quality of the early recommendations for the design of prestressed concrete, few significant changes were made. The Institution of Structural Engineers First Report14 was the first generally available set of design rules, and it is reasonable to assume that, from 1948 onwards, the design process in the IStructE report, or a process very like it, was used. CP 115 was first published in 1959, and later bridge beam design would have followed CP 116, CP 110, BS 5400, and the Department of Transport regulations issued over the last 30 years. The IStructE First Report of 1951 required designers to consider the working load and ultimate load conditions - a limit state approach.

Materials data for the early years are contained in the text of a lecture given by Walley to the PCDG in 1962”.

The main changes in design that are of interest to assessment engineers are in the provision of reinforcement for shear. The First Report and CP 115 did not require nominal link reinforcement for shear. CP 115 suggested the use of links in thin webs, mentioning beams of 2ft in depth and 30ft in span as a limit beyond which links should be provided. In practice, therefore, designers and manufacturers did not put nominal links into inverted T-type beams until compelled to do so by CP 110 in 1972. The change in shear design came from the deliberations of the Shear Study Group16 in the mid- 1950s. The Group, after comparing design rules with a large population of tests, noted that links, when required by Codes, were put in at an excessive level. At the same time, it was recommended that nominal links should be provided at low shear stress levels as a matter of prudence. It is ironic that this measure has caused so much trouble in assessment, when it was insti- gated as a measure to improve upon past practice, not to condemn it. The use of grillage analysis which could consider torsion effects and the development of design rules for torsion also acted as a driver for change, in this case for the provision of longitudinal and vertical torsion steel that previously was not considered.

The discussion of the period 1948-1973, the first half of the 50 years, saw the completion of the standard bridge beam ranges from the first PCDG beams.

The PCDG inverted T-beam endured, but the I-beam fell into disuse. The box beam was next in 1963. The box beam was used extensively in the 1960s, and a variety of transversely post-tensioned and in situ reinforced stitched versions were made. Notable was the Maunsell ‘top hat’ shear-con- nected box beam, used in the Westway in 1965 (Fig 3) and elsewhere. The box beam was liked, particularly by the railway authorities, as it provided an instant access deck and is still used in box or solid ‘log’ form to this day. Box beam construction is less popular with highway agencies and, in the writer’s view, is best avoided, as the inside surfaces are uninspectable.

The M-beam was introduced in the late 1960s”. This beam, conceived to provide a voided deck with an in situ infill above the bottom flange and beam or slab deck, became a mainstay for spans longer than the PCDG inverted T could reach. The reassessment of the M-beam and the introduction of the Y-beam will be covered later in this paper. The U-beam, a true voided beam, was developed and introduced in 1973, although the first beams were not used from standard steel moulds until 1975.

The full range of standard beams was therefore in place by 1973, halfway through the 25-year period. The longest beam of which the author has evi- dence of manufacture in that period was 41.5m. The 1973 standard range comprised the inverted T-box, M- and U-beams, and it was this range that endured, with no additions until the new Y-beam was introduced in 1990. This range and some of the earlier sections mentioned in this paper are shown in Table 1.

Finally, 1973 was marked with the founding of the Prestressed Concrete Association (PCA), the product association of the bridge beam manufacturers.

The standard bridge beam range: 1973-present The second half of the jubilee years saw development from a period of stability of both product and concept to one of more rapid change that we have experienced in the 1990s. The extent of development of design and of backup research, however, in no way approaches the achievements of the 1950s and 1960s.

With the range of beam types set, at first the impetus was to encourage more standardisation, first in design and detailing of beams and secondly in the design of standard bridges.

Somerville & Tiller” and Green2’ worked with users and industry in dis- seminating material that was helpful in encouraging the use of robust eco- nomical details. The practical advice given by Green holds good to this day and is still neglected by some engineers. This area is being addressed currently by CIlUA, and this will provide a valuable reminder of the benefit of using standard, proven details in terms of economy of design effort in manufacture and in resultant performance.

The largest standardisation project in the 1970s was the development of standard bridges. The 1979 brochure of the standard bridges2’ describes this as a ‘not inconsiderable’ project which went as far as producing complete designs, manufacturing, construction and specification material for eight bridges. Of these, three were in situ concrete, four using precast bridge beams and one with steel beams. The precast designs were of single-span inverted T, multispan inverted T (two and four spans), and M- and U-beam deck, both with two and four spans. The designs were carried out by teams from Atkins, Gifford, Maunsell and CONSTRADO, each with a RCU part- ner. Even now, designers have to give reasons on an AIP form for not using a standard bridge.

The brochure remarks that the bridge elevations were all accepted by the Royal Fine Art Commission. The M-beam deck is of particular interest, as this had spaced M-beams, not contiguous, and no special treatment of the edge beam; a UM edge beam with a vertical external face was not used.

Without knowing the numbers of bridges that were constructed to the standard designs it is impossible to be precise about the success of the project. In the period after their introduction, the writer noted a number of U- beam contracts that had a common detail to produce an in situ part depth end diaphragm. These were probably influenced by the standard U-beam design which incorporated such a detail.

At some time between 1975 and 1985 a sea change took place in the way in which bridge design was considered. From a period when concern was primarily with the economy of new build, there was a change to an appre-

TABLE 2 - The Y beam: Reasons for develoument

To replace M-beam which had small flanges which could not accept links bent to new standards

Design To eliminate need for link in top flange required by torsion design To allow the top of the beam to be easily profiled in elevation without loss of the complete top flange

I To allow all the beams in the range to be cast from a single mould To speed steel fixing and mould set-up Manufacture

To have a shape which is optimum for good mould filling TO allow greater covers than the M-beam To have a steep slope at the top of the bottom flange, giving a finish appropriate for spaced beam construction To have a deeper bottom flange giving room for continuity steel in continuoushntegral decks To have a narrow bottom flange to allow beams to be sDaced apart, giving access for inspection

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Paper: Taylor

TABLE 3 - The Y beam rc

Beam Section Span range Notes

Introduction in 1994. Using the same 750mm pallet as the.Y-beam, allows infill and beam & slab construction at the short-span range 7.5m - 17.5m

Y E 14m - 31m Introduced in 1990. Designed to replace the M-beam

SY 1 Introduced in 1992. Using the same shutter pallet as the Y-beam, extends the concept to the long-span range 24m - 45m

I - - - -

'-l l \. c, P

800

10% kxceedance line from designs between 1971 - 1976 (see ref.25)

400

Fig 4. Norwich Southern Bypass, rhePrsr Y-beam bridge l I I I I I I

10 15 20 25 30 Span (m) ciation of the importance of lifetime performance. The increase in road

salting from 1955 onwards, and its detrimental effect on highway structures of all materials, is well

As the highway bridge stock grew, the introduction of bridge management systems to drive maintenance and to respond to the introduction of heavier vehicles resulted in a further emphasis on performance. As far as concrete bridges are concerned the report by Wallbank of Maunsell for the Depart- ment of Transport2' was a turning point. This work, and the previous inves- tigations carried out by DTp referenced by Wallbank, was the start of the move to continuity and integral construction that is so important today.

A parallel development, that of the Y-beam, also took place at the same time. The introduction of the Y-beam and the thinking behind it is covered in the papers written for its l a u n ~ h ~ ~ ~ ~ . * ~ . The Y-beam used a cross-sectional shape which was similar to those used by the first pioneers, the WR and the SBB beams. The reasons why this reversal in practice took place are important, and the rationale behind the Y-beam is briefly reestablished in Table 2. The first Y-beam bridge at Norwich Southern Bypass by Maunsell, with deck design by Rowland Structural Design, is shown in Fig 4. The Y- beam was followed in 1992 by the long-span SY-beam and the short-span version for infill decks, the TY-beam. There always was a span range that was too great for the inverted T-beam but too short for the M-beam. The new TY-beam solves this problem by being suitable for both beam and slab and infill deck construction. The current TY-beam orders have been divided evenly into these two forms of construction. The fully-beam family is illustrated in Table 3.

In the paper which introduced the Y-beam in 199025, the result of a parameter study of M-beams in two periods, 1971-76 and 1981-86, was shown in figure form.

The information for M-beams and Y-beams in the period 1996-97 is shown in Figs 5 and 6. There was a tendency, between the 1971 and 1981 periods, for depths and prestress to become greater at all spans. This does not seem to have continued in the next 15 years. The graphs show the 10% exceedance lines from 1981, and it can be seen that, despite the greater

(a) M beams 1996 - 1997

1400

1200

h E 1000 - E 5 d 800 Q

600

400

- 4

3 /

(b) Y beams 1996 - 1997

Fig 5.1996-1997parameter study based on span VS depth

410 The Structural Engineer Volume 76/NO 21 3 November 1998

Paper: Taylor

/ 0

0

- from designs between 10% exceedance line

1971 - 1976 (see ref.25)

I I I I 1 200 400 600 800 1000

PS (tonnes)

(a) M beams 1996 - 1997

30

25

20

v E c 15 ma v)

10

10% M beam exceedance line 1971 - 1976

I I I I l 200 400 600 800 1000

PS (tonnes)

(b) Y beams 1996 - 1997 Fig 6. 19961997parameter study based on load VS span

weight of they-beam over the M-beam and its relative inefficiency, in practice the resulting structures are broadly similar.

The SY-beam reintroduced an interest in stability that required new research. I-beams in the 1960s must have had high top flange slenderness for their intended span range, but little research or design guidance appears to have been produced in that era. Some work was available from America, but more assurance was sought when the SY-beam was first used. The SY- beam can have top flange slendemesses in the order of 125 at the long-span end of the range. Initially, a simple check of the lateral torsional stiffness of a SY-beam was carried out, and a stiffening transport frame was devised (Figs 7,8). Subsequent research by Stratfordz8 has provided an excellent cor-

Fig 7. SY-beam, lateral torsional displacement test

roboration with the result of a tilt test (Fig 9), and extensive further analysis has produced a general solution to the lateral buckling problem. This has shown that the SY-beam, made with normal construction imperfections, has an adequate factor of safety against lateral buckling in handling and that the transport frame provides a useful further margin to deal with dynamic and static forces during transportation.

The Wallbank survey work previously referred to demonstrated the need for continuity in bridge decks to remove deck joints which, when they fail, allow salt water to reach the bridge substructure. This requirement for continuous decks at inner supports led to the logical conclusion of requiring integral construction. The interest in these developments from the point of view of bridge beams is two-fold.

The Wallbank work covered a range of bridge types, including 32 steel decks. Of the 200 bridges, 74 used prestressed concrete. There seem to be no particular problems with precast concrete bridge beams, although in some bridges leakage between the outer and second beam of the deck was noted and was probably due to failure of the waterproofing layer in the deck at the kerb or in a service duct. The prestressed concrete bridge beam appears to be a survivor! Further research to look at concrete mix and production issues particular to bridge beam manufacture may be of more than narrow interest.

Wallbank also noted the deck joint as a primary cause of bridge deterio- ration. The move towards integral construction is a sensible further step.

Integral construction is not just a prestressed bridge beam issue. Bridges of all construction materials benefit. Prestressed concrete brings some added complications in design, with respect particularly to long-term effects. These were pointed out and partly addressed in they-beam launch paper^^.'^.''. The Prestressed Concrete Association (PCA) has commissioned further work in this area, to the extent that design advice is now a ~ a i l a b l e ~ ~ ' ~ ~ ' ~ ' .

There .is a considerable body of research from the USA on these issues


Paper: Taylor

Fig 8. SY-beam transport frame

l Prediction for beam without frame 0

60

ow I

-L”

Fig 9. SY-beam: results of lateral torsional displacement analysis and test

which are referenced by Clarkz9. The final and useful reference on integral construction is the report of the study tour of the Concrete Bridge Develop- ment Group in the summer of 19973z.

Development overseas Mag~~el’~ describes a number of exciting early bridges constructed in France using the Freyssinet system and in Belgium using his own, Magnel, stressing system. Construction was well advanced and ahead of practice in the UK with respect to post-tensioned construction; Magnel acknowledges the pretensioned method and mentions the early UK rail bridges.

Magnel designed the first prestressed bridge in the USA in 1949 -Walnut Lane in Philadelphia. This bridge had a 47m span and was made from site precast, post-tensioned I-beams. Unfortunately, the grouting was inade- quate and the bridge was demolished some years later.

The first AASHTO girders I-IV were adopted in 1958-1959 (Fig 10). The wide-top flanged type V and V1 were introduced in 1960-1961. AASHTO girders tend to be used at greater spacings than equivalent M- or Y-beams because US bridge loading is much less than that in the UK and has less prestressed reinforcement. Further development in the USA has come from rationalisations of changes to the standard section that were generated by a number of State departments of transport.

In 1980 a second attempt at deriving national standard shapes was made by the Federal Highway Administration, and a bulb T design was finally adopted (Fig 10). Rival versions of the bulb T are still developed, and were compared recently”. The more recent changes were justified on grounds of efficiency of cross-sectional area and minimum material cost, not the more holistic analysis used in the development of the Y-beam in the UK. A further reason for change in the USA is to enable precast beams to be post-tensioned onsite to extend their span range beyond the transportation limit. A discussion of the US bridge beam is given in the PCDG study tour report”, and an example of such a bridge is illustrated.

412

Apart from the UK, the European countries have developed standard bridge beams to some extent. Most countries have some form of I- and box beam, with some developing a large top flanged bulb T. The notable excep- tion is Germany where bridge beams used are often post-tensioned. Belgium and France have recently published design guides, with the French guide being up-to-date in addressing secondary continuity moments for precast bridge beams made

The future Eurocodes A suite of Eurocodes must be used to design prestressed concrete bridges: the basic concrete Eurocode EC2, EN 1992-1-1, its section on precast concrete, ENV 1992-1-3, the section on concrete bridges, EN 1992-2, and the one on loading, EN 1991-2, will be the minimum requirement. There will also be a product standard for precast bridge beams. All of these documents exist currently either in ENV form or as committee drafts and, in most cases, NADs exist.

NADs (National Application Documents) tend to be written to give design results similar to current designs to UK Codes and have been devised to sim- plify trial design. The final form of the Eurocodes, which may not have accompanying NAD material, is as yet unclear, and it is the author’s view that it is better, from the point of view of understanding the implications for bridge design, rather than understanding the design approach, to study the Codes with as little NAD intervention as possible.

New materials Despite the excellent performance of steel reinforced bridge beams, it would clearly be beneficial if non-metallic reinforcement could be used both for longitudinal stressing and for shear. Advantages would come from an assurance of chloride resistance over the long term.

ff( %

L-J Type I 4in

C m .- E

3ft 6in

E

I-------? 3ft 6in

Type v ’ - 2ft 4in - ’ Type

3ft 6in I c[x{xg[I I

.- S W

- 2ft 2in %‘ W

BT-54 BT-63 BT-72 2ft 2in

Fig 10. AASHTO sections types I-N and PCA bulb Ts

The Structural Engineer Volume 76/No 21 3 November 1998

Paper: Taylor

e High performance polymer - Aramid

I

I I Unsuitable I

for prestressing I and inefficient I for reinforcing I

Mild steel e

High yield steel e .

Stabilised and deformed strand

I I I

1000 2000 3000

Ultimate strength (N/mm2)

Fig 11. Cost of reinforcing and prestressing on a pellformance basis

A number of materials have been considered, including glass, carbon fibre, and high performance aramid polymers. Glass has well-known alkali-resistance problems and even in its alkali-resistant form is unsuitable for prestressing. Carbon and aramid tendons can be used but would require care- ful design to take advantage of their very different properties from steel and their cost. B u r g ~ y n e ~ ~ has given an excellent review of the need for a new approach to design using advanced high performance materials. The new materials have lower E values and higher strength than prestressing steel, pronounced creep rupture properties, and tend to fail abruptly without yield. In some cases, bond can be too good, not allowing slip at a crack and not, therefore, producing a ductile failure. Unbonded systems have been used but these have the disadvantages of no gain of strength from first crack to failure and the cost and difficulty of anchorages. It has been shown by Lees3’ that intermittent bond can be a suitable form of design for beams using pretensioned aramid reinforcement. This, and some additional confinement of the compression zone, can produce a load displacement curve not unlike that for a beam prestressed with steel with large deflections and warning of failure.

Shear also requires a new approach if non-metallic links are to be used, perhaps with novel reinforcement layouts.

Finally, there is the problem of cost. A bridge beam using an advanced polymer stressing tendon may be 10-15 times more expensive than one using steel. This does not reflect as unfavourably when a complete bridge deck is considered, and it is possible that the new design approaches, cou- pled with more appropriate methods of manufacture (making beams in full bed lengths and cutting them afterwards to the required length, for example) could result in the cost differential being reduced. What is needed is more work on shear, followed by design studies to develop schemes that take account of the new material properties in a rational way followed by a first real project.

Demonstration bridges using both carbon fibre and aramids as tendons in the pretensioned beams have already been constructed in Japan, and a first design Code for the use of these materials exists. This can be used as a starting point, but the design approach would still benefit from a radical rethink.

It has to be questioned, however, in view of the excellent performance of pretensioned bridge beams to date, whether the move to a non-metallic tendon and other reinforcement, with the inevitable long-term uncertainties, can be economically justified. The cost position is illustrated in Fig 11. Here, the various materials used to reinforce or prestress concrete are shown as the basis of cost to carry 1kN over a distance of lm against ultimate strength. The efficiency of high strength prestressing steels is shown, justifying its economic case for use in high volume products such as lintels or short-span flooring.

The zone in the centre of the figure is marked as unsuitable for prestressing or reinforcing. In this case, with flexural components having an ultimate load factor of twice that of the working load, the strains are too high

for crack control and too low for efficient stressing. Glass and aramids have low creep-to-failure limits and, when used for prestressing, the design process and form of structure must take account of the initial prestress levels that would be lower than the 70-75% commonly used for steel. In the author’s view, the reasons for using non-metallic reinforcement in bridge beams must be particularly persuasive, much more than in abutments, decks, and parapets.

Intelligent bridges The microelectronics revolution has resulted in costs of processing data from sensors being significantly reduced. The development of durable sensors is continuing, and it may soon be possible to fit a transducer to a bridge or bridge beam that will be suffrciently reliable to produce data over the extend- ed lives of real bridge structures. A more important issue is to consider what such instrumentation should measure: there is little point in instrumentis- ing a bridge beam and monitoring it for 50 years or more in the hope that a critical bridge-life-endangering event is recorded that would otherwise not be obvious from the road below.

The use of an alternative approach may be worthwhile, similar to that used in offshore structures which may be analysed by spectral methods in which the loading spectrum is transferred into a spectrum of response of the structure at a particular location - e.g. the sensor site. The sensor could measure the specified response by having intelligence that could enable it to act as a datalogger. This spectrum could ultimately tell the bridge owner about the loading experienced by the bridge and, if a damage rule could be devised for the structure with respect to fatigue, for example, it may be possible to make predictions about design life.

Conclusion This paper has traced the development of the bridge beam from its first tri- als to being the major contributor to short- and medium-span bridging over the first 50 years of its use. The power of incremental innovation has been demonstrated in the many improvements in reaction to success and failure of designs and details. The basic premise that pretensioned beams are an inexpensive and durable form of bridging still applies. There will, no doubt, be future changes, but it has to be accepted that these will be from a position of strength.

Acknowledgements The author is not trained as an historian and in describing developments has attempted to present a brief review of a process to which many engineers, some well known and some less prominent, have all contributed. The inclu- sion or omission of contributions and contributors is not intended to be indicative of critical prominence.

The author would nevertheless like to acknowledge valuable discussions with Dr F. Walley, Mr M. Burke, Dr G. Somerville, Mr G. Tootle, Mr K. Wilson, Mr S. Chakrabarti, and colleagues now retired, Mr E. Harby and Mr J. SnasdelI.

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